# Work on object lifted in a gravational field

1. Mar 6, 2008

### sracks

A 1.1 kg book is lying on a 0.80-m-high table. You pick it up and place it on a bookshelf 2.17 m above the floor.

How much work does gravity do on the book?
How much work does your hand do on the book?

W=mgh

(1.1 kg)(9.8 m/s^2)(0.8m)=8.6

The answers that I am getting are wrong

2. Mar 6, 2008

The book is initially on the table and then its moved to the shelf. So it moved (2.17-0.8) m
Also, work done by gravity will be negative.

3. Mar 6, 2008

### sracks

Help

Now I have another problem

A 10 g particle is moving to the left at 29 m/s. How much net work must be done on the particle to cause it to move to the right at 47 m/s?

W=mg$$\Delta$$r

I got 1.7. I don't think I am doing this right.

4. Mar 6, 2008

### tiny-tim

Show us how you got 1.7

(btw, if you type alt-j, it give you ∆ without having to tex! )

5. Mar 6, 2008

### sracks

(.01kg)(9.8m/s^2)(47 m/s - 29m/s) = 1.7

I have a feeling that i am completely doing it wrong

6. Mar 6, 2008

Got it