Impulse and Momentum


The momentum of an object is the product of its mass and its velocity. Momentum is a vector quantity and is measured in Ns or \cfrac { kgm }{ s }

The equation for momentum is: 


p = momentum in \cfrac { kgm }{ s }

m = mass in kg

v= velocity in \cfrac { m }{ s }


A change of momentum is known as an impulse. When a force acts on an object to change its velocity, the momentum of the object will change because the velocity changes. The equations for impulse are:

I=\Delta p={ p }_{ f }-{ p }_{ i }

\Delta p=mv-mu=m(v-u)

Another equation for impulse is:


Example 1:

What is the momentum of a 50kg girl walking north at 2.5\cfrac { m }{ s } ?


p=50\times 2.5

p=125\cfrac { kgm }{ s } north

Example 2:

A 900kg car is travelling west at 8.2\cfrac { m }{ s } before coming to a complete stop at a red light. What is the impulse of the car as it stops at the red light?

I=\Delta p={ p }_{ f }-{ p }_{ i }

\Delta p=mv-mu=m(v-u). Note: we will denote west as positive:

\Delta p=900(0-8.2)

\Delta p=-7380

Therefore, Impulse=7380\cfrac { kgm }{ s } east