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Momentum: GCSE Combined Science Physics practice questions

Foundation and Higher · Grade 7

A second set of questions on momentum. They are different from the ones on the topic page. Try each, then open the answer and compare your working.

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Question 1 Hard

A 1000 kg cannon fires a 20 kg shell at 50 m/s. Work out the recoil speed of the cannon.

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Answer: 1 m/s

  1. Momentum of the shell = 20 × 50 = 1000 kg m/s
  2. 1000 ÷ 1000 = 1 m/s

Question 2 Hard

A collision changes the momentum of a driver by 6000 kg m/s. Work out the force if the stop takes 0.5 s.

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Answer: 12 000 N

  1. force = change in momentum ÷ time
  2. 6000 ÷ 0.5 = 12 000 N

Question 3 Hard

Explain why an airbag reduces the force on a driver in a crash.

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Answer: It makes the stop last longer, so the same change in momentum happens over more time and the force is smaller.

  1. The driver’s momentum change is fixed.
  2. The airbag makes the stop last longer, so the force is smaller.

Question 4 Hard

A 5000 N force acts for 0.1 s. Work out the change in momentum.

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Answer: 500 kg m/s

  1. change in momentum = force × time
  2. 5000 × 0.1 = 500 kg m/s

Quick checks

Short questions with options. Choose one before you open the answer.

Check 1

What is momentum?

  1. How hard it is to stop a moving object
  2. How fast an object is moving
  3. The pull of gravity on an object
Show answer

How hard it is to stop a moving object A measure of how hard it is to stop a moving object. It depends on mass and velocity.

Check 2

What is the equation for momentum?

  1. momentum = mass ÷ velocity
  2. momentum = mass × velocity
  3. momentum = mass + velocity
Show answer

momentum = mass × velocity momentum = mass × velocity (p = m × v).

Check 3

What is the unit of momentum?

  1. N
  2. m/s²
  3. kg m/s
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kg m/s Kilogram metres per second, kg m/s.

Check 4

Is momentum a vector or a scalar?

  1. A vector
  2. A scalar
  3. Neither
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A vector A vector, because it has a direction, the same as the velocity.

Check 5

A 5 kg trolley moves at 4 m/s. What is its momentum?

  1. 9 kg m/s
  2. 20 kg m/s
  3. 1.25 kg m/s
Show answer

20 kg m/s 20 kg m/s, because 5 × 4 = 20.

Check 6

What does a minus sign on a momentum mean?

  1. The object has less momentum
  2. The object has stopped
  3. It moves the opposite way to the positive direction
Show answer

It moves the opposite way to the positive direction The object is moving in the opposite direction to the one chosen as positive.

Check 7

What is a closed system?

  1. A group of objects with no outside forces
  2. A system with no moving objects
  3. A system inside a closed box
Show answer

A group of objects with no outside forces A group of objects with no outside forces acting on them.

Check 8

What is conserved in a collision in a closed system?

  1. The speed of each object
  2. The total momentum
  3. The kinetic energy
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The total momentum The total momentum.

Check 9

Which quantity is not usually conserved when two cars collide and crumple?

  1. Momentum
  2. Mass
  3. Kinetic energy
Show answer

Kinetic energy Kinetic energy.

Check 10

A 1 kg trolley at 4 m/s sticks to a still 3 kg trolley. What is their speed?

  1. 1 m/s
  2. 4 m/s
  3. 12 m/s
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1 m/s 1 m/s, because 4 kg m/s ÷ 4 kg = 1 m/s.

Check 11

Why is the total momentum zero just before a gun fires?

  1. Because momentum is always zero
  2. Both are still
  3. Because the gun is heavy
Show answer

Both are still The gun and the bullet are both still.

Check 12

What is recoil?

  1. The forward movement of the bullet
  2. The sound of the shot
  3. The backwards movement of a gun when it fires
Show answer

The backwards movement of a gun when it fires The backward movement of a gun when it fires, so that the total momentum stays zero.

Check 13

Why does the heavier object move more slowly after an explosion?

  1. Equal momentum, so more mass means less velocity
  2. Heavier objects have less momentum
  3. Heavier objects cannot move
Show answer

Equal momentum, so more mass means less velocity The two momenta are equal and opposite, so the bigger mass has the smaller velocity.

Check 14

A bullet gains +6 kg m/s when a gun fires. What momentum does the gun gain?

  1. +6 kg m/s
  2. −6 kg m/s
  3. 0 kg m/s
Show answer

−6 kg m/s −6 kg m/s.

Check 15

Higher tier. What is the equation for force in terms of momentum?

  1. force = change in momentum × time taken
  2. force = momentum + time
  3. force = change in momentum ÷ time taken
Show answer

force = change in momentum ÷ time taken force = change in momentum ÷ time taken (F = Δp ÷ Δt).

Check 16

What does the symbol Δp stand for?

  1. The change in momentum
  2. The pressure
  3. The power
Show answer

The change in momentum The change in momentum.

Check 17

Higher tier. What happens to the force if the collision time gets longer?

  1. The force gets bigger
  2. The force gets smaller
  3. The force stays the same
Show answer

The force gets smaller The force gets smaller, for the same change in momentum.

Check 18

Name two safety features that make a crash last longer.

  1. A speed limit and a horn
  2. A heavier engine and wider tyres
  3. A crumple zone and an airbag
Show answer

A crumple zone and an airbag A crumple zone and an airbag, or a seat belt.

Check 19

Why do seat belts stretch a little?

  1. To make the stop longer so the force is smaller
  2. To make the person stop faster
  3. To make the car lighter
Show answer

To make the stop longer so the force is smaller To make the stop last longer, so the force on the person is smaller.

Check 20

When two cars collide and crumple, where does most of the lost kinetic energy go?

  1. To the gravitational potential store
  2. To the thermal store and sound
  3. To the chemical store
Show answer

To the thermal store and sound To the thermal store and as sound.

Check 21

A student writes the momentum of a 2 kg trolley moving left at 3 m/s as +6 kg m/s when right is positive. What is wrong?

  1. Nothing is wrong
  2. The mass should be 3 kg
  3. The sign should be negative
Show answer

The sign should be negative It should be −6 kg m/s, because the trolley moves in the negative direction.

Check 22

Higher tier. A force changes a momentum by 500 kg m/s in 0.5 s. What is the force?

  1. 1000 N
  2. 250 N
  3. 500.5 N
Show answer

1000 N 1000 N, because 500 ÷ 0.5 = 1000.

Check 23

Why is a lorry harder to stop than a car going at the same speed?

  1. It has less friction
  2. It has more mass and so more momentum
  3. It is always faster
Show answer

It has more mass and so more momentum The lorry has more mass, so it has more momentum.

Check 24

Why does a gymnast bend their knees when landing?

  1. It makes the gymnast lighter
  2. It stops the change in momentum
  3. A longer landing time gives a smaller force
Show answer

A longer landing time gives a smaller force It makes the landing take longer, so the force on the body is smaller.

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