Physics
The diagram alongside shows a pulley arrangement.
(a) Name the pulleys A and B.
(b) In the diagram, mark the direction of tension on each strand of string.
(c) What is the purpose of the pulley B?
(d) If the tension is T, deduce the relation between (i) T and E, and (ii) E and L.
(e) What is the velocity ratio of the arrangement?
(f) Assuming that the efficiency of the system is 100%, what is the mechanical advantage?
Machines
ICSE
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Answer
(a) A → Movable pulley, B → Fixed pulley
(b) Tension on each strand of string is as shown in the diagram below:
(c) The purpose of fixed pulley B is to change the direction of effort to be applied from upward to downward.
(d)(i) When tension in the string is T then the effort E balances the tension T. So, E = T.
(d)(ii) When the load is balanced by the tension T in two segments of the string and the effort E balances the tension T at the free end, so
L = T + T = 2 T and
E = T,
Hence, 2E = L.
(e) When distance moved by effort is d then distance moved by load is d/2 so the velocity ratio becomes 2.
(f) If the efficiency of the system is 100%, then effort applied is equal to half the load. Hence, it acts as a force multiplier and the mechanical advantage is 2.
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