M-4 PULLEY SYSTEMS
Read the theory section of experiment M-3. Also review the sections on force solution of equilibrium systems in any text.
Pulleys, strings, weights, weight hangers, support rods and hardware.
Pulley systems allow one to lift heavy loads using forces much smaller than the weight of the load. Generally a force applied to a rope supplies the input work. This force usually acts in a direction parallel to the motion of the load.
Figs. A-F illustrate various types of pulley systems. For each of these, do the following.
(1) First calculate the theoretical mechanical advantage by doing a force analysis on the system. Do this by calculating the required input force to just balance the system in equilibrium. In this calculation, consider the pulleys massless (except in system D). Also, calculate the displacement ratio.
(2) Select a heavy load, at least 10 times as heavy as the total weight of the pulleys, and set up the system. Determine experimentally how much force to apply to the input string to balance the system. Apply this force by hanging weights on that string. Compare the experimental value of mechanical advantage with the theoretical value.
(3) Measure the displacement ratio, and compare it with your previous calculation.
(4) Compute the efficiency of the system.
Some of these pulley systems have names:
System A: block and tackle. System B: Spanish burton.
System F has an attachment to the floor. The square block in the diagram represents the load. Predict in advance how this system will behave when you pull the free end of the rope upward.
(1) The theoretical mechanical advantage of a block and tackle (such as system A) is equal to the number of ropes supporting the load. Does this rule work for the other systems?
(2) Why do you suppose system C is sometimes called the "fool's tackle?"
Text and drawings © 1997, 2004 by Donald E. Simanek.