5.22 Momentum and its conservation
9 exam questions worth 25 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the unit of momentum?
Choose one answer.
Correct Not quite: the answer is kg m/s
Mark scheme
A: kg m/s
Model answer
kg m/s (A).
Calculate the momentum of a 70 kg cyclist travelling at 8.0 m/s.
Mark scheme
p = m v = 70 × 8.0 (1); 560 kg m/s (1)
Model answer
p = m v = 70 × 8.0 = 560 kg m/s.
A ball has a momentum of 12 kg m/s and a mass of 0.40 kg. Calculate its velocity.
Mark scheme
v = p ÷ m (1); 12 ÷ 0.40 (1); 30 m/s (1)
Model answer
v = p / m = 12 / 0.40 = 30 m/s.
State the principle of conservation of momentum.
Mark scheme
in a closed system (1); the total momentum before an event equals the total momentum after (1)
Model answer
In a closed system, the total momentum before an event is equal to the total momentum after the event.
A 3.0 kg trolley moving at 4.0 m/s collides with a stationary 1.0 kg trolley, and they stick together. Calculate their speed after the collision.
Mark scheme
momentum before = 3.0 × 4.0 = 12 kg m/s (1); momentum after = (3.0 + 1.0) × v (1); 4.0 v = 12 (1); 3.0 m/s (1)
Model answer
Momentum before = 3.0 × 4.0 = 12 kg m/s. After: (3.0 + 1.0) × v = 12, so v = 3.0 m/s.
A 0.020 kg bullet is fired at 300 m/s from a stationary rifle of mass 4.0 kg. Calculate the recoil velocity of the rifle.
Mark scheme
momentum before = 0 (1); 0 = (0.020 × 300) + (4.0 × v) (1); 4.0 v = −6.0 (1); v = −1.5 m/s (the rifle recoils at 1.5 m/s) (1)
Model answer
Total momentum before is zero. 0 = 0.020 × 300 + 4.0 × v, so v = −6.0 / 4.0 = −1.5 m/s. The rifle recoils at 1.5 m/s.
Two ice skaters, initially at rest, push each other apart. Skater A has a mass of 60 kg and moves at 2.0 m/s to the left. Explain what happens to skater B (mass 80 kg) and calculate B's velocity.
Mark scheme
total momentum before = 0 so after = 0 (1); 60 × (−2.0) + 80 × v = 0 (1); v = +1.5 m/s (1); moves to the right / opposite direction (1)
Model answer
The total momentum is zero before and after. 60 × (−2.0) + 80 × v = 0, so v = +1.5 m/s: skater B moves to the right at 1.5 m/s.
Explain why a gun recoils when a bullet is fired.
Mark scheme
the total momentum before firing is zero (1); the bullet gains forward momentum (1); so the gun must gain an equal momentum in the opposite direction (1)
Model answer
The total momentum before firing is zero, so when the bullet gains forward momentum, the gun gains an equal backward momentum, which is the recoil.
Two trolleys collide and stick together. Total momentum is conserved but kinetic energy is not. Explain what happens to the lost kinetic energy.
Mark scheme
transferred to other energy stores (1); thermal and sound (1)
Model answer
Some of the kinetic energy is transferred to thermal and sound energy stores.
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