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Third-Class Levers in the Human Body

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Updated on Oct 23, 2014

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 makesthe person on the other side of the seesaw go up (output).

There are three main classes of levers.Ifthe fulcrum is in the between the output force and input force as in the seesaw, itis a first-class lever.In a second-class lever, the output force is inbetween 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 barrowrequires theoutput forceto be lifted, and the person at the handle supplies the input force. In a third-class lever, the input force is inbetween the output force and thefulcrum.An example of this class of leveris 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 forceis greater than the output force but the output load is able to move farther.

Examples of Levers

Youhave 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 ftpiece 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 liftthe 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 bicepmuscle on your arm, the paper clip on the forearm represents where the muscle attaches. The distancebetween thethree 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.

Results

When the bucket contains more sand, it takes more effort to lift. What might be more surprising is when the bucket is hanging off the PVC pipe or yardstick; it takes more force to lift it. It feels like there is more sand in the bucket. When you tug at the string between the two paper clips on the model’s forearm, the hand lifts up, covering more distance than the distance you pulled the string.

Why?

When you added more sand to the bucket, you increased its weight, so you needed more input force to lift it. When you hungthe bucket off the piece of PVC pipe or yardstick, you increased the length of the output arm, which also increased the amount of input force you needed to add.In all third-class levers, the length of the output arm is longer than that of input arm. When you added length to your arm by hanging the bucket on the stick, you exaggerated this characteristic. When you tugged at bicep area of your model, you made the hand move farther than the amount if string you pulled.In all third-class levers,the distance moved by the output load is greater than the distance moved in the input force.

Want to investigate your “arm machine” more? Put two more paper clips on the other side of your model upper arm, and another paper clip on the other side of the one on your forearms.Thread another piece of string through the paperclips.You have created a model of your tricepmuscle, which straightens your arm. Tug the bicepmuscle so it bends the arm, then tug the string between the two paperclips on the upper arm.The arm should straighten.

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