How can you lift a heavy object without straining your muscles too much? Using the power of a lever is one way. See how levers make lifting easier by using one you create with a ruler, a pencil, and a plastic bag.

Activity's uses:	Demonstration or small group exploration
Area(s) of science:	Physical Science
Difficulty level:
Prep time:	< 10 minutes
Activity time:	10-20 minutes
Key terms:	Physics, lever, lifting, force, mass, simple machines, building a tool
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Background Information
A lever is a simple machine that can be used to lift heavy objects, as well as to make it easier to do other kinds of physical work. A lever gives the user a mechanical advantage by multiplying the effort the user puts in. Some examples of levers you may see every day are seesaws, scissors, wheelbarrows, and even the human jaw.
All levers have the same functional components, but different classes of levers vary in where the components are located. Seesaws and scissors belong to a certain class of levers, called class 1. These levers usually have a beam that is rigid, long, and thin, like a ruler. Between the two ends of the beam is the fulcrum, or pivot point, from which the beam can balance and move freely up, down, and around. On one end, the user places a load to be moved. On the other end, the user applies effort, or a force, to try and move the load. To lift and balance an object, the effort force multiplied by its distance to the fulcrum must equal the force of the load multiplied by its distance to the fulcrum. Consequently, the greater the distance between the effort force and the fulcrum, the heavier the load that can be lifted.
In this science activity, students can build their own lever using a ruler, a sandwich bag, tape, a pencil, and a bar of soap, and then use their lever to explore how it can lift a load.
For Discussion
This science activity can serve as a starting point for a variety of science and physics discussions. Here are a few examples of questions that can be used to start a discussion:

• What essential components do all classes of levers have? What makes class 1 levers different from class 2 and class 3 levers?
  
• As what part of the lever does the pencil/pen function in this activity? What about the other materials? How do they function as other parts of the lever?
  
• If you double the distance between the effort force and the fulcrum, how do you expect you will need to change the amount of force you apply? Do you think it will be half of what you applied, or a different value?
  
• Come up with examples of levers you use or see on a regular basis. What are some examples of the three different classes of levers?
  

Materials
Needed for preparing ahead:

• Plastic zip-lock sandwich bag (1 per demo or small group)
  
• Scissors (1)
  
• Tape (masking tape, duct tape, packing tape, or other strong tape)
  
• Ruler (1)
  

Needed for each demo or small group at the time of the science activity:

• Ruler (preferably one that is stiff) (1)
  
• Pen or pencil (1)
  
• Tape
  
• Bar of soap (still in its packaging) (1)
  
• Pennies (about 300) or other small, numerous, identical items that are all the same weight, such as marbles or beans (about 700 grams total)
  
• Plastic sandwich bag with a taped hole cut in it (1)
  

What to Do
Prepare Ahead (< 10 minutes)

1. For each demo or small group, reinforce a plastic zip-lock sandwich bag and cut a hole in it. Start by cutting two strips of tape that are each about 2.5 centimeters (cm) longer than the width of the ruler. Put one strip of tape on the outside of one of the sides of the plastic bag, centered about 1 cm below the zipper on the bag and running parallel to the opening. Flatten and press the tape down to make sure it is securely stuck.
   
2. Put the second strip of tape on the inside of that same side of the bag, so that it mirrors the location of the first strip. Again, press down on the tape to make sure it is secure.
   

1. Fold the taped section in half, width-wise. Use the scissors to cut a horizontal slit in the tape, about 1.5 cm from the top of the tape, that is just wide enough for the ruler to slip in.
   
2. Repeat until you have prepared one taped plastic bag with a hole for each demo or small group.
   

Science Activity (10-20 minutes)

1. Each group should have one ruler, one pen or pencil, one bar of soap, numerous pennies or other identical small items, tape, and a plastic bag that was prepared with a reinforced hole cut in it.
   
2. Instruct the students to tape the bar of soap securely to one end of the ruler, with the long side of the soap bar lined up with the end of the ruler.
   

1. Have the students insert the other end of the ruler into the hole in the plastic bag, horizontally, and securely tape the bag to that end of the ruler. Students may need to put tape on the inside of the bag, on top of and under the ruler. Make sure the students don't tape the bag closed.
   

1. To create a fulcrum, tell students to tape a pencil or pen parallel to the edge of a table or desk. They should then place the ruler perpendicularly on the pencil, with the soap bar resting on the table and the plastic bag dangling over the edge of the table.
   
2. Now students can test their levers. Instruct them to move the ruler so that the bag is 6 cm from the pencil. Have students put the pennies or numerous other small objects in the plastic bag, counting all the while, until the weight in the bag just lifts the soap off the table and balances with the soap. How many pennies or other objects did it take?
   

1. Ask the students to empty the plastic bags and move the ruler so that the bag is 12 cm from the pencil, twice the former distance. Have students again put pennies or other small objects into the bag until it just lifts the soap off the table and balances with the soap. How many pennies or other objects did it take this time?
   
2. Finally, instruct the students to move the ruler so that the bag is 18 cm from the pencil and again count how many pennies or objects it takes to lift and balance the soap. How many did it take this time?
   

Expected Results
You should see that as the distance between the bag and the pencil increases, the number of pennies or other objects needed to lift the soap decreases. However, when the distance is doubled, such as when going from 6 cm to 12 cm, the number of pennies needed to lift the soap does not simply become half. The equation below shows how to balance objects using a lever, where the effort force is the pennies, the load force is the soap, and the fulcrum is the pencil.

Equation:
Consequently, the amount of pennies needed to lift the soap is equal to the force the soap is exerting multiplied by the distance between the soap and the fulcrum, all divided by the distance between the pennies and the fulcrum.
For Further Exploration
This science activity can be expanded or modified in a number of ways. Here are a few options:

• What is the graphical relationship between how many pennies are needed to lift the soap when the soap is at different distances from the pencil? Have students repeat this activity, testing different distances and then making a line graph of their results, putting the distance on the y-axis and the number of pennies on the x-axis.
  
• What happens if the load is doubled by adding two soap bars? Ask students to investigate whether twice the number of pennies is needed to lift the two soap bars. What if three or four soap bars are used?
  
• In this activity, students built a class 1 lever. Students could try to build a class 2 lever as well, and then compare the class 1 and class 2 levers. Do they require the same effort to lift the load? Be sure to keep the distance between the fulcrum and the effort end the same when comparing the levers.
  

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