# Homework Help: Two boxes connected to a string over an ideal pulley?

1. Mar 13, 2012

### algar32

1. The problem statement, all variables and given/known data
Two blocks are connected by a string that goes over an ideal pulley as shown in the figure and pulls on block A parallel to the surface of the plane. Block A has a mass of 3.00 kg and can slide along a rough plane inclined 30.0° to the horizontal. The coefficient of static friction between block A and the plane is 0.400. What mass should block B have in order to start block A sliding up the plane?

2. Relevant equations
box A- sum of x & sum of y
box B- sum of y
plug into each other

3. The attempt at a solution
m2=(ms*m1*sin(theta)+m1*cos(theta)
=3.198

Thanks.

2. Mar 13, 2012

### tal444

First of all. What are all the forces acting upon block A?

3. Mar 13, 2012

### algar32

The forces acting on block A are static friction, fnormal, gravity, and ftension.

4. Mar 13, 2012

### tal444

Yes. So the friction force and force of gravity are preventing the block from sliding up the ramp. What's next?

5. Mar 15, 2012

### algar32

ffriction = -.4*9.81*3
fgrav= -sin30*9.81*3
ftension =m*9.81

ffriction +fgrav = ftension

Not sure which part i messed up

Last edited: Mar 15, 2012
6. Mar 15, 2012

### tal444

Friction force is equal to μF$_{normal}$.