This eighth-grade science lesson is about friction. It is the 24th lesson in a sequence of 32 lessons on mechanics. The lesson is 62 minutes in duration. There are 27 students in the class.

00:00:03Okay guys. We're all, uh, looking good. Feeling good?
00:00:12Very good.
00:00:20What we're going to do first- what we're going to do first is huddle. So let's move up for huddle.
00:01:14Adjusting microphone. Okay. All right I'll start talking to you while these are being passed around. What you need to do is to take, uh, one copy.
00:01:25This is going to be yours so would you start there and pass them around that way, and David, you start there.
00:01:33Uh, you can start having a look at those as they come around and listen to me at the same time because, uh, I want to ask you a- a rather strange question first of all.
00:01:45And if you want more clarification, if you think you- if you think you can answer it straight away that's fine, go ahead. But if you want more clarification you must ask me some questions, okay?
00:01:59Um, you may not have heard this question before, but I'm not going to tell you the answer straight away. We'll move on and we'll discover the answer as we go.
00:02:11Okay, this is a question that, uh, really was put some hundreds of years ago and people, uh, philosophers, as they were then called, not scientists, uh, thought about it and talked about it.
00:02:24And that is, any solid object that you might consider, whether it be this mass or this trolley or indeed, anything, the question is, what is its natural state? What is its natural state?
00:02:42Now in old days- yeah, you'll be first, just wait a minute.
00:02:46In old days philosophers used to be considered a lot or concerned a lot about considering questions like this of natural state and the perfect shape.
00:02:56Anyone know what was considered to be the perfect shape?
00:02:59A circle?
00:03:01Close. Three dimensional circle.
00:03:06Sphere. That's right. Yes. So they talked about things very philosophically. Didn't do a lot of experiments or measurements. That came quite a bit later.
00:03:17So one of the questions was, to be considered, what's the natural state of a body? Now you were first, Olivia?
00:03:23Uh, I thought it was an object.
00:03:27Uh, a body or an object. That's all right. Yeah.
00:03:29A solid or- //(inaudible).
00:03:31//We- we're talking about a solid. Diane?
00:03:35Untouched, we're getting there. Anyone want to ask me any questions to, uh, clarify the question a bit more? What is the natural state of a body? A body is just an object.
00:03:46As I said, it can be anything. Yourself. You're- you're a body. Any other ideas? Anyone want to- I mean, is that question clear to you?
00:03:56Do you think you know what it means? Do you think I- you know what I'm after? Then you should ask me some questions now. What do you mean by this or-
00:04:08What do you mean by a body?
00:04:09A body, good. You're a body. This is a body. I'm a body. My pen's a body. Any solid object.
00:04:16So where it came- came from before is here.
00:04:21Not from where- not where it came from. We're not concerned about where it came from. We're concerned about its natural state.
00:04:29Um, in a rock? It was in a rock once?
00:04:32Well, a rock is a body, but it's not in a- Yeah?
00:04:34Does that mean like, what's it use?
00:04:37What's it?
00:04:38Its use.
00:04:39No. Not what's its use. I'll give you a clue now with a little demonstration. Um, if I take this mass and just give it a little force, what's it do?
00:04:55It moves. Is it still moving?
00:04:57What's its natural tendency to do?
00:05:01Everything do that?
00:05:05Even a trolley?
00:05:09Eventually. What if you throw a ball?
00:05:12All right, what if you fire a bullet?
00:05:14What if you fire a rocket?
00:05:17Into space?
00:05:20It floats.
00:05:21It floats?
00:05:22There's no gravitation to stop it or pull it down.
00:05:24Once it gets away from earth, does it stop?
00:05:27Yes- oh, no.
00:05:29Ryan says no.
00:05:31Not until it hits something.
00:05:33Not until it hits something.
00:05:34Like a planet.
00:05:36Okay. If- If we can say that about rockets that we fire into space, then it wouldn't- And rockets are bodies in- in orbit, yeah?
00:05:45And- and rockets are bodies. This is a body. Then when we talk about the natural state we're really talking about the natural state of motion.
00:05:57Right? We're talking about the natural state of motion of bodies and- and philosophers used to wonder, is it a natural state for things to come to rest or be at rest or to keep moving.
00:06:09From what we've talked about- talked about so far, what would your conclusion be? Do things tend to come to rest? Keep moving?
00:06:18Or something else that you might like to suggest. Ideas. Doris?
00:06:24They tend to stop?
00:06:25They tend to stop like rockets that are going into space.
00:06:30Well, except for that.
00:06:31Except for that, but we want- do- do we want to include everything? We want a statement that will include everything.
00:06:38And if- if rockets don't stop, if their natural tendency is not to stop, maybe the natural tendency of this mass is not to stop nor of this trolley to stop eventually.
00:06:55All right. How do you stop it, Ryan?
00:06:58Uh, I put my hand there.
00:07:00And by putting your hand there you're applying what?
00:07:01A force.
00:07:02A force. Okay. What tends to stop things?
00:07:06Force and //gravitational pull.
00:07:08Forces. Gravitational forces? Do you reckon? Okay, forces generally. All right. Okay, rocket moving in space, is there any force acting on it if the rocket engines have stopped firing?
00:07:21Mm hm.
00:07:22Uh, let's say it's got well away from earth and it's well away from stars. It's traveling through interstellar space. Long way-
00:07:29No. It's not (inaudible).
00:07:30No forces?
00:07:32Any other opinions?
00:07:34The force is //(inaudible).
00:07:38You think it should have a force on it because it's still moving.
00:07:41Uh huh.
00:07:42Ah, that's good thinking. What do you reckon? If something's moving it's got to have a force on it to keep it moving. What do you think about that?
00:07:53Sounds reasonable, doesn't it?
00:07:54Yeah, but (inaudible).
00:07:55It all sounds feasible. That's what tricked people, you know, when they looked at things like this. Comes to rest. Stops. Must be its natural state of motion is to stop.
00:08:07That's its natural state. Everything should be at rest. Everything tends to come to rest eventually. Sooner or later. Rockets don't. That's the problem.
00:08:19It's good just was well the ancients didn't have rockets to contend with. It would have been a- a real fly in their theory, wouldn't it?
00:08:26Mm hm.
00:08:27Would have mucked up that theory. So now these days we've got to have a theory which explains why they were wrong or why rockets do indeed keep moving after the engines are turned off.
