Integrated Natural Science

Integrated Natural Science

for Detroit Public

Schools

Ropes and PulleysKat Woodring

Key Questions:• 4.1.1 Explain the function of simple

machines.

• 4.1.2 Differentiate input and output forces and diagrams all forces.

• 4.1.3 Calculate the mechanical advantage of a simple machine.

• 4.1.4 Relate the concepts of simple machines to the human body.

• 4.1.5 Calculate advantage mechanical using input and output.

Michigan Content Expectations P3.2B Compare work done in different situations.

P3.2C Calculate the net force acting on an object.

Qualitatively and quantitatively explain forces and charges in motion.

District Outcomes

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 3 weights the bottom block

The bottom block and the weights are the load to be lifted.

Use the force scale to measure the total weight of the load. Record it.

Pulley Investigation #1

Why does the pulley have so many strings?

1. The yellowyellow string is the one we pull to lift the lower block, it then supports the block and transfers the lifting force to the block.

2. Red safety strings support bottom pulley block only when hanging

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, 5 Supporting 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

Key Questions:• 4.1.1 Explain the function of simple

machines.

• 4.1.2 Differentiate input and output forces and diagrams all forces.

• 4.1.3 Calculate the mechanical advantage of a simple machine.

• 4.1.4 Relate the concepts of simple machines to the human body.

• 4.1.5 Calculate advantage mechanical using input and output.

Mechanical AdvantageFor 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.

WorkWe define and measure work

in a very specific way in science.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 - #2 Work

Transfer the data you recorded as your Output Force from investigation 4.1 to the Data Table in 4.2.

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 change 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 (but not equal 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 and distance for 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.