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?

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