Freebody Diagrams and Newton's Third Law

Freebody diagrams often help you to figure out what happens in a dynamics problem. Simply draw the object in question and all the forces on it.
Remember to add forces properly as vectors. It usually helps to split them up into "x" and "y" components before adding.
Sometimes it makes sense to choose "x" and "y" axes which are not the usual horizontal and vertical ones, but instead parallel and perpendicular to an important direction in the problem.
Newton's Third Law states that
```
       When body A exerts a force  F on body B,
       then body B exerts a force -F on body A
```
In other words, for every force which acts between two bodies, there is an opposite and equal force.

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