00:08:42What do you reckon? Which way do we go? Do we try to explain why a rocket will keep moving when the engines are turned off?
00:08:50Or do we try to explain why the ancients got it wrong in saying that everything should come to rest? Let's look at the rocket situation.
00:09:00We were- Who's- who's in agreement on this that things that are moving have got to have a force behind them to keep them moving? Who thinks that's right? Don't be shy.
00:09:12It's okay to think that. All right, who disagrees? All the heretics put their hands up. Oh, this is good. I like the heretics. Murray?
00:09:24Why do you disagree?
00:09:26Because it just doesn't seem right.
00:09:28Why? Tell us why.
00:09:30I don't know. I just don't think it's right.
00:09:32You got a gut feeling about this, have you?
00:09:33Mm hm.
00:09:34All right. Who else said they disagreed?
00:09:41Because there's no, um, there's a force stopping it.
00:09:44What forces stopping what? What? What?
00:09:46Like so when you- like skid it-
00:09:49Yeah. All the-
00:09:49It stops from friction.
00:09:52Like there's no friction or- or when you throw a ball it falls down because there is gravity. But in space there's none of that so it doesn't stop or fall.
00:10:00Very good. Right. Okay. Now look at your sheets. You'll see- where did the- where did the stray ones get to?
00:10:09Um, I don't know.
00:10:10Up there? Here we are. Thank you. You didn't notice that, did you Ryan?
00:10:15You weren't going to help me. Ever heard of Galileo?
00:10:20Well, this is a little bit of a riddle to start with and you should get it easy now. Galileo thought about it. Ever heard of Newton?
00:10:30Good. Newton formalized it which means that he wrote it down, made laws out of it. While we're all affected by it and experience it even if we're not aware of it in scientific terms.
00:10:43What is it?
00:10:44I don't have that sheet.
00:10:45Oh, there you go. Pass that down to Ashley, please. What is it?
00:10:53Rachel doesn't have a sheet //either.
00:10:54//Law of gravity?
00:10:55Oh, Rachel. how can we miss out two lovely ladies like you? There you go. Okay. What is it? Anyone got a clue yet?
00:11:08Yes. That's right. I said it'd be easy. Now let's read on and see what other clues I'd given.
00:11:13It caused people for hundreds of years to think that the natural state of motion is at rest because it makes moving things stop just as Justin pointed out. Okay.
00:11:22Then there's another list- there's a list of things there about, um, friction and some of it's effects and some of it's influences. All right. Put the sheet aside for a minute.
00:11:35Is friction- Is friction important to us? If so-
00:11:46Why? Where? Where- I mean, um, anybody got a pen in their hand at the moment?
00:11:52Got a good grip on it? All right, you're sitting on a seat aren't you?
00:11:58Mm hm.
00:11:59Got a good grip on the seat?
00:12:00No. I could //(inaudible).
00:12:02What- what if the seat was really slippery?
00:12:05You'd need friction to stay on.
00:12:07You'd need-
00:12:10Stay //on.
00:12:10//Stay on.
00:12:11To stick to it.
00:12:12Stay on. Right, very good. What- what- Before we look at, um, friction in our lives some more, what do you think friction is? It is firstly and foremostly what? Yeah?
00:12:27Forces and gravity.
00:12:29Forces and gravity?
00:12:32Let's- let's just say it's a force. How about that?
00:12:35All right.
00:12:36All right? Now we were comfortable with it being a force.
00:12:39Okay. Friction is a force. Now, let's just go- for a moment go back to the rocket and the, um, majority of opinion here that things that are moving need a force to keep them going.
00:12:56Who's still comfortable with that? Want to still support that idea. We've talked about friction. We've talked about things stopping. Diane, you still comfortable with that?
00:13:08Good. Who else? There seems to be a lot less at the moment. What's the matter with you? You giving up so easy. Come on. A little bit of fight in there, //hey?
00:13:19//(No) but there's like evidence to say that there's not like the rocket.
00:13:23Say- say that again. Go on.
00:13:25Um, it's like you can't like- with the rocket there's not like a force pushing it along in the air up there.
00:13:33Air up there. No air up there.
00:13:35There's no air up there.
00:13:36Okay. But there's no force pushing it along. We agreed there was no force acting on the rocket, didn't we? Did we agree on that?
00:13:46Anybody disagree?
00:13:48Diane again. You're a real fly in the ointment, aren't you? I like it. I like it. What-
00:13:55(Pushing it through like the) force.
00:13:56Diane. If- if we're all saying there's no force acting on the rocket as it's traveling through empty space with nothing around it, what force do you think is acting on the rocket?
00:14:04Um, it needs something to make it go.
00:14:07You're still convinced it needs something to make it go?
00:14:09I agree with Diane.
00:14:10You agree with Diane. Who agrees with Diane? Good, okay. Who's changed their mind in the process? Anybody agreed at first and has changed their mind now?
00:14:22Um, one, two, okay, three, right-o. Okay, we'll come back to that a little bit more later on because we need to talk about forces and things moving. Okay?
00:14:38Let's just look at this one more time. Does this take a force to get it moving?
00:14:46Does it take a force to stop it moving?
00:14:50What force was that?
00:14:52Friction. Okay. Now what if there was no friction?
00:14:56It would just slide.
00:14:57It wouldn't stop.
00:14:58It would keep going?
00:14:58It would just keep on going.
00:14:59Yes, it would slide.
00:15:00Okay. Would- Diane, would you agree that after- after I'd given that a force, a shove like that, and I'm not touching it anymore, would you agree that I'm not applying any more force to it?
00:15:11Um, kind of. Yeah but no because-
00:15:13//Yes and no because it-
00:15:15//(inaudible) type force to it (inaudible) friction but (inaudible).
00:15:17Yeah but, I'm just talking about- what- it's sitting there.
00:15:20(You applied force before) and like it loses the force gradually, slowly as it's sliding across the table as it gets slower.
00:15:26Loses the force.
00:15:27Yeah, like- loses energy like-
00:15:29Well, energy is different from force, isn't it? We learned that earlier on. Okay. All right. Is- is- is there any force being applied to this by me at the moment?
00:15:41No. Is there now?
00:15:44All right. I'll just give it a short burst of force. Once it left contact with my hand, are we agreed that I was no longer exerting a force on it?
00:15:56Okay, but would you agree that the force of friction stopped it?
00:16:02Okay. Was there any other force stopping it other than friction?
00:16:08Mm hm.
