# Work problem-blocks,pulley

1. Feb 12, 2009

### james brug

Work problem--blocks,pulley

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

Two blocks are connected by a very light string passing over a massless and frictionless pulley . The 20.0 N block moves 75.0 cm to the right and the 12.0 N block moves 75.0 cm downward.

Find the total work done on 20.0N block if there is no friction between the table and the 20.0N block.

2. Relevant equations
$$w=f\cdot s$$
$$f_k=\mu_k\cdot n$$

3. The attempt at a solution
w=12N(.75m)=9J--mastering physics says it's wrong.

2. Feb 12, 2009

### w3390

Re: Work problem--blocks,pulley

Ya, your attempt is not correct. What you need to do is find out the force the hanging block exerts on the sliding block. Then you can do W=F*d using this force over a distance of 75cm.

3. Feb 12, 2009

### Delphi51

Re: Work problem--blocks,pulley

First, are you sure the blocks are in Newtons? Very odd - would expect kg.
Second, I think the whole thing is more complicated because the blocks are accelerating. A force of 12 N down on the 2nd block will be opposed by the Tension in the string, so
sum of forces = ma
mg - T = ma
T = mg-ma and it will not be the full 12N pulling on the other block.
I think the first step will be to calculate the acceleration of the whole system.

4. Feb 12, 2009

### james brug

Re: Work problem--blocks,pulley

That's probably it. There is a problem nearly identical to this in the book, but it says the blocks are moving at a constant speed.

5. Feb 12, 2009

### w3390

Re: Work problem--blocks,pulley

I believe Delphi51 is correct.

6. Feb 12, 2009

### james brug

Re: Work problem--blocks,pulley

T would give me the force on the 20N block and thus the work, yes?
a=12N/total weight?

7. Feb 12, 2009

### w3390

Re: Work problem--blocks,pulley

I think you mean that a=(Force)/(mass) not weight. For the force, use the T from your first statement.

8. Feb 12, 2009

### james brug

Re: Work problem--blocks,pulley

What first statement?

Last edited: Feb 12, 2009
9. Feb 12, 2009

### w3390

Re: Work problem--blocks,pulley

This statement. You say that solving for T would give you the force you need in the equation to find work.

10. Feb 13, 2009

### james brug

Re: Work problem--blocks,pulley

What about the work done on the other block? Is it the same?

Last edited: Feb 13, 2009
11. Feb 13, 2009

### w3390

Re: Work problem--blocks,pulley

The work will not be the same for the other block. In this case the only force acting on it is mg. Therefore, your work will just be mg*.75m and I think it will be negative.

12. Feb 13, 2009

### james brug

Re: Work problem--blocks,pulley