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4.39: Simple Machines

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When you hear the word machine , do you think of power tools or construction equipment, like the ones pictured above? While both of these examples are machines, you might be surprised to learn that devices as simple as hammers and screws are also machines.

Q : Why are simple tools considered to be machines?

A : Like all machines, they change forces and make work easier.

What Is a Machine?

A machine is any device that makes work easier by changing a force. Work is done whenever a force moves an object over a distance. The amount of work done is represented by the equation:

Work = Force x Distance

When you use a machine, you apply force to the machine. This force is called the input force. The machine, in turn, applies force to an object. This force is called the output force. The output force may or may not be the same as the input force. The force you apply to the machine is applied over a given distance, called the input distance. The force applied by the machine to the object is also applied over a distance, called the output distance. The output distance may or may not be the same as the input distance.  

How Machines Make Work Easier

Contrary to popular belief, machines do not increase the amount of work that is done. They just change how the work is done. Machines make work easier by increasing the amount of force that is applied, increasing the distance over which the force is applied, or changing the direction in which the force is applied. For a humorous introduction to machines and how they make work easier, watch video chapters 1–3 at the following URL:

http://schoolmediainteractive.com/view/object/clip/B658CEF884C8088A6B48EAA7DDA81CB8

Q : If a machine increases the force applied, what does this tell you about the distance over which the force is applied by the machine:

A : The machine must apply the force over a shorter distance. That’s because a machine doesn’t change the amount of work and work equals force times distance. Therefore, if force increases, distance must decrease. For the same reason, if a machine increases the distance over which the force is applied, it must apply less force.

Increasing Force

Examples of machines that increase force are steering wheels and pliers (See Figure below ). Read below to find out how both of these machines work. In each case, the machine applies more force than the user applies to the machine, but the machine applies the force over a shorter distance.

Increasing Distance

Examples of machines that increase the distance over which force is applied are leaf rakes and hammers (See Figure below ). Read below to find out how these two machines work. In each case, the machine increases the distance over which the force is applied, but it reduces the strength of the force.

Changing the Direction of Force

Some machines change the direction of the force applied by the user. They may or may not also change the strength of the force or the distance over which the force is applied. Two examples of machines that work this way are the claw ends of hammers and flagpole pulleys. You can see in the Figure below how each of these machines works. In both cases, the direction of the force applied by the user is reversed by the machine.

Q : If the pulley only changes the direction of the force, how does it make the work of raising the flag easier?

A : The pulley makes it easier to lift the flag because it allows a person to pull down on the rope and add his or her own weight to the effort, rather than simply lifting the load.

Simple and Compound Machines

There are six types of simple machines that are the basis of all other machines. They are the inclined plane, lever, wedge, screw, pulley, and wheel and axle. The six types are pictured in the Figure below . You’ve probably used some of these simple machines yourself. Most machines are combinations of two or more simple machines. These machines are called compound machines. An example of a compound machine is a wheelbarrow (see bottom of Figure below ). It consists of two simple machines: a lever and a wheel and axle. Many compound machines are much more complex and consist of many simple machines. Examples include washing machines and cars.

You can learn more about each of the six types of simple machines at these URLs:

http://www.engquest.org.au/students-sm-up.cfm

http://idahoptv.org/dialogue4kids/season7/simplemachines/facts.cfm

Summary

  • A machine is any device that makes work easier by changing a force.
  • Machines may increase the strength of the force, increase the distance over which the force is applied, or change the direction in which the force is applied.
  • There are six types of simple machines. Machines that consist of two or more simple machines are called compound machines.

Vocabulary

  • machine : Any device that makes work easier by changing a force.

Practice

Watch the video about simple machines at the following URL, and then answer the questions below.

http://nilma18.edu.glogster.com/simple-machines/

  1. Which type of simple machine is a catapult?
  2. What work does a catapult do?
  3. How does a catapult make work easier?
  4. How can you make a catapult with a spoon and ping pong ball?
  5. The pivot point where a catapult turns is called the __________.
  6. Which playground equipment is the same type of simple machine as a catapult?
    1. swing
    2. teeter-totter
    3. slide
    4. all of the above

Review

  1. What is a machine?
  2. List three ways that machines may change a force.
  3. Can a machine increase both the force and the distance over which the force is applied? Why or why not?
  4. A broom is an example of a machine. Where are the input and output forces applied?
  5. Look at the pictures of simple machines above. Which type of simple machine is represented by each of the following tools?
    1. hammer
    2. ladder
    3. knife
    4. doorknob

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Oct 27, 2014
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