Why Are My Calculations for the Window Washer's Pulley System Incorrect?

[VVV]

A window washer pulls herself upward using the bucket-pulley apparatus shown in Fig. 4-48.(heres the pic. http://img126.imageshack.us/i/physics.jpg/) The mass of the person plus the bucket is 70 kg.
(a) How hard must she pull downward to raise herself slowly at constant speed?
(b) If she increases this force by 8%, what will her acceleration be?
For some reason i got wrong answers 686.7 N and 10.59 m/s^2, Why?

 

[Doc Al]

Don't just give your answers--show how you got them.

Hint: What's the relationship between how hard she pulls on the rope and the tension in the rope? How many times does the rope pull up on the "person+bucket" system?

 

[PHYCCHICK]

Hi! I am actually working on a question that is pretty much identical, with different number for weight and % of increase. I was able to get part A.. F=m*a, but then you divide the answer by 2, because the force is split on either side of the pulley. I'm having trouble with B though. I tried to sub in the new force (answer+10.1%... or whatever % was in your question) and solve for acceleration using the same formula F=ma, but I'm still not getting the right answer. Please help!

 

[Doc Al]

I'm having trouble with B though. I tried to sub in the new force (answer+10.1%... or whatever % was in your question) and solve for acceleration using the same formula F=ma, but I'm still not getting the right answer. Please help!

Don't forget that F stands for the net force and that gravity doesn't change.

 

[PHYCCHICK]

I'm confused? If the acceleration remains the same (9.8) then how do I solve for a? And what other forces would be included in the net force?

 

[Doc Al]

If the acceleration remains the same (9.8) then how do I solve for a?

The force of gravity--the weight--remains the same, not the acceleration!

And what other forces would be included in the net force?

Just the weight and the force from the ropes.