00:16:08Gravity (inaudible).
00:16:10Well, um, gravity you say? Okay. Uh, which direction does gravity- gravity act in?
00:16:17Down. Which direction is it moving?
00:16:24To stop something moving that way don't we need something acting that way rather than that way?
00:16:30Air pressure.
00:16:31Air acts against it. Right? Air resistance, we might say.
00:16:36Mm hm.
00:16:37Resistance to air. Good. Okay, anyway, coming back to this argument. After I've set it in motion, let's say there was no air resistance and no friction. Got that?
00:16:47It just-
00:16:48No air resistance, no friction.
00:16:48It just-
00:16:49Who says it will keep moving? Any dissenters? Anyone disagree with that? Right. If it keeps moving, a number of you said it needs a force to be acting to keep it moving.
00:17:05What force is then acting on it to keep it in motion?
00:17:09What force-
00:17:11Can you tell me that? If I've //stopped- I've stopped exerting- I've stopped exerting that force on it.
00:17:12//Force to apply (inaudible).
00:17:17And we said that the- there was no air resistance and no friction but that would tend to stop it rather than keep it moving, so what keeps it moving then? What force is keeping it moving, Diane?
00:17:30Your hand did.
00:17:31No, I've- I've taken my hand away after that initial contact like that. It's no longer there. Gabriel?
00:17:37Tension? That stored energy.
00:17:41Stored energy. Is- is that a force?
00:17:44No. We're talking about two- like apples and oranges, All right? Can't- can't talk about them as being the same. Add one apple and one orange, you don't get two of something.
00:17:53Two of-
00:17:54You get on apple and one orange. So force and energy, different ideas. Okay. All right. We'll talk more about that at the end, okay?
00:18:05What I want us to do now is to consider the question of, "what is friction and what determines how big it is?" Does it matter how big the force of friction is?
00:18:18Are there circumstances where we need friction to be maximized and other circumstances where we need friction to be minimized?
00:18:27(inaudible) //(like storing in a) balloon.
00:18:28//Give me some examples by putting our hand up and not calling out all the time. Give me some examples of where we need to maximize friction. Alvin?
00:18:38In a racing car. Like the wheels have to have more friction so it can handle the corners better.
00:18:43Traction. Right? When turning a corner you need friction between the wheels and the road. Very good. All right. Jasmine.
00:18:48Uh, you also need it on a car when you want to stop.
00:18:52To brake.
00:18:53Yep. Certainly you want a lot of friction there to stop. Yeah.
00:18:55If you have too much it wears the motor.
00:18:57But if you have too much it wears the motor, so we want less friction inside the workings of the motor and more friction between the tires and the road in cornering.
00:19:08All right? But not while we're traveling along. Uh, what about yourself? And personal affects and things like that? Um, if the- Well let's- let's, uh, ask the question about your clothes.
00:19:22How do your clothes stay on you?
00:19:25If they're really tight. Shoulder things. If they're really tight. //(inaudible).
00:19:27//Oh, I can see what you're thinking. Diane?
00:19:30I don't know.
00:19:31Really tight.
00:19:32Really tight. Okay. If things are really tight they tend to have-
00:19:42Okay? If you hold something- just- just borrow that pen for a minute. Now I want you to pull it out of my hand. Okay, easy?
00:19:48Mm hm.
00:19:49Was I holding that very tightly?
00:19:51You need more friction.
00:19:53I need more friction. Let's give it some more friction. I've got some more friction. Pull now. Bit harder now, isn't it?
00:19:57Sweaty hands.
00:19:58You didn't have enough friction. You've got slippery hands?
00:20:00All right? What tends to decrease friction?
00:20:02Water. //It makes it slippery like-
00:20:03//Water. Water- water- what's a- a word that's, um, more general than water?
00:20:11//Liquid, um, L- starts with L- in cars.
00:20:16Lubricants. Very good. All right. Okay. Sometimes we need to maximize friction, sometimes we need to minimize it. Now what determines how big friction is? How large it is. Yvonne?
00:20:33Newton. Newton said it was going to be a certain size and it's going to be a certain size. Um-
00:20:41We're- we're really talking about, um, like Newton discovers laws and ideas, he doesn't make them up. Okay?
00:20:50Newton discovered gravity, he didn't say how big it would be so I don't think Newton got that big a say in the role of the universe. Justin?
00:20:58It's surroundings- Like where it is- Uh-
00:21:02We need to be a bit more specific. We'll come back to you. Jacob?
00:21:07Um, what sort of surface.
00:21:08Oh, what sort of surface. What if this was ice here, would this go more easily?
00:21:16All right. If it were sandpaper-
00:21:19No, it wouldn't.
00:21:20Wouldn't slide so easily. So the nature of the surface is important, yes?
00:21:24All right.
00:21:25It's wet and it slides.
00:21:26Now what else makes a difference?
00:21:30It's flat.
00:21:31See if you can guess.
00:21:33As I start to do this.
00:21:34The flat, hard surface.
00:21:36We've talked about the nature of the surfaces. Something else. Something else determines the size of the force of friction. Jacob?
00:21:43(inaudible) or-
00:21:47Is it the ability of what the thing weighs already?
00:21:50What it weighs. All right, why would it the weight make a difference? Look, I can demonstrate that here. If I give this a little push, it just rolls along like so.
00:22:01If I put some weights on there and give the same little push it doesn't go quite as far, and the more weight I put on there the more force I'm going to exert to get it going, isn't it?
00:22:12And what's more, it will-
00:22:17It's only //(inaudible).
00:22:18//It'll go faster down a hill.
00:22:20Go faster down a hill, yes, but with more weights on here do you think it's going to stop this from going so far?
00:22:29No because-
00:22:30All right. Because it's-
00:22:32It's gravitational pull and you're saying if you push like it harder it'd be the same as pushing it softer because it's got weights on.
00:22:41All right.
00:22:42It still got the same //(inaudible).
00:22:43//Let's- let's ask a different question. Let's say we had a- a flat board here and you had a rope attached and you're going to drag your mate along on that like a slide. A sled.
00:22:53And you had one person on there you're dragging along. Not too bad. That's easy enough. And then your friends turn up and they all jump on. Is that harder?
00:23:01Mm hm.
00:23:05Because it's just (inaudible). They weigh too much.
00:23:06What's it- what have all those extra masses done to the force of friction?
00:23:10Increased it.
