Key concepts

Have you ever driven up a mountain, seen a sign reading "Hazard! Icy Roads Ahead," and wondered why ice makes roads dangerous to drive on? The answer has to do with friction—specifically, the lack of it.

Specifically, in the case of driving a car down the road, the friction that allows the car to move occurs between the tires and the road. This friction normally allows the car to "grip" the road—keeping the tires in contact with the relatively rough road surface, and the driver of the car in control. However, when the road is icy, the friction between the two surfaces decreases, allowing the car to slip—rather than grip. You can replicate these situations much more safely at home using some common objects—regardless of the weather.

Friction is the resistance to motion when two objects rub together. An example of low friction is when you sled down an icy hill. It's easy because both the sled and the ice on the hill are smooth and the surfaces of the sled's runners and the ice slide past one another with little resistance. On the other hand, sledding down a rough, concrete driveway is almost impossible due to the rough surface of the driveway, which provides resistance on the sled's runners. This is an example of high friction.

As you can imagine, friction happens when the rough parts of two objects catch one another as they rub together. The resistance that occurs depends on a value called the coefficient of friction. This is a measure of how much two objects interact with one another when rubbing together, and this depends on the materials that make up the two objects. The friction that takes place also depends on the mass of the moving object in the pull of gravity. All of these factors play a role in determining how much friction there is between two rubbing objects.

•    Two "two-by-four" or similar size wood planks, about one half meter long works best
•    Small plastic tub with lid
•    Sand or another dense material to fill the tub
•    Water
•    Freezer
•    Books
•    Ruler
•    Protractor
•    A carpeted area or a rug that can get damp (or a protective covering, such as a large plastic trash bag)

•    Make sure that the pieces of wood have surfaces that are similar in roughness and are about the same length and width.
•    Take one piece of wood and soak it in water—this can be done by weighing the wood down in a bathtub filled with a few inches of water for at least 30 minutes.
•    Place the soaked piece of wood in the freezer overnight.
•    Fill the plastic tub with sand or another dense material and secure the lid.
•    Use caution (you may want to wear gloves) when handling the wood—especially if it is rough—to avoid getting splinters.

•    On the carpeted area or a rug, build a ramp using several books and the dry piece of wood. Make sure the same part of the wood leans on the books for all of the trials.
•    Place the filled tub on the top of the ramp, with the edge of the tub lined up with the top end of the wood. Release the tub. If the tub slips down the ramp, remove the tub and lower the ramp height by removing a book. If the tub does not slip, remove the tub and raise the ramp height by adding a book. How high is the stack of books when the tub slips? What is the height of the shortest stack needed for the tub to slip?
•    Using the protractor, measure the angle where the ramp meets the carpet when the ramp is set up using the shortest stack of books needed for the tub to slip. What is the angle?
•    When you are done testing the dry piece of wood, retrieve the icy piece of wood from the freezer. (Because the icy piece of wood will thaw out over time, have everything in place and ready for testing it before you remove it.) How does the icy wood feel compared with the dry wood?
•    Using the icy piece of wood, test it on your ramp with the tub just as you did using the dry piece of wood. What is the height of shortest stack of books needed for the tub to slip on the icy piece of wood? Again, use the protractor to measure the angle of the ramp. What is the angle?
•    You can try testing each ramp with different book stack heights multiple times. Are your results consistent, or is there some variation?
•    Overall, which ramp—the one made with the icy or the dry wood—allowed the tub to slip at a smaller ramp angle?
•    Tip: If the icy wood thaws while you are testing it or seems dry, put it horizontally in the freezer and pour a little water all over the top of it. Leave it in the freezer for at least an hour. Test it again using the top side as the surface of your ramp.
•    Extra: How easily would the tub slip on a ramp made of other materials? Make a similar ramp using a plank covered with sandpaper, oil or aluminum foil. Alternatively, cover the bottom of the tub in these materials. How easily does the tub slip using the different materials?
•    Extra: If you know the ramp's height and length, you do not need a protractor to figure out its angle. All you need is to do some math: The equation used to solve the angle of the ramp is sin(theta) = opposite/hypotenuse. In the formula, "sin" stands for sine, "theta" is the angle, "opposite" is the ramp height and "hypotenuse" is the ramp length. Find out more about how to do this calculation at "Finding an Angle in a Right Angled Triangle" from Math is Fun Advanced. Does the calculated angle match the one you measured with your protractor?

Observations and results
In general, did the tub slip down the icy ramp better than the dry ramp? Was the smallest ramp angle needed for the tub to slip on the icy ramp smaller than the angle needed on the dry ramp?

When two objects rub together, there is always friction, and the surfaces of the two objects determine the amount that is generated. In this activity, you should have seen that there was less friction between the tub and the icy wood than from the dry plank. In other words, a smaller ramp angle was needed when using the icy wood than when using the dry wood for the tub to slip.

To calculate the force of friction, one must multiply the coefficient of friction by the force generated by the object, which was the mass of the tub in the pull of gravity. Although the mass of the tub and gravity did not change, the coefficient of friction of the two ramp systems did. The icy plank and the tub bottom had a lower coefficient of friction than the dry wood and the tub. You can easily feel this difference by carefully running your hand along the icy and then dry wood surfaces. The icy plank's frozen coating makes it much smoother than the dry wood.

More to explore
"Friction" from Kidipede, History and Science for Kids, Portland State University
"Moti" from Motion Basics: Friction from Rader's
"Slip Sliding Away: Experimenting with Friction" from Science Buddies

This activity brought to you in partnership with Science Buddies