Finding the mass of a block on an inclined plane

[hansel13]

Homework Statement

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 m_b, attached to the dangling end, for which A remains at rest is...?

Homework Equations

F = ma
F_s = u_s*F_n

The Attempt at a Solution

F_s = .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.

 

[tiny-tim]

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 m_b, attached to the dangling end, for which A remains at rest is...?
…
F_s = .4*10*9.8*cos35 = 32.1N

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?

 

[hansel13]

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

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

 

[hansel13]

Hopefully it is haha..

 

[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...

 

[tiny-tim]

T = F_k-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

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?

 

[hansel13]

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?

ah, Right!

T = F_k-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..