What is work done by gravity when lifting an object?

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Homework Help Overview

The discussion revolves around the concept of work done by gravity when lifting an object, specifically a brick of mass m moved vertically upward by a height h. Participants are exploring the relationship between force, displacement, and the sign of work in this context.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the formula for work as force times displacement and question how the direction of gravity affects the sign of the work done. There are attempts to clarify whether work can be done by a force acting in the opposite direction to displacement and how this relates to the definition of the system.

Discussion Status

There is an active exploration of the implications of work done by gravity, with some participants suggesting that the work done by gravity could be negative. Others express confusion about the concept of net work when lifting the brick, indicating a productive dialogue without a clear consensus.

Contextual Notes

Participants are grappling with the definitions and implications of work in the context of forces acting in opposite directions, as well as the assumptions underlying their interpretations of the problem.

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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