Perpetuum Mobile and Gravitation

[A.T.]

humans with their muscles or with their machines

Human made things can certainly change the momentum of objects, without spending energy. A stick connecting two masses rotating in space changes their momentum.

 

[Nugatory]

I’m asking about the process when we apply a force over an object but no energy is spent.

Consider a heavy object resting on a table. The table is applying an upwards force to the object (otherwise the object would move downwards because of gravity) but spends no energy doing so.

The table can support the weight forever without getting tired.

 

[shlosmem]

Ok, Thank you all.
I really appreciate it.

 

[Nugatory]

So you are basically saying I can open any screw without spending any energy? this is good news.

No, we are not saying that.

To turn a screw, you will apply tangential forces in opposite directions on opposite sides. As the screw turns, the point of application of these forces moves, so the distance in $W=Fd$ is non-zero. The force is obviously also non-zero, so there's work being done.

It's a different story if the screw doesn't turn. If it's so stuck that the force you're applying doesn't move it, then it's just like the table supporting a weight that I mentioned above - you can lean on the end of your wrench all day without doing any useful work. You will get tired because human muscles burn energy just sitting there, but if you were to hang a weight off the end of the wrench it could sit there forever, twisting the bolt but not moving it.