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Megapinchers: How Do Levers Reduce The Force You Apply To Them?

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Author: Janice VanCleave

Problem

How do levers reduce the force you apply to them?

Materials

  • 2 yardsticks (metersticks)
  • masking tape
  • empty box (measuring about 4 inches [10 cm] on each side)

Procedure

  1. Construct the "megapinchers" (giant tweezers) as follows:
    • Hold two yardsticks (metersticks) together.
    • Place a 6-inch (l5-cm) piece of tape over both sticks at one end.
  2. Hold the sticks, one in each hand, about 18 inches (45 cm) from the free ends.
  3. Press the ends of the sticks against the box, one on each side of the box.
  4. Try to lift the box and carry it to a point 12 inches (30 cm) to the left or right.
  5. Grasp the box with your fingers and move it back to its original position.

Results

You must apply more pressure to the yardsticks (metersticks) to move the box than if you were to grasp the box directly with your fingers.

Megapinchers

Why?

Machines are meant to be helpful when doing work. The advantage a machine gives you as you work with it is called mechanical advantage, which is expressed as a number. The number indicates how many times a machine multiplies the effort used. If the mechanical advantage equals one, the machine does not change the magnitude of the effort force. A machine with a mechanical advantage of two means that your effort force is doubled. For example, using a machine with a mechanical advantage of two allows you to lift an object weighing 20 lbs (90 n [Newtons]) with an effort force of only 10 lbs (45 n). To determine the mechanical advantage of a lever, divide the effort arm (the distance from the effort force to the fulcrum) by the load arm (the distance from the load to the fulcrum). MA. is the symbol used for mechanical advantage, and the equation for determining the mechanical advantage is:

A third-class lever (a lever with the effort force between the load and the fulcrum) always has a mechanical advantage less than one. This means that third-class levers, like the megapinchers in this experiment, require more effort to lift the load than if it were lifted directly with your hands. But third-class levers do have advantages. They can be used to handle delicate objects because they reduce the force you apply directly. The effort is applied somewhere between the load and the fulcrum, so the effort arm is always shorter than the load arm.

Let's Explore

  1. Does the position of the effort force affect the results? Repeat the experiment twice, first holding the sticks as close to the taped ends as possible, and then holding the sticks closer to the opposite end.
  2. Would the shape of the sticks affect the experiment? Repeat the experiment, replacing the measuring sticks with dowel rods of equal lengths.

Show Time!

Construct a first-class, a second-class, and a third-class lever using the materials and instructions listed below. Use photographs of each type of lever, as well as diagrams, as part of a project display. Indicate on each diagram the amount of the load force (the weight of the bucket and rocks) and the effort force (the reading on the scale).

    Materials:

      yardstick (meterstick)

      chair

      helper

      bucket with 2 lbs (1 kg) of rocks

      string

      hand-held scale

  1. For a first-class lever, place about 12 inches (30 cm) of the yardstick (meterstick) over the back of a chair. Ask a helper to support the stick while you attach the bucket of rocks to the short end of the stick with a string. Hook the scale to the opposite end of the stick, and pull down with enough force to balance the stick. Record the measurement on the scale.
  2. Megapinchers

  3. To construct a second-class lever, place about 4 inches (10 cm) of the stick over the back of the chair. With your helper supporting the stick, hang the bucket in the center and hook the scale on the end of the stick. Pull up on the scale with enough force to support the stick. Read and record the measurement on the scale.
  4. Megapinchers

  5. A third-class lever can be formed by placing about 4 inches (10 cm) of the stick over the back of the chair. Ask a helper to support the stick while you attach the bucket to the end, and hook the scale in the center of the stick. Pull up with enough force to support the stick. Read and record the measurement on the scale.
  6. Megapinchers

 

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