00:23:12Increased it. Okay. They're the ideas that we're going to test. Turn to your sheets now. And see number one, how do we measure it? What is it again?
00:23:22Friction. Good. There's some instructions there and then over the page there's some tables for you to record your measurements.
00:23:32And number two, what determines how big it is? And then we go on to effects of surface area and then at the end we've got a couple of interesting questions.
00:23:44Does it exist in a vacuum? Does it exist without gravity? We'll talk about those later.
00:23:51First of all we want to accumulate and gather data that supports our suspicions that friction is determined by- that is the size of friction.
00:24:01How much friction there- there is, is determined by the force of gravity on the body that's moving. And secondly the nature of the surface. What about the size of the surface? The area.
00:24:17If we take a block like this and we want to move it along like so, then we want to move it along like so. Would it be easier one way or the other way?
00:24:30Um, it would be easier that way.
00:24:32Along the grain.
00:24:33Easier on the smaller surface?
00:24:34There's not as much surface on the-
00:24:36Okay. Well, you're going to take measurements to test that. All right? You've got your predictions. You've got your ideas now and you're going to find that out.
00:24:46This one might be the little more interesting one of them all. Now bear in mind you're going to be using spring balances and what did I tell you about spring balances yesterday?
00:24:53They're not always accurate.
00:24:54They're not perfect and so when you take measurements with them we've got to see that, uh,
00:25:02A number of measurements that vary by ten or twenty grams when we're measuring five hundred or so, can be- can mean that the measurements are the same.
00:25:12You've got to bear that in mind when we draw conclusions from our results. Okay. Your needs for this activity, are going to involve a block of wood. Thanks, Olivia.
00:25:28Um, these are friction blocks and you can see that that's for checking out the nature of the surface. That comes later in the instructions. So don't take those first.
00:25:39You've got spring balances here. The blocks, we've got scissors, well there's a heap of scissors there and there's- in case you all thieve those, these are tied on.
00:25:52Okay, you could cut them couldn't you?
00:25:56But you're honest people so- so you're going to need to take a- a section of string from here to attach to your block.
00:26:03And these are weights for when you come to the sec- section that deals with adding weights. But the first bit- have you read the instructions for the first bit yet?
00:26:13What are you going to do with the first bit?
00:26:15I don't know.
00:26:16Someone tell me. What are you going to do? Yes, Olivia.
00:26:18I haven't read it.
00:26:19Someone? Who's read it? What are you going to do with the first- what's the first one? Justin?
00:26:22To see how long with the spring balance it takes to pull along the block.
00:26:26Okay. There's two parts to the first one. The one- the first one is how much force does it take to get it started? And the other is how much force does it keep- take to keep it moving?
00:26:37Do you think it's going to be different?
00:26:40Yes. Oh why?
00:26:43I don't know.
00:26:46You don't think it's going to be different?
00:26:48Mm yeah.
00:26:49Who thinks it's going to be- Look, to, um, to get a- a block moving, let's say I was applying a force.
00:26:57I've got a spring balance attached to here, string, and I just stretch gently, slowly, stretch, stretch, stretch, until all of a sudden, boom. It starts to move.
00:27:05And the- the force that I read- the maximum force in grams on the spring balance is the force that it took to get that moving.
00:27:12But if I start it moving and then I just measure it as I move it along at a constant speed and I read the force on the spring balance.
00:27:22Is that going to be different from the one that I read to start with to get it moving?
00:27:28Oh, we have a difference of opinion. There's some yes, some no- some no. That's good. We're going to find that out too. Okay?
00:27:35We're going to do five trials for each of these measurements. Why? Why do we do a number of trials? Jasmine?
00:27:41Um, to get the average. To get the closest to the (average).
00:27:44That's right. Because there's always errors associated with each of our measurements. By taking a number we're more likely to get closer to the truth. Okay.
00:27:54When you think you know what you're doing, you move into your groups. Come and get your apparatus and begin taking the measurements.
00:28:11Yes, you can.
00:28:20Why do we need the string? Oh, to hook it on there.
00:28:26We didn't have any eyehooks so, there you go. Like that.
00:28:38You're making science look boring in Australia? You're making science look boring?
00:28:45Thank you. You can tell me later.
00:28:46No, (actually). Wait, I just want to say, sir- cause people will think, like, Australia is really boring and stuff.
00:28:49Good. Would you like to-
00:28:51Would you like to do the experiment?
00:28:54Well, let's go and do it.
00:28:55Okay, //I was just saying-
00:28:56//We can talk about that later. All right?
00:28:58(I'm not trying to be) (inaudible).
00:29:02You can- you can do this later. All right?
00:29:04I was just saying, cause you make it look boring, make it look like you have fun and (learn) at the same time?
00:29:07Why don't you go and have fun?
00:29:08Yeah, but we always have fun and you're making it look boring. Make it look fun like you normally do.
00:29:14Oh, (we'll get there). Go and enjoy it. Hey, hey Amanda? You think that's appropriate behavior in here?
00:29:25Well, you get sent outside if you do that.
00:29:29Let's get on with our work.
00:29:40Got your gear?
00:29:43What are you waiting for?
00:29:45(inaudible) the string.
00:29:46Oh, so you're getting the string?
00:29:47You read the instructions?
00:29:48Mm hm. Yes.
00:29:49Know what to do?
00:29:50Good. Now one thing you got to consider is the section of the bench you're going to work on. You want it to be almost all the same. Okay? So clear your things away a bit-
00:30:01Do we have to finish all this, today?
00:30:03Do we have to finish all this today?
00:30:04Oh, we may not finish all that today. No. We've taken a bit more time.
00:30:10Okay. You got your string? Start.
00:30:12How're you guys going?
00:30:15Do we do it again?
00:30:23All right. Now that's just getting it started, is it?
00:30:35You slowly stretch it until it starts to move. Now you're getting very small readings. Why don't you get one of the more sensitive balances.
00:30:42Because this is for bigger- when you've got bigger forces to deal with. Okay?
00:30:51How're you guys doing?
00:30:55Relax. You're on-
00:31:00Sir, how do you do this?
00:31:02Now, choose a section of your bench. Here it's all fairly smooth.
00:31:06Yeah, there.
00:31:08You don't need particles on there. Move it away.
00:31:09Aren't we supposed to get this (inaudible)?
00:31:10Now this spring balance- Uh, is- it's going to be a very small measurement. Okay? There are more sensitive ones up there.
00:31:19Yeah, but they're like stretched and breaking //(inaudible), not working anymore.
