Calculating work not given acceleration, work done by gravity

AI Thread Summary
To calculate the work done by the rope on a 20kg mass lifted vertically 80cm, the applied force must be determined, which requires knowledge of the acceleration. The work done by the rope is calculated using the formula W = FΔd, but without the applied force, the solution remains incomplete. For the work done by gravity, the initial calculation mistakenly shows zero work, but since the mass moves against gravity, work is done by gravity, which must be accounted for. The correct approach emphasizes that energy conservation is key, and the work done against gravity is not zero. Understanding these principles is essential for solving the problem accurately.
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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.
 
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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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