Momentum
Physics: Cambridge International GCSE Supplement, topic 1.8.1
Momentum and Impulse
Momentum is a measure of how hard it is to stop a moving object. It depends on both the mass of the object and its velocity. The momentum of an object is its mass multiplied by its velocity (p = mv).
Impulse = change in momentum
- The change in momentum of an object is equal to the impulse applied to it.
- Impulse = change in momentum (Δp).
Impulse equation
- Impulse = force x time.
- Δp = F x t.
Conservation of momentum
- The law of conservation of momentum says that momentum cannot be created or destroyed.
- Momentum is conserved in all isolated systems where no external forces act.
- This means the total momentum before an event is always equal to the total momentum after the event.
- So, if two objects collide, the sum of momentum before collision = sum of momentum after collision.
Change in momentum = mv - mu
- Change in momentum = mv - mu, where m is mass, u is the initial velocity of an object and v is the final velocity of an object.
Impulses Equations
Impulse is the effect of a force acting over a period of time.
The formula for impulse
- Impulse = Force × time
- The symbol is FΔt, where:
- F is the force (in newton, N).
- Δt is the time for which the force acts (in seconds, s).
Impulse-momentum relationship
- Impulse applied to an object is equal to the change in its momentum.
- Equation:
- Impulse = Change in momentum
- FΔt = Δp
Resultant force and momentum
- The resultant force is the change in momentum per unit time.
- This is also known as Newton's second law.
- Equation:
- Force = change in momentum / time force acts
- F = Δp / Δt
Test yourself on Momentum
What are the units of momentum?
Using ideas about momentum, describe how a rocket is launched.
If the change in momentum of a cricket ball being hit by a bat is 17.5 Ns, then what is the impulse?