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 (1/37) (3%)
2. mg down, N up, some other force F to the right (0/37) (0%)
3. mg down, N up, equal-and-opposite forces left and right (0/37) (0%)
4. could be 1 or 3 (36/37) (97%)

A box is moving at constant velocity to the right. Which free-body diagram is correct?

1. mg down and N up (1/37) (3%)
2. mg down, N up, some other force F to the right (13/37) (35%)
3. mg down, N up, equal-and-opposite forces left and right (8/37) (22%)
4. could be 1 or 3 (15/37) (41%)