Conservation of Momentum - Outcomes State the principle of conservation of momentum. Demonstrate the principle of conservation of momentum. Solve problems about conservation of momentum. Discuss conservation of momentum in e.g. collisions, spacecraft, aircraft.

Define the Principle of Conservation The principle of conservation of momentum states that in any interaction, the total momentum before the interaction is equal to the total momentum after the interaction, provided no external forces act on the system. Recall momentum, 𝑝=𝑚𝑣 Usually we only discuss two objects that don’t change their masses, so we get a formula: 𝑚 1 𝑢 1 + 𝑚 2 𝑢 2 = 𝑚 1 𝑣 1 + 𝑚 2 𝑣 2

Demonstrate the Principle Covered by a mandatory experiment.

Solve Problems about Conservation A rocket is launched by expelling gas from its engines. Use the principle of conservation of momentum to explain why a rocket rises. A pendulum bob of mass 10 g was allowed to swing so that it collided with a block of mass 8.0 g at rest on a bench, as shown. The bob stopped on impact and the block subsequently moved along the bench. The velocity of the bob just before the collision was 2 m s-1. Calculate the velocity of the block immediately after the collision.

Solve Problems about Conservation Two shopping trolleys each of mass 12 kg are on a smooth level floor. Trolley A moving at 3.5 m s−1 strikes trolley B, which is at rest. After the collision both trolleys move together in the same direction. Calculate the initial momentum of trolley A Calculate the common velocity of the trolleys after the collision.

Solve Problems about Conservation A spacecraft of mass 50 000 kg is approaching a space station at a constant speed of 2 m s-1. The spacecraft must slow to a speed of 0.5 m s-1 for it to lock onto the space station. Calculate the mass of gas that the spacecraft must expel at a speed 50 m s-1 for the spacecraft to lock onto the space station. (The change in mass of the spacecraft may be ignored.) In what direction should the gas be expelled? Explain how the principle of conservation of momentum is applied to changing the direction in which a spacecraft is travelling.