00:31:21//Yeah, we like this one (inaudible).
00:31:23No. You haven't had a look at them. These are good ones.
00:31:25Okay. (inaudible).
00:31:26All right. Go and get one that's a bit more sensitive because we want a bigger measurement.
00:31:31All right? A bigger reading. That's good.
00:31:33We do it five times (inaudible) once.
00:31:34And then you just stretch it slowly until it starts to move and you've got to watch carefully to see what the maximum force was.
00:31:40Who's recording?
00:31:42Yes, Olivia?
00:31:43Are we going to get new stools?
00:31:45Are you what?
00:31:45Yeah, are we going to get new stools?
00:31:46Yeah. How come the other classes get one?
00:31:50(inaudible), yeah.
00:31:51We'll talk about this furniture later, all right?
00:31:53No. Because (inaudible) we had new stools.
00:31:56There's a special story with that. I'll have to tell you later.
00:31:58//Well I'm special enough.
00:31:59//Tell us now, please.
00:32:00Oh you're special. Yes.
00:32:02Yeah, because we got to sit on them today, yeah.
00:32:06Yeah, but no, we're aren't special-
00:32:08I'll see what I can do for you with the stools, all right?
00:32:10Can I please go to the toilet?
00:32:11Yeah, go on.
00:32:13Mr. (inaudible)?
00:32:14Yes, girls. You've got the wrong block to start with. You want one of the ordinary plain blocks to start with. This is a friction block. Okay? Did you remember the one that I-
00:32:25Where do you get the blocks?
00:32:26Just up in the box up there. In the tray. Is there one left there?
00:32:33Yep, they're there. Just over there.
00:32:35This one?
00:32:36Yeah, anyone will do. Anyone.
00:32:45Okay, know what to do?
00:32:50Well, have you read?
00:32:52You did? All right. Now you've got to tie that on there but do that when I finish talking.
00:32:57And then tie the other end on here. Now you're going to just slowly pull on this keeping it flat.
00:33:04Pull on it until the block suddenly lurches forward and then where that gets to, read it off on the grams side here. That's going to be your maximum force to get it started.
00:33:17And you record it in your table over here. That's number one trial. Then you do it again for a number two trial. And start from the same place each time.
00:33:28Okay? And then you repeat it, but this time- for this one, you get it moving first and you read its measurements. Okay?
00:33:46How're you going, boys?
00:33:47Pretty good. We, um,-
00:33:48Got some measurements?
00:33:49Yeah //we've got the first one.
00:33:50Now wait, wait, wait, wait. What are you writing up there for?
00:33:53Because it says. Record your results here below.
00:33:55Yeah. See, I draw a nice table for you and you want to write over there.
00:33:59//Why not?
00:34:00//How do you know which one's different like that?
00:34:02Well you just keep it on one surface first, all right, and you record in that table and you're doing five trials.
00:34:08Okay. I'll- I'll do the next trial, okay?
00:34:12Now, this spring balance is a big one. Why don't you get him one that's more sensitive to these measurements because it won't go far on the scale there. This is for when we put the weights on it.
00:34:22All right?
00:34:23Yeah, I'll go get another one.
00:34:26Where's your group, Justin?
00:34:28Have you got any results yet?
00:34:30Why are you using this spring balance rather than a more sensitive one? This is for when we put weights on there. This is when you need bigger forces.
00:34:39All right. I'll go get it.
00:34:40Do we get a small one?
00:34:41Uh, a small one would be good. Yeah. It'd be, uh, a bit more sensitive to it.
00:34:52Fifty, 50, 100. What's happened here?
00:34:55(Two hundred).
00:34:57Was it- was it all that different?
00:34:58No it wasn't. It was 175.
00:34:59One seventy-five. One seventy-five.
00:35:01No, I saw 201 (exactly)-
00:35:03Oh. Guys, guys. You've got the spring balance that measures the bigger forces. This is the one we put weights on here. Why don't you get the little spring balance that's more sensitive.
00:35:13Yeah. Think about those things, all right?
00:35:20Why didn't you point that out to the guys?
00:35:23You didn't think about that, did you?
00:35:24I didn't think about it.
00:35:26All right.
00:35:33Okay, you rebels. Trouble makers down here. Hey. Why-
00:35:41You must have now noticed it.
00:35:42Why have you got this spring balance? Why don't you get a more sensitive one because look, it's not reading very much.
00:35:47Because (inaudible).
00:35:48Oh, like it's my fault?
00:35:51Yeah, this spring balance just reads up to 100. All right? You've taken one measurement already, have you? 50.
00:35:59Do you- do you understand why I'm saying that about the spring balance?
00:36:02Well, why'd you put them out for?
00:36:04Because when we add weights later one we'll need ones that go to bigger readings.
00:36:11You should have put them in separate containers and told us.
00:36:13They're in separate piles and I wanted you to think about it.
00:36:16Well that's-
00:36:17I wanted you to figure that out.
00:36:18Yeah. I got you on that one, didn't I?
00:36:25Is that 100 grams? That's it. Use that one.
00:36:32Um, when we do it, right, do we-
00:36:34No, no, no, no. Did- I- maybe I didn't tell you guys. You want a more sensitive spring balance for this one. These read to a kilogram. All right?
00:36:44And you're only going to pull down to about there. All right? And there's a lot more error in here. Okay? This comes later when we put weights on it. Okay?
00:36:56So whip- whip up and get the little balance. 100 grams. Even a 500 would be all right. Now what were you going to ask, anyway?
00:37:04Um, when you do it- when you pull it and it like goes- it goes to a high point and then it goes back to a low point, do you do the higher point or the low?
00:37:11The higher point because it's what's needed to just get it started.
00:37:15Okay? Now if you use that one it's going to be more sensitive.
00:37:27Okay, now. These- these are in Newtons. Now to convert them to grams we just multiply by 100, so that'll be 50 grams, 100 grams, 150, 200.
00:37:38That's- this goes up to two fifty grams.
00:37:39Oh, okay.
00:37:42All right? So it's just a confusing one, isn't it?
00:37:50(inaudible) did you read it?
00:37:51That was like- there.
00:37:53Got up to there, did it? See, I wasn't looking. Let's try that again. Let's try it again. You yell when it starts to move. I'll do this very slowly.
00:38:06Oh, got up to about there didn't it?
00:38:08Mm hm.
00:38:09All right. What do you reckon that reading is? That's one- one point four?
