ENERGYENERGY

THE DRIVING THE DRIVING FORCEFORCE

What is Energy?What is Energy?

• The ability to do workThe ability to do work

• The ability to cause changeThe ability to cause change

• Measured in Joules, just like workMeasured in Joules, just like work

• Formula Symbol: EFormula Symbol: E• as in E = mcas in E = mc22

Forms of EnergyForms of Energy

• Mechanical EnergyMechanical Energy

• Describes energy stored and Describes energy stored and released according to function or released according to function or forceforce

Forms of EnergyForms of Energy

• Electromagnetic EnergyElectromagnetic Energy

• Waves of mostly invisible energyWaves of mostly invisible energy

• Includes light, radio waves, X-raysIncludes light, radio waves, X-rays

Forms of EnergyForms of Energy

• Chemical EnergyChemical Energy

• Stored and released in the bonds Stored and released in the bonds between atomsbetween atoms

Forms of EnergyForms of Energy

• Nuclear EnergyNuclear Energy

• Stored and released from the nucleus Stored and released from the nucleus of an atomof an atom

Forms of EnergyForms of Energy

• Thermal EnergyThermal Energy

• Heat in all its forms and flowsHeat in all its forms and flows

Two descriptions of EnergyTwo descriptions of Energy

• Potential EnergyPotential Energy

• Energy stored by position or conditionEnergy stored by position or condition

• Kinetic EnergyKinetic Energy

• Released energy or energy of motionReleased energy or energy of motion

Potential EnergyPotential Energy

• The easiest quantified PE is The easiest quantified PE is Gravitational PEGravitational PE

• This is the PE stored in an object by its This is the PE stored in an object by its height above another reference pointheight above another reference point

• PEPEGravitationalGravitational = m g h = m g h

Joules = kg●(m/sJoules = kg●(m/s22) ●m) ●m

Remember, g = 9.8 m/sRemember, g = 9.8 m/s22

Potential Energy, revisited…Potential Energy, revisited…

• Another form is Elastic PEAnother form is Elastic PE

• This is the PE stored in an object by its This is the PE stored in an object by its being disturbed from its natural state,being disturbed from its natural state,

• and how much it wants to return to that and how much it wants to return to that natural statenatural state

• Good examples:Good examples:

• Springs, swings, pendulums, rubber Springs, swings, pendulums, rubber bandsbands

Kinetic EnergyKinetic Energy

• This is the KE released by an object by This is the KE released by an object by its motionits motion

• KE = ½ m vKE = ½ m v22

• Joules = kg●(m/s)Joules = kg●(m/s)22

ProblemsProblems

• A 3.0-kilogram toy dump truck moving with a speed of 2.0 m/s starts up a ramp. How high does the truck roll before it stops?

• A 2.0-kilogram ball rolling along a flat surface starts up a hill. If the ball reaches a height of 0.63 meters, what was its initial speed?

ProblemsProblems

• Find the potential energy of this roller coaster when it is halfway down the hill.

• A 2.0-kilogram ball is tossed straight up with a kinetic energy of 196 joules. How high does it go?

Law of Conservation of EnergyLaw of Conservation of Energy

• Energy can not be created or destroyed Energy can not be created or destroyed in normal reactionsin normal reactions

• Energy can be changed from one form Energy can be changed from one form to anotherto another

• The total amount of energy must be the The total amount of energy must be the same before and after a change in same before and after a change in energyenergy

• Most common energy conversion is from Most common energy conversion is from potential to kinetic energypotential to kinetic energy

• Gravitational potential energy of an Gravitational potential energy of an object is converted to kinetic energy as object is converted to kinetic energy as the object falls.the object falls.

• Potential energy can become kinetic and Potential energy can become kinetic and vice versa.vice versa.

• Example: PendulumsExample: Pendulums

Mechanical Energy in a PendulumMechanical Energy in a Pendulum

• EinsteinEinstein’’s famous equations famous equation

• E = mcE = mc22

• m = massm = mass• c = speed of light (3 x 10c = speed of light (3 x 1088 m/s) m/s)

• Shows that matter can be converted Shows that matter can be converted into energy and energy can be into energy and energy can be converted into mass. converted into mass.