Question

Figure shows a pulley arrangement.

1. Name the pulleys P and Q.
2. In the diagram, mark the direction of tension on each strand of string.
3. What is the purpose of the pulley Q?
4. If the tension is T, deduce the relation between (i) Tand E, and (ii) E and L.
5. What is the velocity ratio of the arrangement?
6. Assuming that the efficiency of the system is 100%, what is the mechanical advantage?

Answer 1

(a) P is a movable pulley; it rises along with the load L. Q is a fixed pulley.

(b) The two strands supporting movable pulley P exert upward tensions T on it. The effort E is applied downward at the free end of the string.

(c) The function of fixed pulley Q is to change the direction of the effort, so that the effort may be conveniently applied downwards.

(d) Relation between:

(i) T and E:

T = E

(ii) E and L:

The movable pulley is supported by two strands, each having tension T.

Since T = E,

L = 2E

`E = L/2`

(e) The velocity ratio is:

V.R. = 2

(f) Mechanical advantage at 100% efficiency:

η = `(M. A.)/(V. R.) xx 100`

Since η = 100%,

M.A. = V.R. = 2

M.A. = 2

Therefore, the pulley arrangement has a mechanical advantage of 2.