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Heavy Lifting with a Lever

A fun physics challenge from Science Buddies
bsh kids on seesaw



iStockphoto/warrengoldswain

Key concepts
Physics
Lever
Force
Tools
Mass

Introduction
Have you ever wondered how ancient people could lift very heavy objects, such as large stones, to build pyramids? A lever is a simple machine that can help people do just this. It can also help make other kinds of physical work easier by giving the user a mechanical advantage.

Common examples of levers you might see around you are seesaws, scissors, wheelbarrows and even the your own jaw. Although all of these levers have the same functional parts, they vary in where the different components are located. How much effort does it take to lift a heavy load using a common type of lever?

Background
Seesaws and scissors belong to a certain class of levers, called class 1. Class 1 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, which is the point at which the beam can balance and move freely up and down around. On one end, the user places the load to be moved. On the other end, the user can apply effort, or a force, to try and move the load.

The way levers work is by multiplying the effort exerted by the user. Specifically, to lift and balance an object, the effort force the user applies multiplied by its distance to the fulcrum must equal the load force multiplied by its distance to the fulcrum. Consequently, the greater the distance between the effort force and the fulcrum, the heavier a load can be lifted with the same effort force.

Materials
•    Plastic ziplock sandwich bag
•    Scissors
•    Strong tape
•    Ruler (preferably one that is stiff)
•    Pencil or pen
•    Bar of soap (still in its packaging)
•    Pennies (about $3 worth) or other small, numerous items that are all the same size, such as marbles or beans (about 1.5 pounds—or700 grams—total)
•    A table

Preparation
•    Securely tape the packaged bar of soap to one end of the ruler, with the end of the bar of soap lined up with the end of the ruler.
•    Cut two strips of tape that are each about 2.5 centimeters longer than the width of the ruler.
•    Put one tape strip on the center of one side of the plastic bag, about one centimeter below the bag's zipper and running parallel to the opening. Press down to firmly attach the tape.
•    Put the second tape strip on the inside of the bag, on the same side and location as the first piece. The tape strips should completely overlap. Again, press down to firmly attach the tape.
•    Fold the taped section in half, width-wise. Use the scissors to cut a horizontal slit in the tape, about 1.25 centimeter from the tape's top, just wide enough for the ruler.
•    Insert the empty end of the ruler into the hole in the plastic bag, horizontally, and securely tape the bag to the end of the ruler. Make sure the bag doesn't get taped closed.

Procedure
•    Tape a pencil or pen to the edge of a table, running parallel to the table's edge. Place the ruler perpendicularly on top of the pencil, with the end with the soap bar resting on the table and the end with the plastic bag dangling over the edge of the table.
•    You now have a functional lever. What part of the lever do you think the soap is? What about the ruler and the pencil?
•    Move the ruler so that the bag is six centimeters from the pencil.
•    Slowly fill the bag with pennies or other small numerous objects until the soap just barely lifts off the table and looks like it is balanced with the bag of pennies. How many pennies did it take to lift the soap?
•    Empty the bag and move the ruler so that the bag is now 12 centimeters from the pencil. Again, slowly fill the bag with pennies or other small objects until the soap lifts off the table and balances. How many pennies did it take to lift the soap this time? How does this seem to correlate with the change in distance from the pencil?
•    Empty the bag again and move the ruler so that the bag is now 18 centimeters from the pencil. Again, slowly fill the bag until the soap lifts off the table and balances. How many pennies did it take this last time to lift the soap? Is it what you expected based on the previous two times you tested the lever?
•    Extra: What is the graphical relationship between the amount of pennies needed to lift the soap when it is at different distances from the pencil? You can try this activity again but test different distances and then make a line graph of your results, putting the distance on the y (vertical) axis and the number of pennies on the x (horizontal) axis. What does the graphical relationship look like?
•    Extra: What happens if the load is doubled? You can test this by trying this activity again, but this time use two soap bars. Is twice the number of pennies needed to lift two soap bars? What if three or four soap bars are used?

Observations and results
Were less than half the pennies needed to lift the soap when the bag was 12 centimeters from the pencil compared with when the bag was six centimeters from the pencil? Was less than half this amount needed to lift the soap when the bag was 18 centimeters from the pencil?

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 six to 12 centimeters, the number of pennies needed to lift the soap does not simply become half. To balance objects using a lever, the effort force (the pennies) multiplied by its distance from the fulcrum (the pencil) must equal the load force (the soap) multiplied by its distance from the fulcrum. Consequently, the force exerted by the pennies needed to lift the soap is equal to the force the soap exerts multiplied by the distance between the soap and the fulcrum, all divided by the distance between the pennies and the fulcrum.

More to explore
"No Tipping," from Scientific American
"Levers," from V. Ryan at Technologystudent.com
"Escape from Merlin's Maze," from CYBERCHASE Thirteen/WNET, Educational Broadcasting Corporation, New York
"Levers," from Petervaldivia
Give It a Lift with a Lever," from Science Buddies


This activity brought to you in partnership with Science Buddies and Cyberchase
ScienceBuddies

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