Potential Energy

[DingDongD]

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So, I am trying to clear up potential energy concept in my head.

If I apply 20J of energy to lift up a book, then I changed the potential energy of the system by 20J.

Delta PE = W = +20J

But, why isn't gravity taken into account? Wouldn't it turn 0 due to gravity.

I assumed that gravity does -20J of work on the book since force is in the opposite direction of motion. So, -20J + 20J equals to 0.

If you could help clear this out, it would be really helpful.

 

[TheRhymenocerous]

The kind of potential energy you're referring to is gravitational potential energy. It's the force of gravity that gives the objects their potential energy, so by definition it's already part of the equation. If you apply a force to the book, but gravity is exerting the same force in the opposite direction, the book won't move at all. By lifting the book, you're necessarily exerting a force greater than the force of gravity, so you'd use the total force you exert minus the force of gravity (the net force on the object) to calculate the work being done.

 

[DingDongD]

But, if you are lifting at constant speed, the net force is 0. I assumed that Wnet = Fnet * h. So, the Wnet should be 0, right?

Since Wnet is 0, the change in mechanical energy of the system should be 0.

My question is how can Wnet be 0, if my applying the force on the book is an external force?

 

[TheRhymenocerous]

Work=change in KE, not PE.