Two workers pulling on ropes over pulleys to raise a platform

haruspex · Nov 3, 2013

lollikey said:

I got it ! the answer is (4f-7mg) / 7m

Good show.

Poetria · Jan 25, 2016

Well, I can't understand why there are 4f and not 2f. I thought it was mechanical advantage but these pulleys are fixed to the ceiling, aren't they? I am obviously missing something.

haruspex · Jan 25, 2016

Poetria said:

Well, I can't understand why there are 4f and not 2f. I thought it was mechanical advantage but these pulleys are fixed to the ceiling, aren't they? I am obviously missing something.

Draw a line across just below the pulleys. How many ropes cross that line? What is the tension in each?

Poetria · Jan 25, 2016

I see. There are four ropes and each of them has tension f.

haruspex · Jan 25, 2016

Poetria said:

I see. There are four ropes and each of them has tension f.

Right.

Poetria · Jan 25, 2016

I got it. Many thanks. :)

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