TBR Chapter 3 Passage 1

[ravupadh]

---

Members do not see this ad.

I was working through this passage and it got me thinking. If the people moving the piano up the ramp are doing 1000 J of work on it (since W = (500)(2)) and gravity is doing -1000 J of work on the piano (since W = (100)(-10)(2)) shouldn't the total work when the piano reaches the top of the ramp be zero since both works cancel eachother out?

Yet it's not because when you do W = Ef - Ei you get W = mgh or 1000J. Can someone please explain this to me? Was the chemical energy of the people moving the piano up the ramp converted into potential energy or something?

Also, I'm getting confused about the signs in these energy problems. When is and isn't g negative? Because clearly as in the above paragraph using g as positive 10 works out. I'm assuming other vectors like F, velocity, and displacement still follow the rules of using negative for opposite directions in these energy and work problems?

 

[Rabolisk]

Yet it's not because when you do W = Ef - Ei you get W = mgh or 1000J.

Yes it is. Work is 0 because it cancels out. W = mgh is the work done by gravity.

 

[ravupadh]

Really? Cause if you do W = Ef - Ei or W = (1/2mvf^2 + mghf) - (1/2mvi^2 + mghi) you get W = (1000) - (0)