MomentumEdexcel IGCSE Physics: Revision notes
Section 1
What is momentum?
Momentum is a property of any moving object, combining how much mass it has and how fast it is moving.
p = m × v
where p is momentum (kg m/s), m is mass (kg) and v is velocity (m/s). Momentum is a vector quantity — it has direction as well as size, so a momentum in one direction is treated as positive and momentum in the opposite direction as negative.
A 900 kg car travelling at 20 m/s has momentum p = 900 × 20 = 18,000 kg m/s.
Section 2
Conservation of momentum
In any collision or explosion, provided no external forces act, total momentum before = total momentum after. This is the conservation of momentum, and it applies to the whole system, not to each object individually.
To solve problems: add up the momenta of all objects before the event (taking direction into account with + and −), and set this equal to the sum of momenta after the event.
A 1000 kg trolley moving at 4 m/s collides and sticks to a stationary 1000 kg trolley. Total momentum before = 1000 × 4 = 4000 kg m/s. After, combined mass = 2000 kg, so v = 4000/2000 = 2 m/s.
Forgetting direction is a common error — momenta in opposite directions must be given opposite signs before adding.
Section 3
Force, momentum and time
Newton's second law can be written in terms of momentum change:
F = (mv − mu) / t
where mv − mu is the change in momentum and t is the time taken for that change. This shows that a force is needed to change an object's momentum, and that the same change in momentum produced over a longer time requires a smaller force.
This is the physics behind vehicle safety features: airbags, crumple zones and seatbelts all increase the time over which a collision brings a person's momentum to zero, which reduces the force experienced by the body.
When asked to explain a safety feature, always link it explicitly: longer time → smaller force → less injury, using the equation F = (mv − mu)/t.
Section 4
Newton's third law
Newton's third law states that when object A exerts a force on object B, object B exerts an equal and opposite force on object A. These two forces act on different objects, are equal in size, and act in opposite directions.
This explains why momentum is conserved in collisions: the force each object exerts on the other is equal and opposite, and acts for the same time, so the momentum lost by one object equals the momentum gained by the other.
Think of two ice skaters pushing off each other — they exert equal and opposite forces on each other and move apart with equal and opposite momentum changes.
Must Know
- momentum, p = m × v, measured in kg m/s, and is a vector quantity
- conservation of momentum: total momentum before a collision/explosion = total momentum after (no external force)
- F = (mv − mu) / t links force, momentum change and time
- increasing the time of a collision decreases the force experienced — the basis of car safety features
- Newton's third law: equal and opposite forces act on the two interacting objects, explaining why momentum is conserved
- always assign + and − signs to opposite directions when combining momenta
That's the notes covered.
Carry on to the next subtopic.