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 SA be the distance of the block A from the datum line and SC, SB 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 = SA + 2SB +SC. Differentiating the equation with respect to time we have that V+ 2VB + VC = 0 and 10 + 2V+ 20 = 0
VB = -15 ft/s = 15 ft/s (in upward direction).
VB = VC + VB/C and -15 = 20 + VB/C,
VB/C = -35 ft/s = 35 ft/s (in upward direction)

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