+Pulleys and

ForceAll about force and how pulleys

can help reduce it

September 2010

+Pulleys & Force

Learn about pulleys and pulley systems

Learn how using multiple pulleys can dramatically reduce required force

Learn how pulley systems are used in machines and impact everyday life

Learn about teamwork and problem solving in groups

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Objectives

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Basics of Pulleys: Two orientations

Movable PulleyFixed Pulley

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Basics of Pulleys The tension in the rope, T, is

always the same everywhere

Fixed pulley allows for change in direction of applied force

Sum of the forces: vertically

2 T = 100 N T = 50 N

Compound Pulley

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Mechanical Advantage

Mechanical Advantage (MA) isthe factor by which a mechanism multiplies the force or torque put into it.

Ideal MA:

Actual MA:

This movable pulley system has a mechanical advantage of 2

+Pulleys & Force

Work is the amount of energy transferred by a force acting through a distance

Work = Force x Distance

A bigger mechanical advantage decreases the force required, but increases the distance over which it must be applied

The total amount of work required to move the load stays the same

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Work

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The ratio between Actual and Ideal mechanical advantage is Efficiency

Frictionless system = 100% Efficiency

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Efficiency

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Pulleys in the World

Pulleys have long been used on sailing ships to handle the rigging and move the sails

Even with large mechanical advantages, it still takes many people to do the work!

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Pulleys in the World

Pulleys are used in elevators to change the direction of the tension in the cable, reduce power required of lift motor

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Pulleys in the World

Industrial cranes lift large loads for construction and transportation

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Measuring Tension

Spring Scale

Calibrate: Hold spring scale at eye-level and turn adjustment screw until the internal indicator is precisely aligned with the top zero line

Measure: Create a loop in the end of the rope you want to measure tension in; attach spring scale to loop. Hold the spring scale steady and read off the tension measurement.

+Pulleys & Force

Groups of 2

Develop 2 systems to lift a filled soda bottle 10cm with

1 pulley

2 pulleys

Build your systems

Measure the distance the soda bottle moves and compare it to the distance you had to pull

What is the actual mechanical advantage?

Measure the force you must exert on the string and compare it to the force that is finally transmitted to the soda bottle

What is the ideal mechanical advantage?

Calculate the efficiency of each system

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Your Turn

+Pulleys & Force

Now join with one other group at your table

Develop 2 different systems to lift a filled soda bottle 10cm with all 4 pulleys

Build both systems

What are their actual mechanical advantages? Ideal?

Which one has a better

efficiency? Why do think that is?

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Your Turn

+Pulleys & Force

Which system required the least amount of force to lift the bottle? How did this system rank in its mechanical advantage?

Do you think the size of the pulley makes a difference in the ideal mechanical advantage? Actual?

How could you further increase the efficiency of your most efficient pulley system design?

What other engineering problems were solved with pulleys or pulley systems?

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Conclusion

+Spring Scale

www.arborsci.com

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