# Simple Machines

## Introduction to six different devices that make work easier by changing a force.

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Human Levers

### Human Levers

Credit: Michael Duxbury
Source: http://www.flickr.com/photos/michaelduxbury/4750867331/

You use several levers when you play tennis or other sports. In fact, you use levers whenever you move during normal daily activities. The levers are part of the human body.

#### News You Can Use

• A lever is a simple tool. It consists of a rigid rod, or arm, which turns about a pivot point, called the fulcrum. Force, or effort, is applied to the lever arm to move a load, also called the resistance.
• There are three classes of levers. Depending on its class, a lever may increase the effort and/or apply the force over a greater distance and therefore more quickly.
• Muscles and bones act together to form levers in the human body. Most human body levers are third class levers.
• Credit: cvasquez84
Source: http://www.flickr.com/photos/cvasquez84/4489237879/in/photostream/

The muscles of the leg and the knee joint combine to form a large lever [Figure2]

#### Can You Apply It?

1. Describe human body levers and how they work in general terms.
2. Identify a first class lever in the human body. What are the parts of this lever?
3. Describe a second class lever in the human body. What does it allow you to do? Identify the fulcrum and lever arm. What muscles apply the effort, and what is the resistance?
4. When you bend your arm at the elbow to lift a book, what type of lever are you using? Identify its parts, and explain how it works.
5. Identify and describe the type of upper body lever used by a swimmer doing the freestyle or back stroke.
6. Many levers are used in sports. A tennis racket is one example. What type of lever is a tennis racket? How is it used, and how does it change the effort force?

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