Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Applications of the conservation of momentum


Apparatus and materials

Water rocket

CO2 capsule rocket 

Spare CO2 capsules will be required.

Health & Safety and Technical notes

See the manufacturer’s instructions. 
With CO2 capsule rockets: Wear eye protection. Make sure the capsule is firmly fixed to the toy truck. Do not stand in front of the truck.

Various kinds of water rocket are obtainable from toy shops. Follow the manufacturer’s instructions. 
Also available from toyshops are CO2 capsule rockets. A simpler arrangement is to attach a CO2capsule (as used for soda syphons) to the top of a toy truck. The capsule should be horizontal with its neck facing the rear of the truck. Sellotape is satisfactory for fixing but it may be better to attach an aluminium tube and then fix the capsule inside it. 
The capsule can be broken with a round nail and a sharp blow from a hammer. The truck will then move at high speed across the floor. 
Other examples include: 

  • toy cars driven by inflated balloons 
  • a sausage-shaped balloon taped to a straw which is then threaded onto a horizontally fixed thread. When the balloon is released it travels along the thread.

Rocket and trolley

Teaching notes

1 How do the rocket motors push the rocket to make it accelerate? The hot gases in space rockets, or the gas or water in toys, simply travels out of the back of the rocket. Any molecules that happen to be travelling backwards go from the tail and any molecules travelling forward hit the front wall of the rocket motor inside the rocket, bounce off it and are then travelling backwards and will then probably escape. All molecules which escape carry away momentum and the rocket gains an equal amount of forward momentum. The molecules which bombard the front wall propel the rocket. 

2 The rocket plus its fuel do not form a closed system because it is attached to the Earth by the gravitational field. You can take account of that by subtracting the weight of the rocket from the upward thrust produced by the motors.