Work Kinetic and Potential Energy Supplemental Text Material Pages 326 333 Work Lifting load against the force of the weight of the object Twice the distance results in twice the work Twice the weight is twice the work Work W Fd Transference of Energy Work Force x distance W Fd Work W Fd Twice the weight Twice the distance Work W Fd Weight lifter expends energy to keep the potential energy in the barbell But he does no work on the barbell after it is lifted 1 Work W Fd Units of force Newtons kg m s2 Force x distance Newton meters Work W Fd Units of work are Joules Work is energy kg m kg m 2 m s2 s2 Joules Work W Fd Nothing about time in definition Slow or fast Same force same distance same work Power Work W Fd Power work time Units Joules second kg m 2 s 2 s kg m 2 s s2 kg m 2 1 kg m 2 watt s2 s s3 Power P energy time kg m 2 watt s3 Half the time Twice the power Twice the time Half the power Power P energy time Fuel burn Biodiesel http www alternativefuels com au Biodiesel dragster htm http www lilligren com Redneck redneck lawnmower htm 2 Work vs Energy Same units Work occurs with transfer of energy Work occurs when you store potential energy Work vs Energy Lift heavy ram of pile driver Work transfers energy to lift into potential gravitational energy Work vs Energy Energy stored in bow Work is done to create the potential energy Work vs Energy Mechanical energy Moving things has two forms 1 Potential mechanical energy Waiting to work 2 Kinetic mechanical energy Work being done Potential Energy Potential Energy Fuel is chemical potential energy http www alternativefuels com au Biodiesel dragster htm http www howstuffworks com inside clock htm http www lilligren com Redneck redneck lawnmower htm http www himalayan pdx edu virtualjourney slideshow se photos web pages Boy 20with 20Slingshot 2C 20J htm 3 Gravitational potential energy Gravitational potential energy Due to object s position Relative to a surface weight x height mgh Work done for object to gain potential energy Gravitational potential energy Gravitational potential energy EP mass x acceleration of gravity x height Height is above some reference level Potential energy is always referenced to a zero level defined in the system Gravitational potential energy EP mgh mg weight h height Gravitational potential energy EP mgh Path to the height is not factor in EP 4 Gravitational potential energy Work Power and Energy EP mgh Horizontal distance is not factor in EP How much work is done when you carry a 75 N bowling ball across the room Not any no change in height so no change in potential energy Work Power and Energy Potential energy only important when it changes Change of EP does work W Fd EP transformed to another form of energy Work Power and Energy EP transformed to another form of energy Kinetic energy of motion Work Power and Energy Water behind the dam Potential energy Kinetic Energy of Motion EK mv2 Work is a change in kinetic energy W EK Delta change 5 Kinetic Energy of Motion W EK Work energy theorem Net work Due to net force Conservation of Energy Transformation from one form to another Potential energy of stretched rubber of slingshot Transformed to kinetic energy of rock flying through air Conservation of Energy Rock transfers its kinetic energy to the object it hits May be transformed to heat upon impact Kinetic Energy of Motion Heat Sound Electricity and light Conservation of Energy Kinetic energy of rock flying through air Potential energy of stretched rubber of slingshot Transformed from potential to kinetic Conservation of Energy Energy cannot be created or destroyed it may be transformed form one form into another but the total amount never changes 6 Conservation of Energy Conservation of Energy EP 10000 J EK 0 J EP 7500 J EK 2500 J EP 5000 J EK 5000 J EP 2500 J EK 7500 J EP 0 J EK 10000 J Conservation of Energy Sun s energy from fusion of hydrogen to helium 4 H He energy 1 1 4 2 Sun s energy converted to chemical energy by plants Sun s heat converted to potential energy when it evaporates water http science howstuffworks com roller coaster htm Conservation of Energy Does a car use more fuel when its lights are on What about when the air conditioner is on How about using the radio when the engine is off 7
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