An 11-kilogram cart is pulled with a horizontal force at a constant velocity of 8.0 meters per second, as shown in the diagram. The force of friction between the cart and the floor is 12 newtons. What is the magnitude of the horizontal force?

The cart is pulled at a constant velocity, so it is in equilibrium. In particular, the net horizontal force is zero, and consists of only the pulling force and friction such that

p - f = 0

where p and f denote the magnitudes of the pulling force and friction.

So we have

p - 12 N = 0

p = 12 N