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.

Consider some basic examples.

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

A box is moving at constant velocity to the right. 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

In both cases above, mg and N are equal in magnitude and opposite in direction. Are these the equal-and-opposite forces discussed in Newton's Third Law?

1. Yes
2. No