00:38:12One forty.
00:38:14One hundred and forty. Very good. That's what you'd record. Then you do- push it back and do it again.
00:38:19You said it would only be something like 80 or 70 grams.
00:38:22Oh I was guessing. I was a guessing.
00:38:25Oh, this one read similar so (inaudible).Yeah, it read sim- similar.
00:38:30Well that means it's a good balance, but we can't be guaranteed of that. And errors in here- see if you make an error of, uh, ten in a hundred, what percentage is that? Ten percent. Okay?
00:38:45But the errors involved, you know, this- The spring in here is just as stiff as the spring in there. So while this moves a distance here, it makes a bigger reading.
00:38:56There's a bigger amount of grams in the same distance move there, than there is in there, isn't there? So that's why this one's got a bigger percentage error.
00:39:03You learn all about percentage error as you go on in science.
00:39:09Well, ladies, how're we going?
00:39:11Very good.
00:39:11All right.
00:39:12Very good. That means you've got some very consistent results and by golly, look at those results. That's brilliant. That's brilliant.
00:39:18Thank you.
00:39:19Excellent. Who was reading those?
00:39:23I was taking them down.
00:39:24Ha ha. Well, you guys are going to be scientific experimenters, aren't you? Look at that. Beautiful!
00:39:30And hey, look! These are generally looking a bit higher, are they?
00:39:34Yeah, but then-
00:39:35A bit higher than those.
00:39:38And this was the one when it was already moving.
00:39:40And you're reading that.
00:39:41It went up and then it went down, we didn't know which we're supposed to write down.
00:39:45Oh. I'll show you. Let's-
00:39:46It went up and then went down.
00:39:47It's- it's better to see it happen and we'll see if you've done it the way I mean you to do it. Just move this around a bit. Now you get it moving and you read it while it's moving.
00:40:00What's that reading?
00:40:02About 100.
00:40:04You've got a 120 or something like that, haven't you?
00:40:06Let's do it again.
00:40:07I didn't read that one, that was Ondine.
00:40:10What's it?
00:40:10A hundred twenty. Ha ha.
00:40:11Now- No. Down there.
00:40:15All right, I can't read it. I'm not (inaudible).
00:40:18How about 80?
00:40:18I've got 90.
00:40:20No //because they're all like in between-
00:40:21//You got to- you got to keep it moving at the same-
00:40:24They're like 30 or 20 off, and you said that all-
00:40:27Yeah. We will- we'll worry about those when we look at the averages. All right? //So-
00:40:32//What does it read?
00:40:33I'm doing that now. One-twenty.
00:40:35Look at this. I'm getting an 80 reading again. You've got 120- you- you were reading this-
00:40:43The first one was Ondine, the rest were me.
00:40:45All right. Okay. Would you- would you like to-
00:40:47No, Sabrina.
00:40:48Would you like to try those again?
00:40:50What is it?
00:40:51Would you- you reckon you might like to try them again? Just check it out? Okay.
00:40:56And remember you've got to get it moving first, and you've got to keep it moving at a constant rate and then read what it says. Got it? Good on you. Okay.
00:41:09Sir. This (inaudible) the same.
00:41:13How's the results going, guys?
00:41:15We got 110 that one.
00:41:17All right. And this one's less.
00:41:22I'm proud of you, guys.
00:41:24You're supporting the theory. Love it. Is that what you would have thought?
00:41:31You would have thought it was the other way around?
00:41:34What about you, Alvin?
00:41:35Yeah the same as Brian.
00:41:36Well, I want to hear why later on. Okay?
00:41:44What have we got?
00:41:45(One twenty-five).
00:41:46Oh, this is starting to look lower. You reckon it should be lower?
00:41:49Not really.
00:41:50You reckon it should be higher? The same?
00:41:52I reckon it should be the same.
00:41:53The same?
00:41:54I think it should be lower.
00:41:55Why do you think so?
00:41:57Because like- when you tug it needs more force to move it and then when it starts moving, it- it- you use less.
00:42:04Why do you use less but? This is- this is not an easy question to answer, I might add, so I'm interested in- in why you think that.
00:42:16I dunno, we'll find out.
00:42:17We'll talk about it later. Yeah, this is something for you to worry about now. All right? But don't worry too much. All right.
00:42:23Get your results down first because all science is based on your results. All right? Data. You take measurements and then you draw a conclusion. Okay. Keep going.
00:42:34Come on guys. How're we going? Let's have a look at your results.
00:42:36We're working out the average.
00:42:37Mm hm.
00:42:39A hundred and ten, 150, a hundred and si - whoa, you're all over the place aren't you?
00:42:43I reckon the quicker you go the, um, more-
00:42:46They should have about-
00:42:47Yeah, but this- this one's just for getting it started, isn't it?
00:42:50All right.
00:42:50Except the quicker you go like //the quicker-
00:42:52//But what do you mean the quicker you go?
00:42:53The quicker you //pull it, instead of going like this, you go-
00:42:54//No, no, no, no, no. No, no. Did you not hear me say that you should- hop off there for a minute. You should do it like this. You just slowly, gently increase and- let it go, until it moves.
00:43:11All right?
00:43:12Slowly, gently and then when it moves, boom. That's what the reading is.
00:43:16One hundred.
00:43:17//Okay? And some of yours are pretty high, aren't' they?
00:43:21Maybe you'd better do that again now you know what to do. Good.
00:43:26You guys hear that? Listen- listen to that?
00:43:28(I work out).
00:43:30Yeah, but you're the mathematician, all right? You've got to get the results right first. You got to- you got to go for the //technique first, all right?
00:43:39//It's hard sometimes.
00:43:41Ask Louis what I've just shown him.
00:43:43It goes all the way down there and then it (inaudible).
00:43:48Let's see how my favorite ladies are going now.
00:43:52Say that louder so (inaudible) can hear.
00:43:55Fifty- //What- what-
00:43:56//(Kind of) serious, huh.
00:43:57What's this point two here? Point two?
00:44:00Oh, it's like 20 grams.
00:44:01Twenty grams. //That's how it read-
00:44:02//Okay, well write it- yeah write it all as- as grams. Write it all as grams, okay? You got 50, 20, 40, 50, 50. All right. Okay. See how you go with this one.
00:44:18Now you know what you've got to do with this one, right?
00:44:20I'll show you. You've got to get it moving first. Hop- hop up. Rachel, hop up. Hop out of the way a bit. Okay.
