Foundations of Physical ScienceWorkshop: Ropes & Pulleys -

Work

Ropes & Pulleys - Work

CPO Science

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

Simple Machines Include: rope and pulley

wheel and axle systems

gears

ramps

levers

screws

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

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