Question

I really need help with this one: What is the least amount of work to do, when we want to roll 2.5 m length curtain that weights 10 N onto a thin tube, that is above the window? Ignore friction.

Answer 1

A = Pl
l = ? I get that it should be something about circle and circular motion, but I can't figure out how to get radius or something to start off with...

Answer 2

I'm thinking W = Fd (work = force * displacement) where the force is 10 N and the minimum displacement is 2.5 m

Answer 3

i agree with @peachpi

Answer 4

@Zyberg do you understand why?

Answer 5

@priyar I understand the formula, but why is the displacement 2.5? Shouldn't it be more? The curtain would be rolled up around a few times after all.

Answer 6

I think this can be simplified.

The centre of mass of the curtain starts 1.25m below where it ends

so the distance moved by the centre of mass = 1.25m

W=F * d

Answer 7

It is evident that the top of the curtain moves less - and the bottom moves more - but the net effect is to raise the centre of mass by 1.25m

Answer 8

@MrNood I think that your answer to this problem is the right one, one of my friends that helped me solved this using calculus (something about weight per meters and such) and got the same answer.
Could you explain a little bit more why is it that the net effect is only to raise the center of the mass?

Answer 9

The question format is relatively simple - it doesn't give you information about the rollere - other than it is 'thin - which I take to mean you can ignore any 'rolling' calculation.