00:44:29Now you get it- you get it moving and then you- while it's moving- Oh, this is a bit jerky here. Get it moving and there- Now see the reading there? That's what we want.
00:44:43I see it, sir.
00:44:45Well, you saw what I was doing right then, didn't you?
00:44:48You've got two eyebrows.
00:44:49Yeah, I know. All right. You got the idea?
00:44:54Rachel's not watching.
00:44:56Who's going to do this?
00:45:00Who's going- you are? Good. All right. Then you see what I'm doing?
00:45:05So it's, um-
00:45:06See what I'm doing?
00:45:07Yeah, it's-
00:45:08Point six.
00:45:09It keeps on moving. We've got to move ours properly.
00:45:10You've got to keep it going at a constant speed, yeah.
00:45:13Yeah, you were going-
00:45:15Yeah, okay thanks.
00:45:16Well, what do you- you see what happens is that the surface changes a bit from there to there and so you want to make sure you get it on a section of surface that's fairly uniform.
00:45:27Get me some results.
00:45:31Get me some results.
00:45:32Get yourself some results.
00:45:35Do we have to do number two?
00:45:36You what?
00:45:37Do we do number two?
00:45:38Yes. You keep moving on.
00:45:40Well, where do we get the, um, weights from.
00:45:42Oh, up there in the tray. You saw the ones I put on the trolley?
00:45:45There's a whole tray of them up there. Let me have a look and see what you've got here. Eight, 10, oh, these are quite low. Okay, but they're smaller than those, right?
00:45:56Should that be so? //Should be, should it? Do you really feel convinced about that?
00:46:04Well, we'll talk about why later, okay? Um, these are quite high compared to these. You sure you did those right? Read the scale right?
00:46:16Okay. All right. You're ready to try adding weights to your block?
00:46:21Okay, those weights up- and add each of those weights that I showed you, they're 500 grams. Add 500 grams at a time and work up for each of your trials. All right?
00:46:31And the force here is in grams, so it's 50- sorry, 500, then kilogram, then one half and so on. Okay. You got to stack them on top of that.
00:46:41You might need that bigger spring balance. The one you had earlier on to replace that once the force gets too large and this goes off the scale.
00:46:47The kilogram?
00:46:48The one kilogram one. Yeah.
00:46:54Is this answer enough?
00:46:57Because one is (consists of) (inaudible). Ah, we're going to talk about that.
00:47:01All right? You can have a bit of a guess of it now. But don't try to answer those just yet.
00:47:04All right.
00:47:05Concentrate on getting our data down. Okay. (inaudible). Oh, very well done, girls. That looks good. Okay. You don't know why it looks good, do you?
00:47:15Because it's right?
00:47:17Yep. That's all right. You don't have to know why it looks good at the moment. It's just very pleasing to my eye. Would you have predicted that these readings should be smaller than those?
00:47:28You would have thought what? The same?
00:47:30Mm. Sort of nice to get a surprise once in a while, isn't it? Science is like that. Full of surprises. That's what keeps people interested. Carry on.
00:47:40We need some blocks now, don't we?
00:47:43How many weights do we use?
00:47:45Well, you've got five trials? You just keep stacking on weights. Each trial is a different weight.
00:47:52And you can see why we need that one for this one now, don't we?
00:47:54All right, okay.
00:47:56Just to get it started or, um-
00:47:57Well what's it say? Try adding weights to the (block) and repeat part C.
00:48:01Oh, okay.
00:48:02What was part C? Constant motion.
00:48:06Sir, we're getting like complete different results.
00:48:08Are you?
00:48:09We got one two five and two two five.
00:48:12One two five and two two five.
00:48:27Now it's- now it's 100.
00:48:29Is his surface different from yours, do you think?
00:48:33It's rougher.
00:48:34It could be a bit different, couldn't it?
00:48:36Do you remember me saying that you should always- you- you got to first of all make sure you've got fairly smooth surface so you don't have jerky motion.
00:48:44And secondly, when you repeat your trials you do it over the same section of bench.
00:48:49Oh, okay.
00:48:50Ah. So are you doing it over the same section of bench?
00:48:54Uh, probably not.
00:48:56All right, try that. You'll get more consistent results. Okay?
00:49:07Well guys, let's have a look and see what you got here. A hundred and fifty- all right. These are smaller than those.
00:49:14What's the meaning of this?
00:49:16Uh, it takes more force to get it going.
00:49:18Why- why do you think that would be?
00:49:22Did you expect that?
00:49:24You did? Ah, why?
00:49:28Bit of a gut feeling or what?
00:49:32Oh, pretty much.
00:49:34Mm hm.
00:49:36It takes like more force to get something going than to keep it moving.
00:49:40What's told you that from your previous experience? Something has told you that, hasn't it?
00:49:47All right, you can't remember what? All right, think about it a little bit. I'd be interested to know where you got that idea from. Okay. Carl, did you expect that?
00:49:57You did expect that. All right. Okay. That's good.
00:50:04(inaudible) for the rest of the year now?
00:50:05You what?
00:50:07Can I sit here for the rest of the year now?
00:50:09Oh, we might be able to swing a deal on that later on. Yeah. What do you got to offer?
00:50:14Two seventy.
00:50:15Increased effort?
00:50:17I don't //know.
00:50:18//My effort can never be increased.
00:50:19Better results?
00:50:20Yeah, okay.
00:50:21Okay. That sounds like a good deal to me. How are results going here? Eighty, 90, 100-
00:50:27Two seventy.
00:50:30These look to be very similar to those, would you agree?
00:50:34Hm, okay.
00:50:35Yes, but these because we just need to have a greater force to get it to start moving.
00:50:43But this is not saying that. This is saying that you need the same force to get it moving as to keep it moving.
00:50:50No, that's 88 grams.
00:50:54Eighty-eight to 92. Do you think that's a significant difference?
00:50:57No. Not really.
00:50:58No, I'd say they're virtually the same.
00:51:01Because of the inaccuracies in our spring balance. All right. That's all right, you're allowed to get those results if they're the results you got.
00:51:10Right-o. You, um, will be interested perhaps to know that a number of other groups are getting //quite a bit of difference between these two.
00:51:16//They're taking a break throughout.
00:51:20All right, but we'll talk about that later on at the conclusion of taking our measurements.
00:51:25Watch it when it moves, you guys.
00:51:31Oh, have we ground to a halt here, boys, or what?
