Yet another work sign convention question.

[September24]

Hey, my brain threw me for a bit of a loop and I got confused.

When work is done on an object, an objects energy increases correct?

Well, how does this relate to potential energy? When something is raised, it increases in energy (Potential energy). When we drop something, it increases in energy (KE). Is work being done in both cases? Does an object increase in energy in both cases?

Im guessing when raising an object, we do positive work but gravity does negative work.
When an object is dropped, gravity does positive work...but something else does negative work?

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[inasensegone]

Hey, my brain threw me for a bit of a loop and I got confused.

When work is done on an object, an objects energy increases correct?

Well, how does this relate to potential energy? When something is raised, it increases in energy (Potential energy). When we drop something, it increases in energy (KE). Is work being done in both cases? Does an object increase in energy in both cases?

Im guessing when raising an object, we do positive work but gravity does negative work.
When an object is dropped, gravity does positive work...but something else does negative work?

Work is a measure of energy. When you do work on an object, you are putting energy into it, so in a way you could say that the object's energy increases (so long as it does not release that energy somehow).

When you raise something up, you have increased it's (potential) energy by an amount equivalent to mgh. When you then drop it, that potential energy is converted to kinetic energy (1/2 mv^2)... it is not correct to say that it increases in energy because this would imply that it generated energy somehow. Remember, it is the conversion of energy that gives it kinetic energy in this instance.

I suppose it is a matter of perspective when asking about the work done by gravity. Gravity isn't really doing work so much as you did work on the object, and now once you drop it, gravity is converting that work into a different type of energy (potential to kinetic). Gravity itself is not putting in any energy, that was all done by you when you raised the object.

 

[DrknoSDN]

To add to what inasensegone said, gravity is a conservative force in that it only converts energy to another form. In that example if you raise the object you are doing work on the object and it gains it in the form of potential energy (W=mgh). When released, gravity does no work, only converts PE to KE, so gravity is conservative because no energy is lost in the conversion and no work is done.

Work is really the change in potential energy, kinetic energy, and internal energy of a object.
If you have a object sliding down a frictionless surface no work is done either.
If the surface had friction then work would be done because the internal energy of the object would change, and the potential energy would not be directly converted to kinetic energy.

Also if you push (slide) an object along a surface with friction you do work in the W=Fdcos(theta) form, and the work done by friction will eventually bring it to a stop.