# Need help with problem

1. Oct 4, 2007

### Xiongster

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

Hi, I have a problem here and I need some help on it. The problem is this:

block m$$_{1}$$ weighs 875 N. The coefficient of static friction between the block and the table is 0.24 and the angle theta is 25.0$$\circ$$. Find the maximum weight of block m$$_{2}$$ for which block m$$_{1}$$ will remain at rest.

2. The attempt at a solution

So far I've drawn an FBD for m$$_{1}$$, m$$_{2}$$, and the knot that connects them.

For the block m$$_{1}$$, I have T$$_{1}$$ = $$M$$$$_{s}$$N in the x-direction, and N = m$$_{1}$$g for the y-direction.

For block m$$_{2}$$, I have T$$_{2}$$ = m$$_{2}$$g in the y-direction.

For the knot, I have T$$_{1}$$ = T$$_{3}$$*cos(25) in the x-direction and T$$_{2}$$ = T$$_{3}$$*sin(25) in the y-direction.

Since T$$_{1}$$ = T$$_{3}$$*cos(25), and T$$_{1}$$ = $$M$$$$_{s}$$N, I set T$$_{3}$$*cos(25) = $$M$$$$_{s}$$N and solving for T$$_{3}$$, I get T$$_{3}$$ = ($$M$$$$_{s}$$N)/(cos(25))

In the knot, we have T$$_{2}$$ = T$$_{3}$$*sin(25). I plug in the T$$_{3}$$ derived earlier and T$$_{2}$$ = m$$_{2}$$g into this equation to get m$$_{2}$$g = $$M$$$$_{s}$$*m$$_{1}$$g*tan(25)

When I plug in numbers, the answer I get is about 98, and that just doesn't seem right to me. Did I do something wrong? Thanks for the help in advance.

2. Oct 5, 2007

### learningphysics

I get the same answer as you. 97.9N. Why do you think it is the wrong answer?

3. Oct 5, 2007

### Xiongster

I've tried putting that in as the answer, but the computer says it isn't correct.

4. Oct 5, 2007

### learningphysics

Did you put in 98 or 97.9? Maybe they want the mass and not the weight? I'm not sure...

5. Oct 5, 2007

### Xiongster

When I put it in, it wants the units as newtons, so I'm sure it's weight. But now that I have a second opinion, I'll just argue with the professor if it's wrong =]. Thanks!

6. Oct 5, 2007

### hagar852

Did you neglect the static friction force?

which would be 210N.. So whatever Tension you assigned to the rope being pulled horizontally would be a maximum of 210N

7. Oct 5, 2007

### hagar852

Yes it seems you did cause I also get 97.9N as the answer....

If it helps I am also a Physics teacher for the sake of your arguement.

8. Oct 5, 2007

### AmaniKaleo

I did the problem, and I also got 97.9 N. Just to let you feel more secure about it :)

9. Oct 5, 2007

### Xiongster

Alright! Thanks everyone!