Chapter 11Chapter 11

ENERGYENERGY

11.1 A Model of the Work-Energy 11.1 A Model of the Work-Energy Theorem Theorem

Throwing a ball: Throwing a ball: – Force and motion are in the same direction, Force and motion are in the same direction,

therefore work is positivetherefore work is positive– Work is positive and the energy of the ball Work is positive and the energy of the ball

increases by an amount equal to W increases by an amount equal to W

Catching a ball:Catching a ball:– Force is opposite the direction of motion, Force is opposite the direction of motion,

therefore work is negativetherefore work is negative– Work is negative and the energy of the ball Work is negative and the energy of the ball

decreases by an amount equal to W decreases by an amount equal to W

Kinetic Energy: Kinetic Energy: – K = ½ m vK = ½ m v22

– Measured in JoulesMeasured in Joules

Stored EnergyStored Energy– mass is a form of stored energy!mass is a form of stored energy!

E = mcE = mc22

Gravitational Potential EnergyGravitational Potential Energy

–Choosing a Reference LevelChoosing a Reference Level

At the reference level, the At the reference level, the potential energy is defined to be potential energy is defined to be zerozero

The reference level is arbitraryThe reference level is arbitrary

Only changes in energy can be Only changes in energy can be measuredmeasured

Elastic Potential EnergyElastic Potential Energy

–Energy stored by distorting an Energy stored by distorting an objectobject

Ex: string in a bow and arrow, Ex: string in a bow and arrow, rubber ball, rubber band, rubber ball, rubber band, slingshot, trampoline, bending a slingshot, trampoline, bending a pole vaulting polepole vaulting pole

11.2 Conservation of Energy 11.2 Conservation of Energy

Choosing a systemChoosing a system– Conservation of mechanical energyConservation of mechanical energy

TME = K + UTME = K + U

KKbeforebefore + U + Ubeforebefore = K = Kafterafter + U + Uafterafter

– Loss of mechanical energyLoss of mechanical energyEnergy can leave the system in a number Energy can leave the system in a number of ways.of ways.

Analyzing CollisionsAnalyzing Collisions

–Elastic: kinetic energy is conservedElastic: kinetic energy is conserved

–Inelastic: kinetic energy is not Inelastic: kinetic energy is not conservedconserved

Carefully identify the system. Make Carefully identify the system. Make sure it is closed.sure it is closed.Identify the initial and final states of Identify the initial and final states of the system.the system.Is the system isolated?Is the system isolated? – If there are no external forces acting on If there are no external forces acting on

the system, then the total energy of the the system, then the total energy of the system is constant. Esystem is constant. Ebeforebefore = E = Eafterafter

– If there are external forces, then If there are external forces, then E Ebeforebefore + W = E + W = Eafterafter

PSSPSS

Identify the forms of energy in the system.Identify the forms of energy in the system.– If mechanical energy is conserved, then there If mechanical energy is conserved, then there

is no thermal energy change. If it is not is no thermal energy change. If it is not conserved, then the final thermal energy is conserved, then the final thermal energy is larger than the initial thermal energy.larger than the initial thermal energy.

– Decide on the reference level for the potential Decide on the reference level for the potential energy and draw bar graphs to showing initial energy and draw bar graphs to showing initial and final energy.and final energy.

Check your answer.Check your answer.