Proving initial speed of a particle

[paradiselost]

Homework Statement

Particle A of mass 1kg is at rest 0.2m from the edge of a smooth horizontal table 0.8m high. It is connected by a light inextensible string of length 0.7m to particle B of mass 0.5kg. Particle B is initially at rest at the edge of table closest to A but then falls out. Assuming B's initial horizontal velocity is zero, prove that speed which A begins to move is 1.04m/s

Homework Equations

F = ma
Momentums : m(v-u) etc.

The Attempt at a Solution

I solved it before, but I can't seem to remember how to =(

 

[housemartin]

do you know this equation:
$$v^{2}=v^{2}_{0}+2gh$$
which can be derived from equation of motion in straight line with constant acceleration. You can use it to find speed of B at the moment when it starts to pull ball A. When you have this velocity, use conservation of momentum.