MomentumEdexcel GCSE Physics: Revision notes
Section 1
What is momentum?
Momentum is a property of a moving object, combining its mass and velocity:
p = m × v
where p is momentum in kg m/s, m is mass in kg, and v is velocity in m/s. Momentum is a vector quantity — it has a direction (the same direction as the velocity).
A heavy, slow-moving object can have the same momentum as a light, fast-moving one.
A 1000 kg car moving at 10 m/s has momentum p = 1000 × 10 = 10 000 kg m/s.
Section 2
How does momentum apply to collisions?
In any collision (or explosion), momentum is conserved in a closed system provided no external forces act — the total momentum before the event equals the total momentum after it.
Examples of momentum in collisions:
- Two vehicles colliding and moving off together (or apart)
- A snooker ball striking a stationary ball, transferring momentum to it
- A rocket engine ejecting exhaust gas backwards, gaining forward momentum itself (momentum is conserved because the gas gains equal and opposite momentum)
Direction matters: momentum in one direction is treated as positive and the opposite direction as negative when adding momenta together.
When comparing momentum before and after a collision, examiners want you to state clearly which direction you are treating as positive.
Section 3
How does Newton's second law relate force to momentum?
Newton's second law can be written in terms of the change in momentum:
F = (mv − mu) ÷ t
where mv is the final momentum, mu is the initial momentum, and t is the time over which the change happens. This means:
force = change in momentum ÷ time
This version of the equation explains why a longer collision time reduces the force experienced for the same change in momentum — the basis of safety features like airbags, crumple zones and seatbelts, which extend the time of impact.
A 900 kg car's momentum changes from 18 000 kg m/s to 0 in a 0.6 s crash: F = (0 − 18 000)/0.6 = −30 000 N, so a force of 30 000 N acts to stop it.
Must Know
- Momentum: p = m × v, measured in kg m/s, and is a vector
- Total momentum is conserved in a collision or explosion (closed system, no external forces)
- F = (mv − mu) ÷ t, i.e. force = change in momentum ÷ time
- A longer collision time reduces the force for a given change in momentum
- Momentum has direction — opposite directions must be given opposite signs when combining momenta
- Newton's third law underpins conservation of momentum in collisions
That's the notes covered.
Carry on to the next subtopic.