00:51:33No, we've done like two pages already. Or one page.
00:51:36Yes, well you've still got some to go haven't you? What are you up to now?
00:51:39Uh, the (inaudible).
00:51:40No, worry about that later. Go onto the next one here. Try different surface areas, okay? Those other questions I've inserted there we'll discuss.
00:51:49Do we need another block?
00:51:50No, what did I say you could do with that one?
00:51:54Sir, can I go to the toilet? I really need to go now.
00:51:55Oh, yes, yes. Go, go.
00:51:56Like that? (Go like that).
00:51:57Yes. Yeah, but if you're going to do it like that, you're going to wrap a piece of cord around there, aren't you, and link it up there- get some more string?
00:52:04We only got three trials so we can do that, that, that.
00:52:12That- that's all you'll need to do. Yep.
00:52:14(He reckons) you got too much weight.
00:52:26You got to get it moving and keep it moving don't you? It's like part C. No, no. Can I suggest-
00:52:32Can I suggest that you're being a bit awkward with that. What you need to do to make it steadier- Move it back to where you started it from. Thank you.
00:52:41Now, if you can just grab it like that and just quickly get it moving and you want to keep it moving like that. No, we're running across some uneven surface there by the looks of it.
00:52:55Take it right back.
00:52:56We've got more than 500 here people.
00:52:57Okay. It's going to be a bit more awkward and difficult with (those hills), but see how I'm holding it like this? So that you can get it moving quickly and keep it moving but-
00:53:08That's a bit difficult, isn't it? Right. Take it back again, and you need a little bit of practice. Okay. Fast- there you go. See how you can do it?
00:53:18Five fifty.
00:53:21Well, you're taking advantage of my measurement, aren't you? All right. Whip it back. You have to be quicker on this. Okay, you have a go at that.
00:53:34Yes, hurry up. What have we got up to, girls?
00:53:39Jean (inaudible).
00:53:42We'll all be on TV.
00:53:44Why'd you show-
00:53:44Well, you better tell me your results, quickly.
00:53:47Um, we just did that number C.
00:53:50Part C. Point six. Okay, that's 60 grams isn't it? Fifty grams, 60 grams. Fifty- Twenty- Forty. These are bigger than those. Hm, interesting. You sure about that?
00:54:04Who's taking the measurements? Who's doing the-
00:54:06Me and Yvonne.
00:54:08Okay. All right. Keep going.
00:54:17Yes, girls?
00:54:19Do we- What do we do?
00:54:21No, this one, you're using that block and it's to do with surface area. Remember, I said you could do it like that. You could do it like that and then there's another one like that.
00:54:30And do you need these on it?
00:54:31No, no, no, no, no. //Okay.
00:54:34And you only- you only got to do three trials. It's- Oh, sorry.
00:54:39Five trials for each surface. But- that's why we got three tables there. Three surfaces. Okay.
00:54:47When we pulled it that way with the weights and when we pulled it that way, two different- like when we pulled it that way the recordings were higher, but when we pulled it that way they were lower.
00:54:58Good. All right, why do you think?
00:55:01Different surfaces.
00:55:02Well those are two- two same surfaces that you were dealing with. Huh? What you've touched on is very important for understanding friction. All right? Can I just have that block a bit?
00:55:14This is just one clue. If you run your fingers over that, that way and then that way. Does it feel the same?
00:55:23[ Student nods head for negative response ].
00:55:24Ah, and you were talking about pulling it in different directions, weren't you? What about the- is it rougher that way or that way?
00:55:30That way.
00:55:31That way, you reckon.
00:55:33All right, and if it's rougher, why would wood be rougher in one direction than the other?
00:55:40Just because of the grain and the way it's cut.
00:55:42Okay. So if it's rougher in one direction, that's going to make the frictional force, the size of it, directional, isn't it? You've picked up something very important. Very good observation.
00:55:59You're doing physics in upper school, aren't' you? We want you.
00:56:11Is that (inaudible) targeting (inaudible) back because of too much weight on it?
00:56:14Oh, yes, yes, good. But look, don't spend too much time on those. In fact, get onto that one. We won't do the measurements. We've only got a couple of minutes left. Okay?
00:56:27Three minutes by you is it? It's five minutes by me. Are you right?
00:56:33Of course I'm right, sir. I'm always right.
00:56:35Okay, we- we better call it all in now, I think.
00:56:39All right folks, stop please. Thank you. Stop now. Now I know that no one has finished and that's fine because it's a long activity and we still have yet to conclude.
00:57:03We'll have to do that, uh, tomorrow. So what I need you to do now is to return all your gear neatly and, uh, quickly to the front, and then resume your seats.
00:57:13If we've got a few minutes left, we'll talk some more.
00:57:38Strings, sir?
00:57:39Yeah, we'll keep the strings.
00:57:55You got onto the friction block, did you? Oh, you didn't. Oh.
00:58:12Oh, Louis, you're a worry, Louis.
00:58:15Thanks sir.
00:59:12Okay, listen in for a minute. Now we've got just a few minutes left before lunch.
00:59:22I'd like you to- from what you remember from your results, the first one where you were measuring the force to get it started, uh, get the block started as opposed to keep it moving.
00:59:35Who found that there was a larger force required to get it started than to keep it moving? Hands up, please. Right. Okay. Thank you. Who- Put your hands up if it was significantly larger.
00:59:49That is 10 grams or more. Ten, 20 grams more.
00:59:54Good. Thank you. Right-o. That makes it more reliable. Okay. Who got to doing those different surface areas? One group? Alvin, okay. Austin.
01:00:07What if- tell us what you've found so far in the- those measurements.
01:00:13Well, like it varies.
01:00:16Yeah, but are they- is the force required to drag it along at a constant speed on one surface area significantly different?
01:00:22Yeah. Oh, It's like- it's different, but it's like- it's about 10 grams.
01:00:27All right, so it might not be that different.
01:00:30Okay. So you're not convinced one way or the other yet. All right. Okay. What about adding weights? Who found that adding weights meant it took a lot more force to keep it moving?
01:00:43Okay, who was surprised by that? No one. I hope not. You were? Ah, okay.
01:00:53Let's just consider-
01:00:57[ bell ]
01:00:58I'll leave you with one question which we'll need to discuss next time when we're rounding all this off, and that is, how would you define the force of friction?
01:01:07How would you actually make a statement that says what it is? Okay? Okay, lunchtime, folks.