energy, work, and transfer of energy physical science chapter 12
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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