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 

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