Year 9: Rocket Physics


Learning Objectives:

Be able to explain how water rockets create thrust by ejecting water, and extend this principle to liquid fuel rockets
Be able to launch bottle water rockets safely using bucket launch platforms
Understand the relationships between distance, time, speed and acceleration, and appreciate that graphs can be used to display these relationships.
Understand the meanings of the terms speed and acceleration, and know that speed=distance/time
Appreciate how the ideas of speed and acceleration can be applied to rockets
Recall a qualitative explanation of acceleration
Appreciate that an unbalanced (resultant) force creates a change in motion (acceleration)
Optimize the launch parameters for a bottle rocket by online experimetation in a systematic way
To appreciate that the effect of a force depends on the area to which it is applied
Know how to use the quantitative relationship between force, area and pressure
Recall some practical applications that use the theory of pressure
Appreciate the way in which frictional forces, specifically air resistance, affect motion
Consider aerodynamics in rocket design to achieve a maximum flight time

Introduction to Rocket Physics

A squid propels itself by filling its body with water and ejecting it backwards in order to move forwards. This is the principle used by Rocket Engineers.

Space rockets use fuels which are burned in a chamber shaped rather like a bottle with a neck pointing backwards. The combustion produces a large volume of gas expanded by heat and this is ejected at high velocity forcing the rocket in the opposite direction.


Practice launching water rockets safely using the Patana rocket launching system.

When you have perfected it, take a video of a successful launch, and save it in your student folder. You will need this in a few lessons time.

Mr Roff's example video


Your challenge over the course of this topic is to learn and apply the basic principles of rocket design to construct a water rocket that stays in the air as long as possible.

Extension Task:

Draw a diagram to show how a rocket propels itself


Construct a timeline for the history of rockets and space flight. Use the resource centre and these web sites:


Don't try this at home!


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Distance, Time and Speed

Speed is the distance that something moves in a given time.

20 metres per second means that something moves 20 metres in one second.

What about 50 miles per hour? Or 600 kilometers per minute? - They're all speeds.

In school we use metres per second


If a rocket travels 5000 kilometers in 30 seconds what is its speed?


It is important to know the speed that rockets travel at because...


Task: How to display speeds

Use a handheld data logger to draw graphs of distance against time for...

1) Acceleration

2) Constant Speed

3) Rest

4) Deceleration

Draw the graphs for each of these motions, and describe their general shape.

Draw a distance-time graph for a water rocket when we launch it?

Ref: Co-Science: Physics pp12-14, 16-19, 24-25

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Acceleration and Newton's Laws

Acceleration is the rate at which something changes its speed.



What force must a rocket over come if it is to accelerate off the launch pad?


For a rocket to accelerate upwards, it must first overcome the downward force of gravity.


Task: Produce a ticker tape for a lump of plastecine is dropped in the Earth's gravitational field.

Draw a diagram of your experiment, and stick down the ticker tape you have produced

What do you notice about your tape when you compare it to other peoples'?


Gravity is the same everywhere on the Earth, and accelerates all things at the same rate (g=10m/s2). This means that everything falls at the same rate no matter how heavy it is!

Weight is a measure of how much something is being pulled towards the Earth.

It is given by...

W=mass x gravity

W= mass x 10 (On Earth!)


Experiment: Investigating Newton's second law.


Results Table

acceleration of trolley (units)

mass of trolley x acceleration (units)

mass dropped (units)

mass dropped x gravity (units)














What can we say about the force acting on the trolley and the force acting on the mass?


Force = mass x acceleration


Explain in words how we have come to this conclusion

Extension Task:

What happens if there are many forces acting on an object? Adding forces worksheet


What is the initial acceleration of our water rockets?

Using loggerpro's movie analysis package, and the videos we made of our launches we can find out...

Ref: Co-Science: Physics pp22-23, 26-27

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Rocket Thrust

For a rocket to take off, the upward force generated by its engines must be greater than the downward force of gravity (its weight).

It must have a resultant force upwards

The greater the upward force generated by the rocket engines, then the greater the upward resultant force (because the rocket's weight remains the same). We need to maximise the upward force generated by the water rocket.

To do this we need to optimize the rocket launch parameters...

Task: Experiment with the online flash experiment to determine the optimum launch parameters for your rocket...

Hint: Approach this in a methodical way, all variables constant except the one you are testing. When you have found the optimum value for that variable move on to the next one.

Write up this experiment in completeness, with results for each variable you tested, and an explanation of the final optimum launch parameters you have determined.

Ref: Co-Science: Physics pp28-29

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Testing Rocket Launch Parameters

Time to test the results of your theoretical work...

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Pressure as how a force is spread out over an area,

Pressure = Force / Area

It is measured in N/m2 or Pa


Task: Explain using the idea of pressure which would be more painful?



The pressure in the model rocket body at launch is about 180000 N/m 2 and the area of the jet through which the water is forced out is 1 cm2.


What is the thrust of the rocket?

How could we use the idea of pressure to make our rockets go higher?


Do an experiment to determine your pressure when balanced on different parts your body, by weighing t yourself and using graph paper to work out the area of different parts of your body.


Which of these objects need low pressure and which need high pressure? Explain.

Snowboard    Snowshoes    Camels' feet    Tractors' tyres   Skating blades    Drawing pins    Sharp knife


Estimate how much pressure is exerted by each of the objects listed in the above question.

Task: Analyse the quality of different trainers using the ideas of pressure in the workbooklet


Ref: Co-Science: Physics pp38-39

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Practical Assessment

Title: Factors Affecting the Depth Cubes Sink into Plasticene

The rest is up to you...

Practical assessment markscheme (make sure you use this when you do your write up to get top marks)

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Recap what the aim of our course is...

NB: Maximise flight time. So we need to maximise height. Streamlining and minimizing drag are important...

Task: A qualitative exercise to improve the rocket design

Apart from maximizing height reached, how else could we go about achieving this aim?



Design and conduct an experiment that investigates how the size and/or shape of a parachute affects the time it takes to fall.

Aim, Method, Diagram, Table of Results and Conclusion needed.



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Adapting Rockets

Final Test- Practical

Final Test 2- Examination

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anrophysics 2009