2Elastic PotentialElastic PE is the energy stored in elastic materials as the result of their stretching or compressing.
3Elastic Potential Energy Rubber band—stretching it stores energyPaddle 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 ballNotice the ball compressingand expanding
9An Elastic SpringA spring is an example of an elastic body that can be deformed by stretching.xFA restoring force, F, acts in the direction opposite the displacement of the oscillating body.F = -kx
10Hooke’s LawWhen a spring is stretched, there is a restoring force that is proportional to the displacement.FxmF = -kxThe spring constant k is a property of the spring given by:The spring constant k is a measure of the elasticity of the spring.
11Example: Bow Energy Transfer ProcedureHold up bow and put arrow on stringPlace fingers on string and pull string backAnchor string and hand under the chinTake aimRelease the stringArrow 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.
12Bow Energy Transfer (solution) ProcedureHold up bow and put arrow on stringPlace fingers on string and pull string backAnchor string and hand under the chinTake aimRelease the stringArrow hits target (hopefully!!!)Main Energy transferChemical in arm to kinetic in arm, string & limbsKinetic in arm & string to elastic potential in limbsElastic potential in limbs to kinetic in string, limbs and arrowKinetic in arrow and sound in limbsKinetic in arrow to heat and sound in targetString is relatively inelastic – it’s the limbs that store the energy
13Bow Energy Transfer (solution) Sankey Diagram Showing LossesKinetic in string; sound in limbs and string; heat in limbsChemical in armKinetic in arm, string, & limbsElastic potential in limbsKinetic in arrowHeat and sound in targetHeat and sound of arrow in flightSound in limbs; heat in arms; heat in limbs & string
14If 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.
15Summary: 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.