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Key QuestionsWhat 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
Mechanical AdvantageFor the Lever
Two ways to calculate mechanical advantageOutput 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
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
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