$(14$ Points$)$

The man pulls the boy up to the tree $\underset{?}{lim}bC$ by walking backward. If he starts from rest when $x_A=0$ and moves backward with a constant acceleration $a_A=0\mathrm{.}2\frac{m}{s^2},$ determine the speed of the boy at the instant $y_B=4m.$ Neglect the size of the limb$.$ When $x_A=0,y_B=8m,$ so that A and $B$ are coincident, i$.$e$.,$ the rope is $16$ m long.

Ans. $v_B=1\mathrm{.}41\frac{m}{s}$uarr