$12-217.$ The block $B$ is suspended from a cable that is

attached to the block at $E,$ wraps around three pulleys,

and is tied to the back of a truck. If the truck starts from

rest when $x_D$ is zero, and moves forward with a constant

acceleration of $a_D=0\mathrm{.}5\frac{m}{s^2},$ determine the speed of

the block at the instant $x_D=2m.$ Neglect the size of the

pulleys in the calculation. When $x_D=0,y_C=5m,$ so that

points $C$ and $D$ are at the same elevation.