9-1 Physics/Momentum

Momentum is defined by the following equation:

$p=mv$

$p$ momentum (kilograms meters per second, ${\text{kg m/s}}$)

$m$ mass (${\text{kg}}$)

$v$ velocity (${\text{m/s}}$)

Conservation of momentum

In a closed system, the total momentum before an event is equal to the total momentum after the event. This is known as the conservation of momentum.

Imagine two balls, a red ball and a white ball. The red ball is stationary, so $p=0$. However the white ball is moving. The white ball hits the red ball, causing it to move. The red ball now has momentum and the white ball has less momentum. The combined momentum of both of these objects will be equal to the initial momentum of the white ball.

Changes in momentum

When a force acts on an object that is moving, or able to move, a change in momentum occurs. The following equations (which you should already be familiar with):

$F=ma$, $a={\dfrac {v-u}{t}}$ can be substituted into each other to give us the formula:

$F={\dfrac {m(v-u)}{t}}$which can be simplified to $F={\dfrac {m\Delta v}{t}}$ where $m\Delta v$ is equal to the change of momentum and the force is equal to the rate of change of momentum.