# Elastic Energy Springy!.

## Presentation on theme: "Elastic Energy Springy!."— Presentation transcript:

Elastic Energy Springy!

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

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

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

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

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

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.)

Example: A spring!

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

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.

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.

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

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

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.

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.