# Homework Help: Work Done by the Gravitational Force problem

1. Jul 17, 2006

### kappcity06

this proplem has been giving me alot of trouble. I was wondering if anyone could lead me in the right direction.

You drop a 2.00 kg textbook to a friend who stands on the ground at distance D = 10.0 m below. Your friend's outstretched hands are at distance d = 1.50 m above the ground.

a)How much work Wg does the gravitational force do on the book as it drops to her hands? (i got this answer-166.5J)

b)What is the change U in the gravitational potential energy of the textbook-Earth system during the drop? (I could not find a way to get this answer.)

c)If the gravitational potential energy of that system is zero at ground level, what is its potential energy U when the textbook is released?(196J)

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? (29.4J)

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?(166.5)

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?

g) Find U at the release point when U is 100 J at the ground level?(296J)

h) Find U at the hands when U is 100 J at the ground level? (129.4)

so i need help finding b and f. please and thankyou in advance

Last edited: Jul 17, 2006
2. Jul 17, 2006

### Staff: Mentor

I don't understand how you could do c, d, g, or h, without knowing how to calculate the change in gravitational potential energy near the earth's surface. (Which is all you need for b and f.)

3. Jul 17, 2006

### Andrew Mason

You appear to be hung up on the term "textbook-earth system".

Is the potential energy of the book-earth system any different than the potential energy of the book in the earth's gravitational field? You have already calculated the latter.

AM

4. Jul 17, 2006

### HallsofIvy

If you lift a book a distance h, on the surface of the earth, then you have increased the book's gravitational potential energy (which is how I would interpret "gravitational potential energy of the textbook-Earth system") by gh. That is assuming that h is small enough that the acceleration due to gravity can be taken to be the constant -h.

5. Jul 17, 2006

### kappcity06

i'm still uin sure of the answer. I did the height(10m)*gravity(9.8)*mass(2) and the answer is wrong?

6. Jul 17, 2006

### Staff: Mentor

The book drops onto his hand, not onto the ground.

7. Jul 17, 2006

### Office_Shredder

Staff Emeritus
Keep in mind that the potential energy decreases as the book falls.

You're on the right track

8. Jul 17, 2006

### sdekivit

remember:

$$\{Delta} U = mgh_{2} - mgh_{1}$$

with 2 = situation after movement and 1 = situation before movement.

Last edited: Jul 17, 2006
9. Jul 17, 2006

### kappcity06

would it be mass*gravity*height(10m)-mass*gravity*height(1.5m)

10. Jul 17, 2006

### kappcity06

would it be mass*gravity*height(10m)-mass*gravity*height(1.5m)

this did not work. I'm still getting a wrong answer. i have 166.6.

11. Jul 17, 2006

### kappcity06

i got part b which was -166.6. but part f is still giving be trouble. should i just and 100 to the answer for b

12. Jul 17, 2006

### Staff: Mentor

Part f still asks for the change in gravitational PE.

13. Jul 17, 2006

### sdekivit

so you were only looking for the minus sign

if you did b correct, than f should be no problem.

Last edited: Jul 17, 2006
14. Jul 17, 2006

### kappcity06

i tried adding one hundred to get -66.6 but that was wrong. should i subtract 100 to get -266.6

15. Jul 17, 2006

### Staff: Mentor

Hint: The 100 J is irrelevant in finding the change in PE.

(It's like asking what's the difference in height from your head to your toes if you are standing on Mt. Everest. Do you need to know the height of Everest to answer?)

16. Jul 17, 2006

### kappcity06

thank you doc al the answer was the same as b. thanks to all who helped me too