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What will happen to this bungee jumper when he reaches the end of the bungee cord? He won’t come to an abrupt stop. If he did, he might be seriously hurt. Instead, the cord will stretch while slowing his fall.

Q : What will happen next?

A : The cord will only stretch so far, and then it will snap back to its original shape, causing the jumper to bounce back up. Bungee cords are useful for this purpose because they are elastic.

Elasticity and Elastic Force

Something that is elastic can return to its original shape after being stretched or compressed. This property is called elasticity . As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force . The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape. You can watch a demonstration of elastic force at this URL:

http://www.youtube.com/watch?v=fFtM9JznLh8

You can do some virtual bungee jumping at this URL:

http://www.youtube.com/watch?v=eaICwFMHcaY

Q : What force stretches the bungee cord after the jumper jumps? When does the bungee cord snap back to its original shape?

A : After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape, and the jumper bounces upward.

Using Elastic Force

Elastic force can be very useful and not just for bungee jumping. In fact, you probably use elastic force every day. A few common uses of elastic force are shown in the Figure below . Do you use elastic force in any of these ways?

Q : How does the resistance band work? How does it use elastic force?

A : When you pull on the band, it stretches but doesn’t break. The resistance you feel when you pull on it is elastic force. The farther you stretch the band, the greater the resistance is. The resistance of the band to stretching is what gives your muscles a workout. After you stop pulling on the band, it returns to its original shape, ready for the next stretch.

Springs like the spring toy pictured in the Figure below also have elastic force when they are stretched or compressed. And like stretchy materials, they return to their original shape when the stretching or compressing force is released. Springs are used in scales to measure weight. They also cushion the ride in a car.

Q : Can you think of other uses of springs?

A : Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground.

Summary

  • Elasticity is the ability of a material to return to its original shape after being stretched or compressed. When an elastic material is stretched or compressed, it exerts elastic force. This force increases the more the material is stretched or compressed.
  • Elastic force has many uses, from hair scrunchies to bed springs.

Vocabulary

  • elastic force : Counter force exerted by an elastic material when it is stretched or compressed.
  • elasticity : Ability of a material to return to its original shape after being stretched or compressed.

Practice

At the following URL, read the short article on elasticity and watch the earthquake animation. Then answer the questions below.

http://scign.jpl.nasa.gov/learn/eq5.htm

  1. Which of the following have elastic properties?
    1. pencils
    2. rocks
    3. rubber bands
    4. all of the above
  2. Materials with elastic properties have a point at which any additional force will permanently change the object's shape. That point is called the
    1. elastic force.
    2. rigid structure.
    3. elastic limit.
    4. breaking point.
  3. Elastic rebound occurs when materials
    1. return to their original shape.
    2. stretch beyond their normal limit.
    3. permanently change their shape.
    4. become brittle and break.
  4. Which type of material is most likely to withstand an earthquake without breaking?
    1. rigid material
    2. compressed material.
    3. elastic material.
    4. deformed material.
  5. In an earthquake, seismic waves are generated by
    1. rock displacement.
    2. elastic rebound.
    3. elastic breakage.
    4. plastic deformation.

Review

  1. What is elasticity?
  2. How does an elastic material exert elastic force?
  3. Explain how elastic force is used to shoot an arrow from a bow.

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Difficulty Level:

At Grade

Grades:

7 , 8

Date Created:

Nov 01, 2012

Last Modified:

Sep 05, 2014
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