Energy, Work, and Energy, Work, and Transfer of EnergyTransfer of Energy

Physical Science Chapter 12Physical Science Chapter 12

Potential EnergyPotential Energy The stored energy an object has because The stored energy an object has because

of the position or condition.of the position or condition. The energy that has not been released The energy that has not been released

yetyet EX: stretching a rubber band (elastic PE)EX: stretching a rubber band (elastic PE) EX: an apple on a tree (gravitational PE)EX: an apple on a tree (gravitational PE)

Calculating PECalculating PE

PE = (mass)(gravity)(height)PE = (mass)(gravity)(height) Gravity = 9.8m/sGravity = 9.8m/s22

Calculate the PE of a 20 kg object that is Calculate the PE of a 20 kg object that is sitting on a shelf 5 meters above the sitting on a shelf 5 meters above the ground.ground.

PE = (20)(9.8)(5)PE = (20)(9.8)(5) PE = 980 PE = 980 JoulesJoules

Kinetic EnergyKinetic Energy

The energy of a moving objectThe energy of a moving object KE depends on mass and speedKE depends on mass and speed

Ex: A small increase in speed produces a Ex: A small increase in speed produces a large increase in KElarge increase in KE

EX: once the apple falls from the tree it is EX: once the apple falls from the tree it is doing work or releasing energydoing work or releasing energy

Calculating KECalculating KE

KE = .5(mass)(velocityKE = .5(mass)(velocity22))

What is the KE of a 44kg cheetah What is the KE of a 44kg cheetah running at 31 m/s?running at 31 m/s?

KE = .5(44)(31KE = .5(44)(3122)) KE = 21,000 KE = 21,000 JoulesJoules

Transfer of EnergyTransfer of Energy

Transfer of EnergyTransfer of Energy

Conservation of EnergyConservation of Energy

Energy cannot be created or destroyedEnergy cannot be created or destroyed Energy will change forms but never Energy will change forms but never

disappearsdisappears Energy can be exchanged out of the Energy can be exchanged out of the

system as friction, heat (thermal energy), system as friction, heat (thermal energy), or sound.or sound.

WorkWork The transfer of energy caused by a force The transfer of energy caused by a force

that makes the object move.that makes the object move.

In order for work to be done, there must In order for work to be done, there must be motion of the object.be motion of the object. EX: If you try to lift the front of a car…you EX: If you try to lift the front of a car…you

will apply a large force, but if the car does will apply a large force, but if the car does not move you have not done work on the not move you have not done work on the car.car.

Work EquationWork Equation Work = Force * DistanceWork = Force * Distance Measured in JoulesMeasured in Joules

Example: A father lifts his daughter Example: A father lifts his daughter repeatedly into the air. How much work does repeatedly into the air. How much work does he do with each lift, assuming he lifts her 2.0 he do with each lift, assuming he lifts her 2.0 meters and exerts an average force of 190 meters and exerts an average force of 190 N?N?

F = 190 NF = 190 N d = 2.0md = 2.0m W = F*dW = F*d W = 190 N * 2.0 m = 380 JoulesW = 190 N * 2.0 m = 380 Joules

Energy and WorkEnergy and Work

Whenever work is done, energy is Whenever work is done, energy is transferred to another systemtransferred to another system

Energy is the ability to do workEnergy is the ability to do work Unit for Energy - JouleUnit for Energy - Joule

Practical Examples of Practical Examples of Energy TransfersEnergy Transfers