In fig., the blocks are of equal mass. The pulleyis fixed. In the position shown, A moves with a speed u and the speed of B is v. Then

See fig.From figurex2+y2=l2∴2xdxdt+2ydydt=2ldldtThere is not vertical motion of the block. Hencedydt=0,dxdt=vx and dldt=vA=uHence  2×vx=2lu or xvx=lu∴vx=lxu=ucos⁡θ.