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 (3/101) (3%)
2. mg down, N up, some other force F to the right (1/101) (1%)
3. mg down, N up, equal-and-opposite forces left and right (0/101) (0%)
4. could be 1 or 3 (97/101) (96%)

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

1. mg down and N up (2/76) (3%)
2. mg down, N up, some other force F to the right (37/76) (49%)
3. mg down, N up, equal-and-opposite forces left and right (0/76) (0%)
4. could be 1 or 3 (37/76) (49%)

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 (90/105) (86%)
2. No (15/105) (14%)