1. Elastic Energy
Springy!

2. Elastic Potential
Elastic PE is the energy stored in elastic materials as the result of their stretching or compressing.

3. Elastic Potential Energy
Rubber band—stretching it stores energy
Paddle ball—elastic band stores energy, then transfers from paddle to ball.
Bouncing balls—rebound off of a hard surface—height of bounce is determined by the elasticity of the ball
Notice the ball compressing
and expanding

4. Elasticity example—bungee!
BUNGEE jumping utilizes a long elastic strap which stretches until it reaches a maximum length that is proportional to the weight of the jumper. The elasticity of the strap determines the amplitude of the resulting vibrations. If the elastic limit for the strap is exceeded, the rope will break.
Photo © Vol. 10 PhotoDisk/Getty

5. Elastic Properties of Matter
An elastic body is one that returns to its original shape after a deformation.
Golf Ball
Soccer Ball
Rubber Band

6. Elastic Properties of Matter
An inelastic body is one that does not return to its original shape after a deformation.
Dough or Bread
Inelastic Ball
Clay

7. Elastic or Inelastic?
An elastic collision loses no energy. The deform-ation on collision is fully restored.
In an inelastic collision, energy is lost and the deformation may be permanent. (Click it.)

8. Example: A spring!

9. An Elastic Spring
A spring is an example of an elastic body that can be deformed by stretching. x F
A restoring force, F, acts in the direction opposite the displacement of the oscillating body.
F = -kx

10. Hooke’s Law
When a spring is stretched, there is a restoring force that is proportional to the displacement. F x m
F = -kx
The spring constant k is a property of the spring given by:
The spring constant k is a measure of the elasticity of the spring.

11. Example: Bow Energy Transfer
Procedure
Hold up bow and put arrow on string
Place fingers on string and pull string back
Anchor string and hand under the chin
Take aim
Release the string
Arrow hits target (hopefully!!!)
Ask if anyone has done archery before.
I will bring in my bow and demonstrate how to pull the string back as a visual prop!!
Can you identify the stages in these energy transfers?
Draw a Sankey diagram to show this.

12. Bow Energy Transfer (solution)
Procedure
Hold up bow and put arrow on string
Place fingers on string and pull string back
Anchor string and hand under the chin
Take aim
Release the string
Arrow hits target (hopefully!!!)
Main Energy transfer
Chemical in arm to kinetic in arm, string & limbs
Kinetic in arm & string to elastic potential in limbs
Elastic potential in limbs to kinetic in string, limbs and arrow
Kinetic in arrow and sound in limbs
Kinetic in arrow to heat and sound in target
String is relatively inelastic – it’s the limbs that store the energy

13. Bow Energy Transfer (solution)
Sankey Diagram Showing Losses
Kinetic in string; sound in limbs and string; heat in limbs
Chemical in arm
Kinetic in arm, string, & limbs
Elastic potential in limbs
Kinetic in arrow
Heat and sound in target
Heat and sound of arrow in flight
Sound in limbs; heat in arms; heat in limbs & string

14. If Robin Hood had been Written by an Engineer
If Robin Hood had been Written by an Engineer....     And Robin didst slowly and with great determination put potential energy equal to the work of his muscles into an elastic storage device.                                                                             
And therewith Robin the Bold and Valiant didst convert this stored energy most quickly, efficiently, and accurately into the velocity of a sturdy and pointed dart (oft called arrow) such that almost all of its former potential energy didst become kinetic. Then this speedy dart didst split an arrow (oft called dart) already buried in most distant target, having been previously hurled there at an equally great speed by a similar conversion of stored energy.

15. Summary: Elastic and Inelastic
An elastic collision loses no energy. The deform-ation on collision is fully restored.
An elastic body is one that returns to its original shape after a deformation.
An inelastic body is one that does not return to its original shape after a deformation.
In an inelastic collision, energy is lost and the deformation may be permanent.