What is work done by gravity when lifting an object?

AI Thread Summary
When lifting an object, the work done by gravity is calculated as the negative of the work done by the hand. Specifically, if the hand does positive work of +mgh while lifting the brick, gravity does negative work of -mgh. This indicates that the force of gravity opposes the upward displacement. The concept of work requires a force component in the direction of displacement, which can result in negative work when the forces oppose each other. Therefore, while lifting the brick, the net work is not zero, as the work done by gravity is counteracting the work done by the hand.
ttttrigg3r
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Homework Statement


I have a brick of mass m in my hand. I move the brick vertically up h meters.


Homework Equations



What is the Work done to the brick by gravity?

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?
 
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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?
 
Doc Al said:
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?

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?
 
dacruick said:
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?
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.
 
Doc Al said:
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.

Roger that.
 
So gravity is down. So is Wgravity= -mgh?
 
ttttrigg3r said:
So gravity is down. So is Wgravity= -mgh?
Right. The work you do (via your hand) is +mgh; the work done by gravity is -mgh.
 
Doc Al said:
Right. The work you do (via your hand) is +mgh; the work done by gravity is -mgh.

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

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