A level Physics · Edexcel 9PH0 · Topic 2C
Momentum
Key points
- Momentum is a vector, measured in kg m s; direction matters, so choose a positive direction first.
- Principle of conservation of linear momentum: in a closed system (no external resultant force) total momentum before = total momentum after.
- Use it for collisions (objects may stick together) and explosions (total momentum starts at zero, so the pieces move off with equal and opposite momenta).
- Newton's second law in momentum form: , the resultant force equals the rate of change of momentum.
- Momentum is always conserved in collisions, but kinetic energy is conserved only in elastic collisions.
- Exam tip: show momentum before and after with signs, then solve for the unknown velocity, and state its direction.
Topic notesPremium
Momentum combines an object's mass and velocity and is the key to analysing collisions and explosions. At this stage you apply its conservation along a straight line; Topic 6 extends it to impulse and two dimensions.
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