- #1

Bipolarity

- 775

- 2

Suppose that you lift a block of mass M an upwards distance of D. The force you applied on the block is a constant F directed parallel to the displacement of the block. Then by the definition of work, you did work equal to FD on the block. This quantity is positive. I think we can all agree on that.

What about the work that the block does on you?

Argument 1:

Since you applied an upward force F on the block, the block exerts a downward force F on you. But your displacement relative to the block is also downwards, and it has magnitude D.

Thus, the work that the block does on you is FD and this quantity is positive.

Argument 2:

You worked on the block and transferred kinetic energy to it. You lost energy, the block gained energy. Since you lost energy, the block did "negative work" on you. This is an immediate consequence of the work-kinetic energy theorem. The magnitude of this work is FD but it is a negative quantity because the block gains energy and you lose energy.

Which of these arguments is valid? Why is the other argument invalid? Hopefully I am not the only one having trouble to understand this...

Thanks!

BiP