Work and Electrostatics - a bit of Clarification, please

[fatcat39]

Work and Electrostatics - Help!

Homework Statement

I need to place equal and oppositve charges on a piano and at the top of a 5.0 meter ramp in order to move a 1000 kg box up a 30 degree frictionless ramp. The box has a charge of +Q and the top has a -Q.

A) What magnitude charges do I need to keep the box at the bottom and not moving?

B) What is the initial energy of the box?

*Why is it negative?

*Or is it zero?

C) How much work does it take to separate the charges from 5 m to infinity?

D) How much work does it take to push the box by hand?
Work = Fd
How do I find the force that's necessary?

Homework Equations

The Attempt at a Solution

A) Solved

I solved B, but I just wanted to check that B and C, the answers should be equal? I got an answer of -24.5 kJ for B and is C 24.5 kJ (positive?)

* Or is the answer to B actually zero? I'm told to "make sure" I define my zeros for potential energy. And I found an explanation on handysolver.com (i don't think it's a trustworthy site) that says that B is zero because the zeros were all there, but doesn't the +Q charge still exert a force?



Thanks a lot in advance, I realize my work is a little scatterbrained...

 

[blochwave]

*But I don't understand the "then" part - can someone explain how to transform the equation?

He set the Fg=Fe and solved for Q

 

[nicksauce]

L outside the radical is the same as L^2 inside the radical...

 

[fatcat39]

And is B zero? I guess someone else had the exact same problem, because I found a solution:

http://handysolver.com/problemsandso...problem_id=418

that says that B is zero, but I really don't think so.