# Potential Energy and Work

1. Feb 19, 2004

### nemzy

i am confused on how to solve these problems..thanks

You drop a 1.80 kg textbook to a friend who stands on the ground 10.0 m below the textbook with outstretched hands 1.50 m above the ground

(d) If the gravitational potential energy of that system is zero at ground level, what is its potential energy U when the textbook reaches the hands?

(e) How much work Wg is done on the textbook by its weight as it drops to your friend's hands if U is 100 J at the ground level.

(f) What is the change U in the gravitational potential energy of the textbook-Earth system during the drop if U is 100 J at the ground level.

(h) Find U at the hands when U is 100 J at the ground level.

2. Feb 19, 2004

### HallsofIvy

Staff Emeritus
"(d) If the gravitational potential energy of that system is zero at ground level, what is its potential energy U when the textbook reaches the hands?"

Surely you know that the difference in potential energy is "mgh" where m is the mass, g is the acceleration due to gravity (9.81 m/s2 in MKS) and h is the height. (Be sure to calculate the distance between the original height of the book and the height of the person's hands.)

"(e) How much work Wg is done on the textbook by its weight as it drops to your friend's hands if U is 100 J at the ground level."

Work done by gravity is the change in potential energy. Caution: since only the change is important whether U= 0 or 100J at ground level is irrelevant!

"(f) What is the change U in the gravitational potential energy of the textbook-Earth system during the drop if U is 100 J at the ground level. "

Someone is having a little fun with you! The answers to these three problems are all exactly the same! They are all really asking for change in U (and the 100 J is still irrelevant).

"(h) Find U at the hands when U is 100 J at the ground level."

Finally a problem where that "100 J" is important. Again, change in U is mgh. Here, of course, h is the 1.50 height of the hands above the ground. Multiplying mgh will give you the change in U from the ground to the hands. Adding the "base" 100J to that gives the actual potential energy, U, at the hands.