# Homework Help: Finding the mass of a block

1. Nov 10, 2008

### hansel13

1. The problem statement, all variables and given/known data
Block A with a mass of 10 kg rests on a 35degree incline. The coefficient of static friction is 0.40. An attached string is parallel to the incline and passes over a massless, frictionless pulley at the top. The largest mass mb, attached to the dangling end, for which A remains at rest is....?

2. Relevant equations
F = ma
Fs = us*Fn

3. The attempt at a solution
Fs = .4*10*9.8*cos35 = 32.1N

That's all I've got so far. :rofl:

Not sure why I'm struggling so hard on this problem.

2. Nov 10, 2008

### tiny-tim

Hi hansel13!

Have you drawn an fbd for the block?

There's the friction (which you've caclulated), the tension, the normal force, and the weight.

So what is the tension?

3. Nov 11, 2008

### hansel13

lets calls the mass of block B, B for now.

I think I figured it out, so:
T = Fk-mg*sin30
= 32.1N -10kg * 9.8m/s2*sin30 = 24.1

And we know that T = B*g

So B = T/g 24.1/9.8 = 2.46 kg

I'm pretty sure that's right, is it?

Thanks for the help! Lots of times I get lost and forget what I'm actually looking for, when you mentioned that I needed to find Tension I just drew it up and figured it out.

4. Nov 12, 2008

### hansel13

Hopefully it is haha..

5. Nov 13, 2008

### hansel13

If someone could let me know if this is right or not, I'd really appreciate it. I'm not too confident in my physics skills yet and this is an extra 5 points to my exam correct, so yeah...

6. Nov 13, 2008

### tiny-tim

Hi hansel13!

Yes, that's more-or-less right, except …

i] isn't it 35º?

ii] shouldn't you add the weight force to the friction force?

7. Nov 13, 2008

### hansel13

ah, Right!

T = Fk-mg*sin30
= 32.1N + 10kg * 9.8m/s2*sin35 = 88.2

And we know that T = B*g

So B = T/g 88.2/9.8 = 9kg

Thanks! Dumb mistakes on my part..