TBR Physics - Work & Energy Q35

[muhali3]

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Q39.

When the box has a mass of 90kg, what is true of the total work associated with raising the box compared to the work needed to raise the counterweight and return the lift plate to the base? (The counterweight is 50kg.)

A. The work needed to raise the box exceeds the work needed to lift the counterweight.
B. The work needed to raise the box is less than the work needed to lift the counterweight.
C. The work needed to raise the box is equal to the work needed to lift the counterweight.
D. No work is needed to raise the box.

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I put choice A, because the question asks what is the "total work" associated with raising the box. The counterweight and people are both performing work on the box to lift it.

But TBR says choice B is right, because the counterweight removes 50kg from the box. But shouldn't the work done on the box by the counterweight also be included since it is asking for "total work"???

I understand how it would be easier for a person to lift up the box. But the question isn't asking for how much work a person would have to do on the box vs. counterweight. It's asking for the total work, which should always be more for a 90kg mass than 50kg mass, assuming everything else is constant (which it is).

Seems like a bad question.

edit: sorry this is question 39, not 35.

 

[Catburr]

I'm not sure if I'm picturing the setup right...my TBR physics is a different version than yours I guess, since my question 35 in work and energy is about cars.

But I think the idea is, the work to get the counterweight of 50kg to the ground is 50N x (distance it travels down) to the ground. The work to get the load of 90kg up to some platform is 90N x (distance it travels up) to the platform. Because of the different directions, you get opposing signs for work done. You can call either one negative, they just have to be opposing. When you combine them for total work done, you get 40N x (distance) worth of work.

To contrast, when you lift the counterweight back up, with no load on the platform, the only work is that of lifting the counterweight. 50N x (distance).

EDIT: Lol, I see your 35->39 edit, but my 39 is still different too. 🙂 Hope this helps anyway.

 

[muhali3]

I see your point. I was thinking that the downward force of the counterweight will pull down on the rope causing an upward tension on the side with the lift plate. But thank you for responding, despite not having my book.👍