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# Compound Machine

## Introduction and examples of a machine that consists of more than one simple machine.

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Pedal Power

### Pedal Power

Credit: felixtsao
Source: http://www.flickr.com/photos/felixtsao/7869684482/

Did you know that the bicycle is the world’s most widely used transport machine. Worldwide, bicycles outnumber automobiles almost two to one. Why is the bicycle so popular?

#### Amazing But True!

• Believe it not, the bicycle is the most efficient transport machine ever invented. You can go farther for the same amount of energy on a bike than you can with any other means of transport.
• A bicycle goes 3,000 miles on the energy equivalent of one gallon of gasoline. The most fuel-efficient cars go only about 100 miles on a gallon of gas.
• A person riding a bike can also travel farther on the same amount of energy than any other animal on the planet.
• Biking is more efficient than walking, running, swimming, or even skating. It takes about three times as much energy to walk, for example, as it does to bike the same distance.

Credit: Created by Cartemere. Edited by Robert Lopez.
Source: http://commons.wikimedia.org/wiki/File:BikeFrameMtb.png

[Figure2]
• What makes a bicycle so efficient? Look at the bicycle drawing above.
•  A bicycle has wheels that roll over the ground. Rolling friction is weaker than other types of friction, needing less energy to overcome.
• Modern bikes have wheels with spokes instead of solid wheels.
• They also have hollow frames.
• These features make bicycles lightweight so it doesn’t take much energy to move them.
• Bicycles are also aerodynamic. Their long, narrow design reduces wind resistance.
• Modern bikes have gears, which you can see clearly in the second drawing. Gears allow the rider to keep pedaling at a comfortable and efficient rate even when traveling uphill or against a headwind.
• Watch this video to learn more about gears and how to use them to maximize efficiency when riding a bike:

#### Show What You Know

1. How would you define the efficiency of a machine?
2. How are a bicycle's gears changed to make peddling more efficient?
3. How have bicycles changed over the years to become even more efficient?
4. A bicycle is more efficient when it is going downhill than when it is traveling on level ground. Why?

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