If a block A is moving downward at 10 ft/s while block C is moving down at 20 ft/s, determine the relative velocity of block B with respect to C.

1) 32 ft/s
2) 30 ft/s
3) 35 ft/s
4) 40 ft/s

Let S_A be the distance of the block A from the datum line and S_C, S_B be the distances of the blocks C and B from the datum line. The total length of the cord L can be written as L = S_A + 2S_B +S_C. Differentiating the equation with respect to time we have that V_A + 2V_B + V_C = 0 and 10 + 2V_B + 20 = 0
V_B = -15 ft/s = 15 ft/s (in upward direction).
V_B = V_C + V_B/C and -15 = 20 + V_B/C,
V_B/C = -35 ft/s = 35 ft/s (in upward direction)

So, velocity of B relative to C is 35 ft/s (upwards).