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Third-Class Levers in the Human Body

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Author: Beth Touchette

A lever is a type of simple machine where a rigid arm is arranged around a fixed point or fulcrum. Input, the force you put in, directed into an output force. The classic example of a lever is a seesaw. The fulcrum is in the middle, and when you push down on your side of the seesaw (input), it makes the person on the other side of the seesaw go up (output).

There are three main classes of levers. If the fulcrum is in the between the output force and input force as in the seesaw, it is a first-class lever. In a second-class lever, the output force is in between the fulcrum and the input force.  An example of a second class lever is a wheelbarrow. The fulcrum is the wheel, the load of stuff in the wheel barrow requires the output force to be lifted, and the person at the handle supplies the input force. In a third-class lever, the input force is in between the output force and the fulcrum. An example of this class of lever is a baseball bat. The handle of the bat is the fulcrum, you supply the input force near the middle, and the other end of the bat that pushes the ball with the output forces. In a third-class lever, the input force is greater than the output force but the output load is able to move farther.

Examples of Levers

You have several third-class levers in your body. One that is easy to investigate is your forearm.

Human Arm Lever Diagram

Problem: How is your arm a third-class lever?

Materials

  • Table
  • Bucket with strong handle
  • Sand or other material to put in the bucket.
  • Helper
  • 2-3 ft piece of one-inch wide PVC pipe, or a strong yardstick
  • Cardboard
  • Scissors
  • 3 paperclips
  • String
  • Small weights

Procedure

  1. Fill the bucket halfway with sand.
  2. Place your arm flat on the table. Your hand and about four inches below your wrist should extend over the table’s edge. The inside of your elbow should be facing upward. 
  3. Have your friend hang the bucket across the palm of your hand.
  4. Keeping your elbow on the table, lift the bucket up.
  5. Repeat, this time with the bucket full of sand. Does this require more or less effort?

Human Arm Lifting Sand

  1. Remove sand until the bucket is again halfway filled with sand.
  2. Put the handle of the bucket around the PVC pipe or yardstick. 
  3. Set your arm on the table as before, but hold the PVC pipe or yardstick with the bucket hanging off of it. The bucket should be 2-3 feet farther from the edge of the table.
  4. Lift the bucket again. How does the effort to lift the bucket on the long stick compare to the effort of moving it when it was in your hand? Does it feel like the bucket contains more or less sand than the first trial?  
  5. Slide the bucket to different places on the long stick and note the different amounts of effort.
  6. Now, you'll make a model of your forearm as third-class lever. Measure the length of your upper arm and forearm and outline your model on the cardboard. Keep the upper arm and forearm pieces separate, and at the end of the forearm, trace your hand.
  7. Cut out your model arm from the cardboard.
  8. Use a brad to join the cardboard upper arm and forearm. This is your elbow joint and the fulcrum of your lever.
  9. Tape three paper clips on to the arm model, and thread the string through.  The two paperclips on your upper arm represent the bicep muscle on your arm, the paper clip on the forearm represents where the muscle attaches. The distance between the three paperclips represents the length of the input effort arm of your lever.
  10. Tie one end of your string to the thumb of your cardboard hand.  Tie or tape the other end to the top of the forearm (where the shoulder would be).  The total string length represents the output arm of your lever.  
  11. Now, lightly pull the string between the two paperclips on the upper arm. This represents the contraction of the bicep. What happens? How does the distance the hand moves compare with the distance you moved the string?

Completed Human Arm Model

  1. If your model seems sturdy, try adding a bit of weight tied with a string around the paper hand and tug the muscle again.  
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