Levers Are Useful Tools!
Levers are devices that increase the force you use them with. Each lever has three main parts: the fulcrum, resistance arm, and effort arm. Do the following activity to experience the relationship between the fulcrum, resistance arm, and effort arm.

Materials:

• Pencil
• Ruler
• Five pennies

Safety concerns: As with all science lab activities, the most important safety rule is to follow all teacher directions.

Procedure:

1. Place the pencil on a desk or table top.
2. Lay the ruler at a 90° angle to the pencil (with the middle of the ruler over the pencil.)
• Place one penny at the far end of one side of the ruler.
• Place a second penny at the opposite end on the other side of the ruler.
• Notice how the ruler balances on the pencil.
3. Now, experiment with moving the ruler so the pencil is no longer in the middle of the ruler.
• Can you get the ruler to balance using different numbers of pennies placed on either side?
• Can you get the ruler to balance by moving the pennies so they are not the same distance away from the pencil?

Analysis:

1. Were you able to get the ruler to balance even though it did not have equal distances on either side of the pencil?
2. How did the distance the pennies were placed with from the fulcrum (pencil) affect the way you were able to balance it?

The distance from the effort to the fulcrum is important. For example, you might have used two pennies at a distance of 20 centimeters and found that they exerted a greater force than the same two pennies at a distance of 15 centimeters.

Increasing the distance of the effort arm while decreasing the distance of the resistance arm allows you to gain more "power." There is, however, the challenge that you cannot make the effort arm too long or it is too hard to use!

Now let's look at a pry bar. It is a form of a lever. Use the photographs and see how it can be used as three different forms of levers. Each form of a lever has a different distance between the effort arm and the fulcrum. Notice, if you guess wrong, it will show you the relationship between the effort arm, resistance arm, and fulcrum for that kind of lever.

## The Three Classes of Levers

First class lever

Second class lever

Third class lever

What class of lever is a pry bar?
....It depends on how it is being used!

Think about each photograph and then move your mouse over what you believe is the best choice listed below the image.

Now, let's apply your knowledge about levers to determine the mechanical advantage of a lever. Mechanical advantage refers to how much the lever seems to increase your effort. The formula for mechanical advantage is as follows:

Analysis:

1. If you place the effort two meters away from the fulcrum and the resistance two meters away from the fulcrum, what is the mechanical advantage?
2. If you place the effort two meters away from the fulcrum and the resistance one meter away from the fulcrum, what is the mechanical advantage?
3. If you place the effort one meter away from the fulcrum and the resistance two meters away from the fulcrum, what is the mechanical advantage?

 Check your answers by highlighting the box below. 1. Ma=1, 2. Ma=2, 3. Ma=.5

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