Question

If you push on a crate with a horizontal force of 100 n and it slides at constant velocity, what is the magnitude and direction of the frictional force acting on the crate?

Answer 1

The crate does not move.

The free body diagram shows that two forces act on the crate, the applied force of 490 N and the frictional force that acts in the opposite direction. The lengths of the arrows of both forces indicate that they are equal in magnitude. This means that the applied force of 490 N or less is not enough to overcome the frictional force exerted by the floor. So, the crate doesn’t move.

Answer 2

According to Newton;s Second Law of motion, F = ma. Acceleration is the change of velocity per unit time. Since there is no change of velocity, then acceleration is equal to zero. Consequently, the net force F is also equal to zero. The net force is equal to the sum of all the forces acting on the body. These forces are the horizontal force and the frictional force. They are acting in opposite directions. So, the sum must be

F = 0 = Horizontal - Frictional
0 = 100 N - Frictional
Frictional force = 100 N