# Homework Help: Work to be performed by a pulley

1. Jul 19, 2012

### inverse

Hello,
I have a doubt in this exercise because it does not relate the forces work to be done.

"The system shown is at rest. Determine the amount of work must be developed to apply a downward vertical force on the sphere, so that the block come off the floor."

http://s2.subirimagenes.com/imagen/previo/thump_7853867fisica1.png

Sphere;

$F + P - T = ma$

Block;

$F_E - P = ma$

How should raise it?

Thanks

Last edited by a moderator: Jul 19, 2012
2. Jul 19, 2012

### Staff: Mentor

The link is not working for me...

3. Jul 19, 2012

### Staff: Mentor

so it looks like you could reduce this problem by symmetry to simply lifting a 2kg weight off the ground with a spring attached to the string you're using.

Given that the weight is 2kg you can determine the downward force you must overcome and given the spring K you can use hooks law to determine the upward force as you raise the string. When they are balanced the weight should lift off the ground.

Does that sound right?

4. Jul 19, 2012

### Staff: Mentor

If the system is initially at equilibrium, and the pulley's are frictionless, what is the tension on the string that connects the 2 kg weight to the top of the spring? What is the tension on the string connecting the bottom of the spring to the 4kg mass? How much does the tension in the string connecting the 2 kg mass to the top of the spring have to increase to just begin lifting the 4 lb mass? What will the tension on the string connecting the bottom of the spring to the 4 kg mass have to increase to? For that increase in the tensions, how much will the spring have to stretch? How much work does it take the stretch the spring this amount?