Investigation 12B: Pulleys

Essential questionsHow do you use pulleys to lift a heavy object?
Does a system of pulleys change the work required to lift an object?
Pulleys can be used to change the direction of the force needed to lift an object and also to reduce the force needed to lift it! The block and tackle is a simple machine that can be used to lift heavy objects. In this investigation, you will combine two or more pulleys to form a block-and-tackle system. In each case, you will measure how far you have to pull on a string to lift the mass by 20 cm and record the force required to do so.
Part 1: The mechanical advantage of a block and tackle

How to construct a block and tackle pulley system
  1. Set up the block and tackle machine as shown. The output force is the weight of the bottom pulley block and weights.
  2. Configure the block and tackle with 1, 2, 3, 4, 5, and 6 support strings. For each configuration, measure and record the input force and the output force.
  1. What is the rule that tells you the mechanical advantage of a pulley system?
  2. What was the largest mechanical advantage you could construct? Why is this the limit of this system of pulleys?
  3. Was the actual measured mechanical advantage greater or less than you think it should have been? Propose an explanation for any differences you observed.
  4. Is it possible to build an infinite mechanical advantage using an infinite number of pulleys? Why or why not? What limits they realistic advantage of a machine made with blocks and pulleys?
Part 2: Mechanical work

How to connect two broomsticks together as a set of pulleys
  1. Repeat the block and tackle setups for each mechanical advantage you found.
  2. Measure and record the distance the load (output) rises when you pull the input string 30 cm.
  3. Calculate and record the input and output work for each machine.
  1. How do the input and output work compare?
  2. Do the input and output work agree with the principle that the output can never exceed the input?
  3. Calculate the efficiency of your machine for different mechanical advantages.
  4. Can you explain why the efficiency decreases as the mechanical advantage increases?
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