AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
In a closed system, the total momentum before an event is equal to the…
Full specification wording
Conservation of momentum: In a closed system, the total momentum before an event is equal to the total momentum after the event.
A closed system is one on which no external resultant force acts, so nothing can add momentum to it or take momentum away.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 1 of 3
- 2 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A closed system is one on which no external resultant force acts, so nothing can add momentum to it or take momentum away. In such a system the total momentum of everything before an event equals the total momentum afterwards. The event can be a collision, an explosion or two objects pushing apart. To use the idea, add the momenta before, remembering signs for opposite directions, then set that total equal to the momenta after. If a 2 kg trolley at 3 m/s strikes a stationary 1 kg trolley and they stick, the total before is 6 kg m/s, so the combined 3 kg mass must move at 2 m/s afterwards. Friction and other external forces stop a real system being truly closed, which is why trolley experiments use a level track and low-friction wheels.
What earns the marks
- Defining a closed system as one with no external forces, or zero resultant external force, acting.
- Stating that the total momentum before the event equals the total momentum after it.
- Calculating the total momentum before the event, with signs for direction.
- Equating it to the total momentum afterwards and solving for the unknown velocity or mass.
Common mistakes and how to put them right
Ignoring direction, so a momentum to the left is added rather than subtracted.
Assuming kinetic energy is conserved as well in every collision.
Using mass alone instead of mass × velocity when adding the two objects.
Adding the two velocities together instead of the two momenta.
Exam tips
- Draw a before and after sketch with a velocity arrow and a value on each object.
- State your positive direction, then write any leftward velocity as a negative number.
- For objects that join together, use the combined mass on the after side of the equation.
Try the lesson
The start of the Conservation of momentum: In a closed system, the total momentum before an event is equal to the total momentum after the event. lesson, made from checked teaching notes for this specification point.
Have a go first
A trolley of mass 2 kg moves at 3 m/s and collides with a stationary trolley of mass 4 kg. The two trolleys stick together. What is the total momentum of the two trolleys just after the collision?
Listen
A closed system is one on which no external resultant force acts, so nothing can add momentum to it or take momentum away. In such a system the total momentum of everything before an event equals the total momentum afterwards. The event can be a collision, an explosion or two objects pushing apart. To use the idea, add the momenta before, remembering signs for opposite directions, then set that total equal to the momenta after. If a 2 kg trolley at 3 m/s strikes a stationary 1 kg trolley and they stick, the total before is 6 kg m/s, so the combined 3 kg mass must move at 2 m/s afterwards. Friction and other external forces stop a real system being truly closed, which is why trolley experiments use a level track and low-friction wheels. A strong answer earns marks for these points. defining a closed system as one with no external forces, or zero resultant external force, acting. stating that the total momentum before the event equals the total momentum after it. calculating the total momentum before the event, with signs for direction. equating it to the total momentum afterwards and solving for the unknown velocity or mass. Exam tip: Draw a before and after sketch with a velocity arrow and a value on each object.
See it
Put it in order
Put these steps in the order they happen.
- 1. Add the momenta, using signs for opposite directions
- 2. Total momentum before the event
- 3. Closed system: no external resultant force
- 4. Solve for the unknown velocity or mass
- 5. Set total before equal to total after
- 6. Total momentum after the event
Key words
Say what each word means before you turn the card.
closed system
Try answering aloud or in your head before turning the card.
conservation of momentum
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See every definition
- closed system
- A system on which no external resultant force acts, so nothing can add momentum to it or take momentum away.
- conservation of momentum
- In a closed system, the total momentum before an event is equal to the total momentum after the event.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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