Force & Momentum (AQA GCSE Physics): Revision Notes

When a resultant force acts on an object, it causes it to accelerate. Acceleration is a change in velocity. So if a force causes an object's velocity to change, then it also changes the momentum of the object $(p=mv).$

Equation We can combine the equations for Newton's second law and acceleration to express force as a change in momentum:

Newton's 2nd Law: $F=ma.$

Acceleration: $a =$ $\frac{v-u}{t}$

Combine them to get:

This can be written as:

• $F$ is the Force in Newtons (N)
• $mΔv$ is the change in momentum in kilogrammes (kg m/s). $m$ is the mass and $Δv$is the change in velocity.
• $Δt$ is the change in time.

A faster rate of change of momentum means a bigger force. So if an object changes velocity very quickly, its momentum will change very quickly. Therefore there is a larger force on the object.

Other examples of safety measures include:

• Gymnasium crash mats and cushioned surfaces for playgrounds – They are cushioned so increase the time taken to fall. This decreases the rate of change of momentum and hence decreases the force on the gymnast.
• Cycle helmets – the foam inside the helmet is crushable so increases the time taken for your head to stop.