You are pushing a wooden crate across the floor at constant speed. You decide to double your speed. To maintain the new speed, the force that you apply must be about...

A) twice as large as before.

B) the same as before.

C) half as large as before.

D) something else.

Solution: B. To maintain constant speed the sum of the forces must be zero. The two horizontal forces are the force you exert and the force of friction; these must be equal in strength and opposite in direction. The force of friction depends upon the normal force and the coefficient of friction. Neither is altered by the speed.