Newton's Law of Gravitation
Earth and Space

Rockets

Classroom Activity for 11-14 Supporting Physics Teaching

What the Activity is for

Building a rocket.

This activity is to remind pupils that to escape the Earth's gravitational field a force is needed. Establish that the bigger the mass of the rocket the bigger the force needed.

The activity also provides the opportunity to introduce the principles of rocket propulsion, showing that the force of the rocket on the water produces a force of the water on the rocket (an example of Newton's third law), which lifts it off the ground in a spectacular fashion.

What to Prepare

  • film canisters
  • postcards
  • coloured pens
  • indigestion relief tablets
  • water
  • support sheet (see below)
  • a clip of a rocket lifting off (see below)
  • eye protection

Safety note: Pupils should wear eye protection and stand well back (at least two metres) from the rocket once it has been primed. If the rocket does not lift off the teacher should knock it over with a metre rule before dealing with it.

What Happens During this Activity

Introduce the activity by showing some clips of rockets taking off. Ask why so much fuel is needed and point to the fact that the gravitational force at the surface of the Earth is large. Generating a rocket thrust to overcome the pull of gravity requires a lot of fuel. Tell the pupils that this activity is going to involve making a rocket on a much smaller scale.

Explain that the fuel you are going to use is compressed carbon dioxide. Demonstrate the effect of adding the indigestion relief tablet to the water and ask the pupils what would happen if this was carried out in a sealed space where the carbon dioxide could not escape. Then show what happens if bits of indigestion relief tablet are dropped into a canister filled with water and the lid put on. Gas is released inside the canister and pressure builds until the lid is pushed off and the water shoots out backwards, at high speed. This provides the thrust for the rocket.

You might use an inflated balloon to model the effect and ask why the rocket will be better than the balloon. This is because the water is much more massive than the air inside the balloon and will produce a greater force on the rocket.

Instructions:

  1. Roll the postcard into a tube. Slide the empty film canister in at one end with its top on. Now tape the seams of the postcard together and tape the postcard to the film canister so that the two do not separate.
  2. Cut two triangular, paper fins and tape them to the rocket.
  3. Make a small paper cone and tape it to the top of the rocket for the nose cone.
  4. Hold the rocket upside down and fill one quarter of the canister with water.
  5. Add half a tablet to the film canister (you will need to experiment with the amounts of tablet used with different size canisters) and quickly snap on the lid.
  6. Place the rocket on the ground, lid down, stand back and count down while you wait for the launch.

Resources

Download the support sheet / student worksheet and video clip for this activity.

Newton's Law of Gravitation
is expressed by the relation F=G(m_1)(m_2)/r^2
can be used to derive Kepler's First Law

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