Impulse and Momentum - Learn

Impulse and Momentum

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:

$p=mv$

$p$ = momentum in $\cfrac { kgm }{ s }$

$m$ = mass in $kg$

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

Impulse

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:

$I=Ft$

Example 1:

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

$p=mv$

$p=50\times 2.5$

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