Aamc #9 ps #29

[komtangiee]

Why is the work done in moving a mass up or down a pulley exactly the same?
I thought moving a mass up using a pulley would require more work since you are opposed by the gravitational force pushing downward and therefore you have to exert more force to counter that...while if you are moving the mass downward, the gravitational force is facing the same way and helps you.

F up - F gravity = F down + F gravity

F up > F down

the AAMC explanation is that W=MGH and therefore the work done is the same...
what am I not getting here?

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[futureortho]

Why is the work done in moving a mass up or down a pulley exactly the same?
I thought moving a mass up using a pulley would require more work since you are opposed by the gravitational force pushing downward and therefore you have to exert more force to counter that...while if you are moving the mass downward, the gravitational force is facing the same way and helps you.

F up - F gravity = F down + F gravity

F up > F down

the AAMC explanation is that W=MGH and therefore the work done is the same...
what am I not getting here?

I would consider it similar to the work-energy theorem in which it doesn't really matter the path taken, since the change in height is the same in both case. That's what I reasoned and got it correct.

 

[Barley426]

It isn't asking which would take less work input on your part, it is asking about work done, meaning overall.

When you are raising the pulley, you are doing the work.

When you are lowering the pulley, gravity is doing the work (if you are ignoring the fact that conservative forces can't technically do work since the energy is being converted).

mg=F
Fd=work, the force is the same the distance is the same, ergo the work is the same.

 

[PocketJacks]

Why is the work done in moving a mass up or down a pulley exactly the same?
I thought moving a mass up using a pulley would require more work since you are opposed by the gravitational force pushing downward and therefore you have to exert more force to counter that...while if you are moving the mass downward, the gravitational force is facing the same way and helps you.

F up - F gravity = F down + F gravity

F up > F down

the AAMC explanation is that W=MGH and therefore the work done is the same...
what am I not getting here?

Ya, the question is kind of confusing because of signs. They're basically asking for the magnitude.
What helped me with Work is visualizing it simply as a transfer of Energy. When you pull it up you "give it" some energy = mg*h. When you lower it down you "remove" energy = mg*h.