# Work energy principle

1. Nov 30, 2008

### Bones

1. The problem statement, all variables and given/known data

The system of two blocks is released from rest. The pulley is a solid cylinder of mass 5.00kg and radius of 10.0cm. MA is 10.0kg and MB is 20.0kg. The coefficient of kinetic friction between MA and the table is 0.2. Find the velocity of the blocks after moving 3.00m using work-energy principle.

Block A is on a flat surface and block B is hanging. They are connected by a rope over a pulley.

2. Relevant equations

3. The attempt at a solution

I don't remember how to do this, please help!

2. Nov 30, 2008

### tiny-tim

Welcome to PF!

Hi Bones! Welcome to PF!

Calculate the work done by the friction force on MA and by gravity on MB.

(and remember that the two blocks, and the rope on the pulley, will have the same speed!)

That will equal the change in KE.

3. Nov 30, 2008

### Bones

Work Force of friction: 0.2*98N*(d)(cos180)=-19.6(d)
Change in KE=1/2(20kg)(V)^2-1/2(20kg)(0)

-19.6(d)=1/2(20kg)(V)^2

I don't think I am doing this right...

4. Nov 30, 2008

### tiny-tim

What about gravity? and KE of the pulley?

5. Nov 30, 2008

### Bones

mAgh+0.5mAvA^2=mBgh+0.5mBvB^2

(0.2)(98N)(3.00m)(cos180)=(10kg)(9.8m/s^2)(0)+(0.5)(10kg)(vA)^2=(20kg)(9.8m/s^2)(h)+(0.5)(20kg)(vB)^2

I am not sure about the height of mass B. I don't think this is right either...

Last edited: Nov 30, 2008
6. Nov 30, 2008

### Bones

I think I figured it out. Thanks for your help!

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