# What is work done by gravity when lifting an object?

1. Oct 12, 2011

### ttttrigg3r

1. The problem statement, all variables and given/known data
I have a brick of mass m in my hand. I move the brick vertically up h meters.

2. Relevant equations

What is the Work done to the brick by gravity?

3. The attempt at a solution
Work = Fd The F for my hand on the brick is mg going up and d is h so the work done by my hand to the brick is mgh upward. But now what is the work done by gravity? This concept is still blurry to me. Is Wgravity=mgh downward?

2. Oct 12, 2011

### Staff: Mentor

Work is still just force X displacement. Since the force of gravity is downward, while the displacement is upward, what would be the sign of the work done by gravity?

3. Oct 12, 2011

### dacruick

I thought that work can only be done by a force that is in the same direction as the displacement. So wouldn't the work done just be 0? Or is that just a simple matter of how you define your system?

4. Oct 12, 2011

### Staff: Mentor

No. Work can only be done by a force that has a component in the direction of the displacement; that component may well be negative.

5. Oct 12, 2011

### dacruick

Roger that.

6. Oct 12, 2011

### ttttrigg3r

So gravity is down. So is Wgravity= -mgh?

7. Oct 12, 2011

### Staff: Mentor

Right. The work you do (via your hand) is +mgh; the work done by gravity is -mgh.

8. Oct 13, 2011

### dacruick

that can't be right though can it? That means that in lifting it up there is no net work done.