Conservation of momentum | Exit Quiz | Oak National Academy

Match the following key terms to their definition.

• collisions

• happen when two objects move together and hit each other

• conservation of momentum

• for a closed system, momentum is always conserved in any interaction

• kilogram metres per second (kg m/s)

• momentum

• happen when a single object splits into pieces that move apart

• explosions

• the unit of momentum

Answer explanation

Momentum is mass × velocity so has units of kg m/s. Momentum is always conserved in collisions (where objects hit each other) and explosions (where they break apart).

Starting with the smallest momentum, sort the following objects into order of increasing momentum.

• a lorry stationary of mass 3000 kg

• a motorcycle of mass 200 kg travelling at 20 m/s

• a van of mass 1200 kg travelling at 10 m/s

• a car of mass 800 kg travelling at 16 m/s

Answer explanation

The momentum is found by multiplying the mass by the velocity, so the correct order is the lorry (0 kg m/s), the motorcycle (4000 kg m/s), the van (12 000 kg m/s) then the car (12 800 kg m/s).

Which of the following is always correct for a closed system of objects that interact with each other?

• Total momentum increases in explosion interactions.

• Total momentum decreases in any collision interactions.

• Total momentum is always the same before and after any interaction.

• Total momentum decreases in explosion interactions.

• Total momentum increases in any collision interactions.

Answer explanation

Momentum is conserved, so total momentum is always the same after any interaction as it was before the interaction.

A toy aeroplane of mass 1.5 kg has a momentum of 30 kg m/s. Calculate the velocity of the aeroplane.

• 0.05 m/s

• 20 m/s

• 28.5 m/s

• 31.5 m/s

• 45 m/s

Answer explanation

Velocity = momentum ÷ mass = 30 kg m/s ÷ 1.5 kg = 20 m/s.

Two stationary dynamics trolleys are separated by an explosion. Trolley A (mass = 1.5 kg) travels at 3.0 m/s after the explosion. Calculate the speed of trolley B (mass = 0.5 kg) after the explosion.

Answer explanation

Before: momentum = 0 kg m/s, momentum of A = 1.5 kg × 3.0 m/s = 4.5 kg m/s, so momentum of B = –4.5 kg m/s. After: velocity of B = momentum ÷ mass = –4.5 kg m/s ÷ 0.5 kg = –9.0 m/s, so its speed is 9.0 m/s in the opposite direction to A.

A van (mass = 1600 kg) travelling at 5.0 m/s collides with a stationary car (mass = 900 kg) and they join together. Calculate the velocity at which they move off after the collision.

• 8.8 m/s

• 5.0 m/s

• 3.2 m/s

• 1.7 m/s

• 0 m/s

Answer explanation

Before: total momentum = (1600 kg × 5.0 m/s) = 8000 kg m/s. After: total momentum = 8000 kg m/s and total mass = 1600 kg + 900 kg = 2500 kg so velocity = total momentum ÷ total mass = 8000 kg m/s ÷ 2500 kg = 3.2 m/s.