Momentum
Momentum for Cambridge IGCSE Physics 0625 Extended. Definition, conservation, impulse, and the collision questions that come up almost every session.
What you need to know
- Momentum is mass times velocity. It is a vector, so direction matters and one direction must be taken as positive.
- In any collision or explosion, total momentum before equals total momentum after, as long as no external force acts.
- In a sticky collision the two objects move off together with a shared velocity.
- Impulse is force times time, and it equals the change in momentum.
- A longer collision time means a smaller force for the same change in momentum. That is why crumple zones, airbags and crash mats work.
Equations
- p = mv
- total momentum before = total momentum after
- Ft = change in mv (impulse)
- F = change in momentum / time
Momentum questions students ask
What is momentum?
Momentum is mass times velocity, measured in kilogram metres per second. It is a vector, so direction matters.
Is momentum conserved in a collision?
Yes, as long as no external force acts. Total momentum before the collision equals total momentum after, in every collision and every explosion.
How do you solve a collision question?
Choose a positive direction. Work out total momentum before, set it equal to total momentum after, and solve. Anything moving the other way carries a negative sign.
What happens in a sticky collision?
The two objects move off together as one, so you add their masses and they share a single velocity after the collision.
What is impulse?
Impulse is force times the time the force acts for, and it equals the change in momentum. Ft equals the change in mv.
Why do airbags reduce injury?
They increase the time over which the passenger comes to rest. The change in momentum is unchanged, so a longer time means a smaller force.
Does direction matter in momentum calculations?
Always. If one object travels left and one travels right, one of them has negative momentum. Ignoring this is the single most common mistake in the topic.
How is force related to momentum?
Force equals the change in momentum divided by the time taken. That is another way of writing F equals ma.
Mistakes that cost marks
- Ignoring direction. If one object moves left, its momentum is negative.
- Using speed instead of velocity in a two direction problem.
- Forgetting to add both masses together after a sticky collision.
- Saying the airbag reduces the momentum change. It does not. It increases the time, which reduces the force.
Every Cambridge and Edexcel paper from 2016 to today, with its mark scheme, no login.
Open the paper libraryLive online classes across Egypt and the Gulf. No online payment.
Request accessOnline classes