Physics: Work in Raising vs. Lowering

[Destro96]

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Suppose you have a single mass on a string (think crane lifting an object). First, it is raised 10M. Next, it is lowered 10M.

They basically ask if more work is done while raising the mass, lowering the mass, or if equal work is done in both actions.


My answer is that raising the mass does more work, as the change in h is positive during lifting and negative during lowering. The absolute value of both work products is the same, but the one for raising is greater.

One of the more recent AAMC tests asks a question testing the same principal. The correct answer shown is that the amount of work is the same in both instances.

While work is a scalar, I have seen physics texts that definitely refer to negative work. Is this a gray area?

 

[TheBoondocks]

Suppose you have a single mass on a string (think crane lifting an object). First, it is raised 10M. Next, it is lowered 10M.

They basically ask if more work is done while raising the mass, lowering the mass, or if equal work is done in both actions.


My answer is that raising the mass does more work, as the change in h is positive during lifting and negative during lowering. The absolute value of both work products is the same, but the one for raising is greater.

One of the more recent AAMC tests asks a question testing the same principal. The correct answer shown is that the amount of work is the same in both instances.

While work is a scalar, I have seen physics texts that definitely refer to negative work. Is this a gray area?

No, this is incorrect. When you lift something, you do work. However, gravity does negative since it acts opposite to the displacement. So, the net work is 0. This is something people confuse. The change in Ke= net work. What is the initial velocity? 0. what is the final velocity? 0. So, no net work is done.

Now, when you lower something, gravity does positive work. However, what direction is the force of the crane? It is in the direction of the sky, thus it does negative work. This must be because without this force, the object would fall. So, the net work is 0.