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 $a_D = 0.5 \text{ m/s}^2$, determine the speed of the block at the instant $x_D = 2 \text{ m}$. Neglect the size of the pulleys in the calculation. When $x_D = 0$, $y_C = 5 \text{ m}$, so that points C and D are at the same elevation.