Solving Pulley and Platform Homework w/ Acceleration

[haruspex]

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

Good show.

 

[Poetria]

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]

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]

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

 

[haruspex]

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

Right.

 

[Poetria]

I got it. Many thanks. :)