# A tough pulley cable system problem!

1. Apr 3, 2009

### electricblue

1. The problem statement, all variables and given/known data
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" [Broken]

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?
2. Relevant equations

F= ma
Length of rope = constant

3. The attempt at a solution

F=maaa
2 T = ma2t
T = mat

F=mbab
3/2 T = mbab
3/2 mat = mbab
ab = (3mat)/(2mb)

is it correct for part (i)?

Last edited by a moderator: May 4, 2017
2. Apr 3, 2009

### 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?

3. Apr 3, 2009

### 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.