Second System of Pulleys:

Figure illustrates the pulley arrangements for second system of pulleys. It contains two pulley blocks. Each of block consists of several pulleys. Upper block frame is associated to the fixed support whereas lower block frame may move up and down. Both the blocks can have equal number of pulleys, and upper block can have one pulley more than the lower block. If the upper block contains one pulley more than the lower block, one of end of rope or string is tied to the frame of lower block. The string then goes around different pulleys and effort is applied at the other end. The load is suspended to the frame of lower block.

If both of the blocks have equal number of pulleys, one end of the string is tied to the frame of upper block. Then the string goes round the pulleys and effort is applied at the other end.

A friction is supposed negligible, the tension in the string shall be the same throughout and equal to the effort (P).

Let n is equal to the number of string segments supporting the lower block. For an ideal system,

∴          W = n P

or         W / P  = n

or         Velocity ratio = n.

If weight of lower block is considerable in comparison to the load, it should be taken into account along with the load W.

Let w be the weight of lower block.

 Thus,   n P = W + w

W / P = n - (w/ P)

Mechanical advantage (MA) = W /P

It can be noted that the number of string segments shall be equal to the total number of pulleys in the two blocks.