ln the arrangement shown in figure the ends P and Q of an non-stretchable string move downwards with uniform speed U. Pulleys A and B are fixed. Mass M moves upwards with a speed

As P and p fall down, the length / decreases at the rate of U m/s.From the figure, l2 = b2+v2Differentiating with respect to time2l × dldt = 2b×dbdt+2y×dydt(As dbdt=0, dldt = U)⇒ dydt = (ly)×dldt ⇒ dydt = (1cos θ)×U = Ucos θ