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1.6Motion, Forces and Energy

Momentum — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

10 min read 22 parts Lesson 55 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.6.1 Momentum — IGCSE PHYSICS notes: Define momentum, use p = mv and apply conservation of momentum to collisions and explosions (Extended).Momentum = mass × velocity (p = mv).p is momentum in kg m/s, m is mass in kg and v is velocity in m/s.An object at rest has no momentum.The…

  • Memory aid
  • Change in momentum
  • Closed system
  • Objects that stick together move off with common velocity
  • The parts therefore move off with equal and opposite momenta, so m1v1
  • Resultant force

Part 1 of 8Momentum

Momentum
Momentum mass × velocity (p = mv).
  • p is momentum in kg m/s, m is mass in kg and v is velocity in m/s.
An object at rest has no momentum.
The greater the momentum, the harder the object is to stop or turn.
Memory aid
  • Memory aid momentum is mass in motion; N s is also an accepted unit.

Part 2 of 8Momentum as a vector

Velocity has direction, so momentum has direction too.
Choose one direction as positive and give the opposite direction a negative sign.
A ball approaching a wall has positive momentum and, after rebounding, negative momentum.
Change in momentum
Change in momentum final momentum − initial momentum, so a rebound gives a larger change than just stopping.

Part 3 of 8Conservation of momentum

Conservation of momentum
Principle
Principle in a closed system, the total momentum before an event equals the total momentum after it.
Closed system
  • Closed system a system on which no external resultant force acts.
Two objects of equal mass moving towards each other at equal speed have total momentum zero.
Collisions
  • Collisions m1u1 + m2u2 = m1v1 + m2v2.
Objects that stick together move off with common velocity
  • Objects that stick together move off with common velocity m1u1 + m2u2 = (m1 + m2)v.

Part 4 of 8Explosions and recoil

Explosions and recoil
Conservation of momentum applies to objects pushed apart as well as to collisions.
A system at rest has zero total momentum before the explosion.
The parts therefore move off with equal and opposite momenta, so m1v1
−m2v2.
Gun recoil is an example of this.

Part 5 of 8Method for momentum problems

1
Draw a before-and-after diagram and choose a positive direction.
2
Write the momentum of each object before the event, using a negative sign if it moves the other way.
3
Write the momentum of each object after, with the unknown velocity as v.
4
Set total momentum before = total momentum after.
5
Solve for v and state its size and direction.

Part 6 of 8Force and momentum

Resultant force
Resultant force change in momentum per unit time (F = Δp/Δt).
This is Newton's second law written as the rate of change of momentum.
Impulse
FΔt = Δ(mv), with unit N s equal to kg m/s.
Use F
Δp/Δt for collision, impact and rebound questions and F = ma when mass and acceleration are given.

Part 7 of 8Momentum and safety

Momentum and safety
A large force over a short time has the same impulse as a small force over a long time.
Crumple zones, air bags, seat belts and crash mats increase the time taken to stop.
The change in momentum is fixed, so a longer time gives a smaller force and less injury.

Part 8 of 8More from the specification

1Linear momentum conservation in collisions
  • Linear momentum conservation in collisions tracking product of mass and velocity before and after impacts provides a reliable means to analyze momentum.
2Momentum
  • The product of mass and velocity, p = mv, measured in kg m/s and a vector quantity with direction.
3Units and calculation
  • Convert masses to kilograms and use metres per second, so a 0.06 kg tennis ball at 40 m/s has momentum 2.4 kg m/s.
4Impulse and force
  • Force equals the rate of change of momentum, F = (mv - mu)/t, so a racket applying force for a short time changes the ball's momentum.
5Conservation of momentum
  • In a closed system total momentum before a collision equals total momentum after, as when a moving brick hits a stationary one.
6Direction and sign
  • Choose one direction as positive, so travelling to the right is positive and an object moving back has negative momentum.

Key terms

TermMeaning
Momentump = mv, measured in kg m/s.
ImpulseForce × time, equal to the change in momentum.
Conservation of momentumTotal momentum before equals total momentum after in a closed system.
Closed systemA system with no external resultant force.
ExplosionObjects pushed apart; total momentum stays zero if it starts from rest.
Resultant forceChange in momentum per unit time.

Avoid these mistakes

1
Ignoring direction — momentum is a vector, so use positive and negative signs.
2
Treating a rebound as a change from mv to zero — the sign reverses, so the change is larger.
3
Saying safety features reduce the change in momentum — they reduce the force by increasing the time.
4
Using grams instead of kilograms — convert mass to kg first.

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