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Here's the question. It's about gravitational potential energy, by the way.

** On your desk you have N identical coins, each with a mass m. You stack the coins into a vertical pile to height y.

a) Approximately how much work, in terms of m, g, and y, must you do on the last coin to raise it from the desk to the top of the pile?

b) Approximately how much gravitational potential energy, in terms of m, g, N, and y, is stored in the entire pile?

of course, we have the equation E_g=mg delta y.

I was thinking that for a), delta y would be something like y/N(N-1), y/N being the height of one of the coins. It's because you have to consider that the height of the coins should not include the one that you are raising from the desk. Is it right? and as for b), I have no clue. Can it be zero, or no?

Please help me!