3.1

The 3rd Law of Motion, Momentum and Impulse

Newton’s 3rd Law of Motion

Newton’s 3rd Law is about action and reaction pairs.

The 3rd Law states that every action creates a reaction that is equal and opposite.

The key to understanding this law is to understand that the action-reaction pairs do not cancel each other out. If you push on the ground on a pair of skates, you are able to move forward because the ground is able to match your force in the opposite direction.

Momentum

Why is it that an object with a large mass is harder to stop than an object with a smaller mass which is moving at an identical speed?

The answer is momentum. Momentum is a lot like inertia but it also includes a speed (velocity).

The formula for momentum is p = mv Momentum is measured in kg . m/s

Impulse

If a force is applied to an object and the momentum is changed, this change is referred to as an impulse.

Impulse is caused by a force being in contact with an object for a period of time thereby causing a change in the object’s momentum.

Ft = mv2 – mv1

“Ft” is the part of the equation that stands for momentum, mv2 is the final momentum and mv1 is the initial momentum.

The Law of Conservation of Momentum

One important law to understand deals with how momentum is conserved in an action-reaction pair.

Basically the law of conservation of momentum states that a closed system (outside forces are absent) that momentum is not lost when transferred between objects.

When a cue ball “breaks” in a game of pool, the cue ball’s momentum before contact is transferred into the total momentum of all of the pool balls after contact.