Energy, Work, and Transfer of Energy Physical Science Chapter 12

Potential Energy  The stored energy an object has because of the position or condition.  The energy that has not been released yet  EX: stretching a rubber band (elastic PE)  EX: an apple on a tree (gravitational PE)

Calculating PE  PE = (mass)(gravity)(height)  Gravity = 9.8m/s 2  Calculate the PE of a 20 kg object that is sitting on a shelf 5 meters above the ground.  PE = (20)(9.8)(5)  PE = 980 Joules

Kinetic Energy  The energy of a moving object  KE depends on mass and speed  Ex: A small increase in speed produces a large increase in KE  EX: once the apple falls from the tree it is doing work or releasing energy

Calculating KE  KE =.5(mass)(velocity 2 )  What is the KE of a 44kg cheetah running at 31 m/s?  KE =.5(44)(31 2 )  KE = 21,000 Joules

Transfer of Energy

Conservation of Energy  Energy cannot be created or destroyed  Energy will change forms but never disappears  Energy can be exchanged out of the system as friction, heat (thermal energy), or sound.

Work  The transfer of energy caused by a force that makes the object move.  In order for work to be done, there must be motion of the object.  EX: If you try to lift the front of a car…you will apply a large force, but if the car does not move you have not done work on the car.

Work Equation  Work = Force * Distance  Measured in Joules  Example: A father lifts his daughter repeatedly into the air. How much work does he do with each lift, assuming he lifts her 2.0 meters and exerts an average force of 190 N?  F = 190 Nd = 2.0m  W = F*d  W = 190 N * 2.0 m = 380 Joules

Energy and Work  Whenever work is done, energy is transferred to another system  Energy is the ability to do work  Unit for Energy - Joule

Practical Examples of Energy Transfers