A student librarian lifts a 2.2 kg book from the floor to a height of 1.25 m. He carries the book 8.0 m to the stacks and places the book on a shelf that is.35 m above the floor. How much work does he do on the book?
I understand most of it, but I am unsure of what distance to use. Please help?

Question

anonymous · Answer

the equation for potential energy is:
$$U = mgh$$
where:
U - potential energy
m - mass of object
g - gravitational constant (9.81 m/s/s)
h - diffrence in height of two points in space

The total work done on a object is equal to the change of its potantial energy from starting point to ending point so the only thing you need is the diffrence in height. That is the power of the law of conservation of energy, bacause no matter how complicated some process is, a lot of the time the energies are easy to calculate.

so it's  $$U = 2.2 * 9.81 * (0.35 - 0)$$
0.35 -  wher the book ends up (on the shelf)
0 - where the book started (on the floor)
1.25 - where it was in between does not matter