Foundations of Physical Science Workshop: Ropes & Pulleys - Work

Ropes & Pulleys - Work CPO Science

Key Questions  What happens when you multiply forces in machines?  What is work and how is work measured?  How are force and distance related in machines?

Overview  Calculate the amount of work done by a simple machine – ropes and pulleys  Use units of joules to measure the amount of work done  Analyze the effects of changing force or distance in a simple machine

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

Mechanical Advantage For the Lever  Two ways to calculate mechanical advantage  Output Force/Input Force  Input arm length/Output arm length  We can use this to generate large forces from much smaller forces

Work  We define and measure work explicitly  Work = Force x Distance  One joule of work is accomplished when 1 newton of force is used to move an object a distance of 1 meter

Pulley Investigation - Work  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 as your Output Force in the Data Table

Pulley Investigation - Work —What distances are we measuring? 1.Input :The length of the yellow string that is pulled to lift the block ( L ). 2.Output: The height of the block after it is lifted; the distance it is lifted ( h )

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

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

Data Collection  We will be taking the data at all 6 of the pulley arrangements  Compare the data at each arrangement – What changes and how? What stays the same?  Do the calculations for the last two columns ( Work Output & Work Input ) after all the data has been collected

Work Calculation  The joule is the unit used to measure work in this Investigation  Work Input = String Length x Input Force  Work Output = Height difference x Output Force

Work Relationship  As the # of pulleys used increased, the Input Force required decreased  As the # of pulleys used increased, the Input Length of string increased  Work Output was very close to Work Input

Work : You Don’t Get Something For Nothing  Work = Force x Distance  As the Input Force goes down, the Length of string increases.  It’s a trade off – Force vs. Distance  You can use less force to lift the same weight as the Mechanical Advantage increases, but you have to pull more string to do it

Input vs. Output  The change in Force & Distance for the Pulleys is easy to feel while doing the Investigation  In fact, Work Output is always less than Work Input  Where does it go?  Friction

Power - The Rate at which Work is done  Rates are often measured in terms of time  Speed = Distance / Time  Power = Work / Time  Work = Power x Time

The Work – Energy Theorem  The total amount of Work that can be done is equal to the total amount of Energy available.  Objects cannot do Work without Energy  Energy is the ability to do Work  Energy is also measured in joules - it is stored Work  Energy can be stored for later use

The Work – Energy Theorem  Energy can be converted or transformed from one form to another  Anything with energy can produce a force that is capable of action over a distance