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?
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.
Related Questions
The diagram alongside shows an arrangement of three pulleys A, B and C. The load is marked as L and the effort as E.
(a) Name the pulleys A, B and C.
(b) Mark in the diagram the direction of load (L), effort (E) and tension T1 and T2 in the two strings.
(c) How are the magnitudes of L and E related to the tension T1?
(d) Calculate the mechanical advantage and velocity ratio of the arrangement.
(e) What assumptions have you made in parts (c) and (d)?
In which direction does the force need to be applied, when a single pulley is used with a mechanical advantage greater than 1? How can you change the direction of force applied without altering its mechanical advantage? Draw a labelled diagram of the system.
State four differences between a single fixed pulley and a single movable pulley.
Draw a labelled diagram of an arrangement of two pulleys, one fixed and other moveable. In the diagram, mark the directions of all forces acting on it. What is the ideal mechanical advantage of the system? How can it be achieved?