How Does Block B's Motion Relate to Block A in a Pulley-Cable System?

[electricblue]

Homework Statement

In the pulley-cable system shown, the acceleration of block A, x(double dot), varies with time, t(in seconds), as given by the expression x(double dot) = 2t m/s2. initially the two blocks are 5m apart and both are stationary.

http://img3.imageshack.us/img3/2074/pulleycable.jpg"


i)what is the acceleration, y(double dot), of block B at time t?
ii)what is the velocity of block B when t = 3s?
iii)how far apart are the blocks when t = 3s?

Homework Equations

F= ma
Length of rope = constant

The Attempt at a Solution

F=m_aa_a
2 T = m_a2t
T = m_at

F=m_ba_b
3/2 T = m_ba_b
3/2 m_at = m_ba_b
a_b = (3m_at)/(2m_b)

is it correct for part (i)?

 

[turin]

I don't think that you need Newton's second law. Is the pulley on the left, closer to B, fixed to B or fixed in space?

 

[Dr.D]

What you really have here is a kinematics problem. You need to express the fact that the length of the rope remains constant, in terms of x and y. Until you do that, nothing is going to happen.