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Calculating work not given acceleration, work done by gravity

  • Thread starter KinjaMSTR
  • Start date
  • #1
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Homework Statement


A rope lifts a mass of 20kg vertically 80cm.

How much work is done by the rope on the mass?

How much work is done by gravity on the mass?



Homework Equations


W = FΔd
Eg = mgh


The Attempt at a Solution


How much work is done by the rope on the mass?
W = FΔd
W = F(0.8)

This is as far as I've gotten. I don't think this is the correct solution. For this to work, I would need to put the applied force of the rope. The problem is that I don't know the acceleration of the rope, so I'm thinking this may be the completely wrong approach.

How much work is done by gravity on the mass?
So far what I have is:
W = FΔd
W = mgΔd
W = (20)(9.81)(0)
W = 0

Because gravity doesn't move the mass, the work done by gravity is zero. I believe this is the correct solution.
 

Answers and Replies

  • #2
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I believe this is the correct solution.
It is not.

It does not matter "what" moves the mass. The mass moves against the direction of the gravitational force, that needs work. Energy is conserved, so where does that work come from?

The speed of the process and other things don't matter, you have the correct formulas there.
 

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