Foundations of Physical Science Workshop: Ropes & Pulleys: Mechanical Advantage

Ropes & Pulleys: Mechanical Advantage CPO Science

Key Questions  What are simple machines?  How do simple machines work?

Overview  Build a simple machine using ropes and pulleys  Measure the input and output forces of different pulley arrangements needed to lift load  Compare the input force needed by different pulley arrangements to lift load

Simple Machines Include:  rope and pulley  wheel and axle systems  gears  ramps  levers  screws

Pulleys as Simple Machines  Simple machines can change the direction and/or magnitude of an Input Force

Pulley Investigation #1  Add a few weights (3 or 4) to the bottom block  The bottom block and the weights are the load to be lifted  Use the Force scale to measure the weight of the load - record it

Pulley Investigation #1 —Why does the pulley have so many strings? 1.Red Strings support bottom pulley block only when hanging 2.Yellow String is the one we pull to lift the lower block, it then supports the block and transfers the lifting force to the block

How Can We Lift the Block?  1. We can attach the yellow string to the bottom block and then thread it up and over the top set of pulleys and pull OR….  2. We can attach the yellow string to the top block, thread it down through the bottom pulley set and then up and over the top set of pulleys and pull

Measure the Input Force  Attach the Spring Scale to the pulling end of the yellow string  Pull on the string and lift the load - read the value from the scale as this happens  Lower the load with the string - again read the scale as this happens  Average the two values from the scale - this is the value of your Input Force

Measure the Input Force for Two Supporting Strands  Unclip the yellow string from the bottom block  Thread the string through the lower set of pulleys  Attach the yellow string to the top block  Repeat the Input Force measurement process for TWO supporting strings

Looping the String Around the Pulleys Supporting strings #: 1, 3, 5Supporting strings #: 2, 4, 6

Forces Involved  The Weight of the load does not change, it is the same for each trial  The Output Force will be the force required to hold the load still– it does not change since the weight remains the same  As more strings are added, the Input Force required to achieve the same Output Force decreases

Mechanical Advantage  Ratio of Output Force to Input Force  Follows simple pattern with Ropes and Pulley system

What is the Mathematical Rule?  We found that the input force required to lift the load decreased as the # of supporting strings increased  What is the relationship?  # of strings x Input Force = Weight of load  # of strings = Mechanical Advantage