Conservation of momentum — AQA GCSE Physics
Test yourself on Conservation of momentum with AQA GCSE practice questions.
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Conservation of momentum explained
A closed system is one on which no external resultant force acts, so nothing can add momentum to it or take momentum away.
Read the full explanation
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.
Your focus
- Define a closed system in terms of external forces and total momentum.
- Calculate the common velocity of two trolleys that stick together after a collision.
- Determine an unknown velocity after a collision by equating total momentum before and after, using signs for direction.
Conservation of momentum exam tips
Marking Points
- one mark for defining a closed system as one with no external forces, or zero resultant external force, acting
- one mark for stating that the total momentum before the event equals the total momentum after it
- one mark for calculating the total momentum before the event, with signs for direction
- one mark for equating it to the total momentum afterwards and solving for the unknown velocity or mass
Examiner 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.
Common Mistakes
- 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