AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Students should be able to use the concept of momentum as a model to…
Full specification wording
Conservation of momentum: Students should be able to use the concept of momentum as a model to describe and explain examples of momentum in an event, such as a collision.
Momentum is a model that predicts motion during interactions.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Momentum is a model that predicts motion during interactions. In a collision, total momentum is conserved if no external resultant force acts. To describe an event, identify the masses and velocities before and after, choose a positive direction, and then calculate. For example, a 1200 kg car at 15 m/s hits a stationary 800 kg car and they stick together. Before the collision: 1200 × 15 + 800 × 0 = 18000 kg m/s. After the collision: 2000 kg × v = 18000 kg m/s, so v = 9 m/s forwards. Momentum can also be used to explain recoil and explosions, where an initially stationary object separates into parts moving in opposite directions, keeping the total momentum at zero.
What earns the marks
- Describe the event by listing masses and velocities before and after the interaction.
- Choose a positive direction and give opposing velocities negative signs.
- Calculate total momentum before and after using p = m × v.
- Show that total momentum is unchanged in a closed system.
- Use momentum to explain recoil and explosions, noting that initial momentum is zero.
Common mistakes and how to put them right
Ignoring direction and treating momentum as scalar.
Put it right: Assign signs to velocities before adding.
Assuming momentum is lost in a crash.
Put it right: In a closed system total momentum is conserved; energy may transfer.
Confusing momentum with kinetic energy.
Put it right: Momentum is m × v, kinetic energy is ½mv².
Exam tips
- Define the positive direction at the start of your answer.
- Show the before and after momentum sums separately before equating them.
- When explaining explosions, explicitly state that the total momentum before the event is zero.
Try the lesson
The start of the Conservation of momentum: Students should be able to use the concept of momentum as a model to describe and explain examples of momentum in an event, such as a collision. lesson, made from checked teaching notes for this specification point.
Have a go first
A moving train carriage has a momentum of 25,000 kg m/s. It collides with and couples to a stationary carriage. Assuming no external forces act, what is the total momentum of the two carriages immediately after the collision?
Listen
Momentum is a model that predicts motion during interactions. In a collision, total momentum is conserved if no external resultant force acts. To describe an event, identify the masses and velocities before and after, choose a positive direction, and then calculate. For example, a 1200 kg car at 15 m/s hits a stationary 800 kg car and they stick together. Before the collision: 1200 × 15 + 800 × 0 = 18000 kg m/s. After the collision: 2000 kg × v = 18000 kg m/s, so v = 9 m/s forwards. Momentum can also be used to explain recoil and explosions, where an initially stationary object separates into parts moving in opposite directions, keeping the total momentum at zero. A strong answer earns marks for these points. Describe the event by listing masses and velocities before and after the interaction. Choose a positive direction and give opposing velocities negative signs. Calculate total momentum before and after using p = m × v. Show that total momentum is unchanged in a closed system. Exam tip: Define the positive direction at the start of your answer.
Key words
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Conservation of momentum
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Recoil and explosions
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Vectors and direction
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Modelling motion
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See every definition
- Conservation of momentum
- Total momentum is conserved if no external resultant force acts, so it is unchanged in a closed system.
- Recoil and explosions
- An initially stationary object separates into parts moving in opposite directions, keeping the total momentum at zero.
- Vectors and direction
- A positive direction is chosen and opposing velocities are given negative signs before adding.
- Modelling motion
- Momentum is a model that predicts motion during interactions, such as a collision.
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