Physical Science Lab #5

Simple Machines: Pulleys

Theory

Effort force: The force being applied by the user of a simple machine (the input force) = Fe.
Resistance force: The force being applied by the machine on an object (the output force) = Fr.
Effort distance: The distance over which the effort force is applied = de.
Resistance distance: The distance over which the resistance force is applied = dr.
Mechanical advantage (actual mechanical advantage): The factor by which a simple machine increases the effort force = Ama = Fr/Fe.
Ideal mechanical advantage: The maximum factor by which a simple machine could increase the effort force assuming their were no friction = Ima = de/dr.
Percent efficiency: The percentage of the maximum mechanical advantage that a machine actually attains = E% = Ama/Ima x 100%
OR The percentage of the input work that is turned into work output = Wo/Wi x 100%. You can show that both of these definitions come down to the same equation, E% = Fede/Frdr x 100%

Procedure
You will use the pulley set up at your bench to measure the following.

  1. Hang a mass of 200 or 500 g (note which) from the hook end of the lower pulley while using the force meter to measure the force required to hold the pulley still. Draw a picture of the pulley system labeling the effort and resistance forces using the variable names above and use your measurements to find the values of each.
  2. What is the mechanical advantage of your machine?
  3. Pull the end of the string with the force meter through a distance of between 50 and 100cm. Note the distance traveled at thisd end AND at the end where the mass is hanging.
  4. What is the ideal mechanical advantage of your machine? What relationship do you expect between the amount of string that gets pulled through the pulley and how high it rises? How is this related to your ideal mechanical advantage?
  5. What is the percent efficiency of your machine?
  6. Calculate the input and output work for your machine. Use these to calculate the percent efficiency of the machine and compare this to the value found above.
Hosted by www.Geocities.ws

1