Engineering Mechanics

Chapter 11 : Simple Lifting Machines „„„„„ 207

∴ Efficiency,

M.A. 6.4

0.80 80%

V.R. 8

η= = = =^ Ans.

Amount of friction

We know that amount of friction in terms of load,

F(load) = (P × V.R.) – W = (50 × 8) – 320 = 80 N Ans.

and amount of friction in terms of effort,

(effort )

320

–50–10N

V.R. 8

W

FP=== Ans.

11.13.SECOND SYSTEM OF PULLEYS

In Fig. 11.12 (a) and (b) is shown second system of pulleys containing two blocks, one upper
and the other lower, both carrying either equal number of pulleys or the upper block may have one
pulley more than the lower one.

(a) Different no. of pulleys (b) Same no. of pulleys
Fig. 11.12. Second system of pulleys.
In both the cases, the upper block is fixed and the lower one is movable. There is obly one
string, which passes round all the pulleys one end of which is fixed to the upper block (when both the
blocks have the same no. of pulleys) or to the lower block (when the upper block has one pulley more
than the lower one). The other end of the string is free and the effort is applied to this free end as
shown in Fig. 11.12 (a) and (b). In both the cases the load is attached to lower block.