# Homework Help: One Knot Problem - Find two unknown masses

1. Oct 1, 2009

### Wolfin

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

See the attachment.
We're asked to find the masses of M1 and M2. I'm wondering how this is possible, 'cause there's two variables and only one known? Do I have to use sine/cosine law?

m3 = 0.1998 kg
T3 = (0.1998 kg)(-9.80 m/s^2) = -1.96 N

Vertical angle A (between T3 and T1) = 38.0 degrees
Vertical angle B (between T3 and T2) = 27.0 degrees

2. Relevant equations

F = ma
F = ma sin (Θ)

3. The attempt at a solution

I tried to solve with algebra:

m1 g sin (A) + m2 g sin (B) = m3 g

Gravity cancels.

m1 sin (A) + m2 sin (B) = m3 g

...And that's about as far as I got :(

Any help/hints would be appreciated muchly.. the Physics head decided we should only get two days to do our labs.. it's due tomorrow.

File size:
40.5 KB
Views:
124
2. Oct 3, 2009

### gabbagabbahey

Hint: Force is a vector...it will have both horizontal and vertical components, and both must be zero for the mass m3 not to move....

3. Oct 3, 2009

### Wolfin

Yeah, I got it now. Had to:

Solve for m1 in the equation for the sum of all the x components and the equation for the sum of all the y components.

Then make them equal to eachother, solve definitively for m2, then substitute to solve for m1.

Thanks for the help!