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Ringer: What is a Third-Class Lever, and What is the Advantage of Using One?

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Author: Janice VanCleave

Problem

What is a third-class lever, and what is the advantage of using one?

Materials

  • Scissors
  • Yardstick (meterstick)
  • String
  • Metal screw
  • Ring for quart (liter) canning jars (any metal ring with a 2½-inch [6.4-cm] diameter will work)
  • Glass soda bottle

Procedure

Ringer

  1. Cut a piece of string 1 yard (1 m) long.
  2. Tie one end of the string to the end of the yardstick (meterstick).
  3. Tie the free end of the string to the metal ring.
  4. Stand the soda bottle on the floor.
  5. Wrap one hand around the bottom of the stick, and place the hand you write with immediately above the first hand in the same way you would hold a baseball bat.
  6. Stand so that the metal ring dangles directly above the top of the soda bottle.
  7. Try to hook the ring over the mouth of the soda bottle by moving the measuring stick with the hand on top only.

Results

It is difficult to move the opposite end of the yardstick (meterstick) small distances; thus, it is hard to hook the ring.

Why?

The yardstick (meterstick) is an example of a simple machine called a third-class lever. A lever is usually thought of as a rigid bar that pivots around a fixed point called a fulcrum (see Experiment 1). A third-class lever has the effort force (the force that you apply) between the fulcrum and the load (the object being moved-in this case, the metal ring). This type of lever does not change the direction of the force (the direction you move the stick with your hand is the same direction the metal ring moves). A third-class lever requires more effort because the effort arm (the distance from the fulcrum to the effort force) is always shorter than the load arm (the distance from the load to the fulcrum). Placing the ring on the bottle is difficult because your hand moves the near end of the stick a short distance, but the far end of the stick moves a greater distance.

Ringer

Let's Explore

Ringer

  1. Would moving your hands closer to the end of the stick to which the string is attached affect the results? Repeat the experiment, moving both of your hands closer to the string end.
  2. Would positioning your hands so that they are apart affect the results? Repeat the original experiment, this time moving only your writing hand closer to the far end of the stick.

Show Time!

  1. Third-class levers have their effort force between the fulcrum and the load. Observe and discover other examples of third-class levers, such as a broom, a fishing pole, a rake, and a baseball bat. Display diagrams and pictures of third-class levers as part of a project display.
  2. Your jaw acts as a third-class lever. The fulcrum is where the jaw attaches. Muscles in front of the jaw connection apply the effort force and your teeth apply a load force. The closer the teeth are to the jaw connection (the fulcrum), the stronger the force that they apply. Make a model of a jaw by cutting an empty cake-mix box in half, and taping one of the narrow sides together as shown in the diagram. Shapes of teeth can be cut into the box. Remember when demonstrating this model that only the lower jaw moves.
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