A free-body diagram is a sketch showing an object and all of the forces being exerted on it. Always draw a free-body diagram when you are analyzing forces.

A box is at rest on a flat table. Which free-body diagram is correct?

1. mg down and N up
2. mg down, N up, some other force F to the right
3. mg down, N up, equal-and-opposite forces left and right
4. could be 1 or 3

The best answer is 4.

So, now we have three completely different free-body diagrams, two for our box and one for our object in outer space, and yet in each case the object stays at rest.

Maybe, in reality, these three free-body diagrams are equivalent. How could we possible conclude that?

If you add the forces on each free-body diagram, what happens?

In each case all the forces cancel, so the free-body diagrams are all equivalent - they all show that there is no net force acting on the object.

We can conclude that when an object is at rest and no net force acts on it, the object remains at rest. Its motion does not change.