A truck in driving up the interstate with a constant velocity. Is work being done by a net external force acting on the truck? Justify answer. Why doesn't the work done by the engine pushing it forward make its kenetic energy increase?

Answer 1

The truck is moving at constant velocity - means zero acceleration - that can happen only when the net force acting on the truck is zero.The frictional force opposes the motion and its magnitude is same as that produced by the engine for forward motion (under constant velocity condition).
Since the net force is zero, the work done by it is zero.

The KE of the truck does not increase because the truck is moving at constant velocity. Yet the truck's engine is delivering power that is just sufficient to compensate for the loss of energy due to frictional forces.