An object at rest is being acted on by several forces which add up to zero.
For instance, a box is sitting on an inclined plane. It is not falling down the plan because the force of friction cancels the component force of gravity parallel to the plane, and it is not falling through the plane because the normal force exactly cancels the force of gravity perpendicular to the plane.
If we define work as force times distance, there's no work being done on the box, because the box isn't moving. But can we say that any of the individual forces did work because they prevented the box from moving a given distance?
Also, what if the entire system is moving at a uniform velocity relative to a given frame of reference? Let's say the system is moving towards the center of the earth at uniform velocity. After a certain amount of time, the system has technically moved some distance x, and the force acting in the direction of this movement, gravity, is not zero, so we should get a non-zero value for work. But relative to the system itself, no work was done! Is that really possible?
Best Answer
Whenever you're confused about forces and work, you can bring it back to energy. The real definition of work is the transfer of energy. If no energy is transferred, no work is done. Ever. Force times distance doesn't define work, it quantfies it. The only mechanism for the transfer of energy is force exerted through a distance.
But there doesn't have to be a nonzero net force. If you push a box across a floor against a friction force at constant velocity, both you and the friction force are doing work. Some people would say that equal amounts of positive and negative work were being done on the box. I don't like that. I prefer to say that chemical energy from you is being transferred to thermal energy in the box and the floor. That keeps it real.
In the case of an object or system traveling toward the center of the Earth at constant velocity, energy is being transferred by the force of gravity from the gravitational field to the person or thing applying the equal and opposite force.