A person pushes a heavy box along a horizontal floor at a constant speed. Explain why the person must continue to do work on the box even though its speed is not changing.

The box is moving at a constant speed, so the net horizontal force on it is zero. However, friction between the box and the floor exerts a force that opposes the motion. To keep the speed constant the person must apply an equal and opposite force to balance the friction.

Work is defined as a force acting over a distance in the direction of the force. The person’s force is applied over the distance the box moves, so work is done. This work is not stored as kinetic energy (the speed is unchanged) but is transferred to the box and floor as thermal energy by the frictional force.

Therefore the person must continue to do work on the box even though its speed does not change because the work is dissipated as heat due to friction.