Mechanical EnergyMechanical EnergyCh. 4

EnergyEnergy

Is the ability to do work.

Energy = work

Units = Joules (J)

James Prescott Joule

WorkWorkWork is a force

applied through a distance.

W=Force(N) x distance (m)◦ W=Fxd◦ SI Unit = Joules (J)

If you apply a force, but do not move, you are not doing work.

Joe pushes the mower with 250 N of force for 20 m. How much work has he done?

W = Fxd250 N x 20 m = 5000 J

PowerPowerPower is the rate at

which energy is converted.◦ Or, it’s how fast

you do work.Power = Work (J) /

time (s) = Watts (W)or

Power = Energy (J) / time (s)

SI Unit = Watts (W) ◦ Watts = Joules /

seconds

If a light bulb produces 300 J of energy in 5 s, how much power does it produce?

P = W/tP = 300J / 5 sP = 60 watts

Potential energyPotential energyStored energy – has the potential

to do workElastic PE – energy stored by

stretching or compressing an object◦Ex. – rubber band, spring

Chemical PE – energy stored in chemical bonds◦Ex. – food, gasoline, batteries

Potential energyPotential energyGravitational PE – stored energy due to

gravitational attraction between objects

Depends on mass, gravity, and heightFormula: GPE = mass x gravity x

height◦GPE = mgh

g = 9.8 m/s2

Unit = Joules (J)

If h, then GPEIf m, then GPE

Kinetic EnergyKinetic EnergyEnergy of motion – an object must

be moving to have KEDepends on mass and velocityFormula: KE = ½ mass x velocity2

◦KE = ½ mv2

◦Units are Joules (J)Depends more on velocity than

mass◦v is squared

Work Energy TheoremWork Energy TheoremWork = Change in Kinetic Energy

◦W = Δ KETo change the KE on an object, work

must be done on the object.◦The more KE an object has, the more

work must be done to change that energy. Car accelerating and decelerating examples

Conservation of EnergyConservation of EnergyEnergy can not be created or

destroyed. It can only be transformed from one type of energy to another.

The total amount of energy never changes!!!

Pendulum Example