Energy and Work

DESCRIBE THE CONSERVATION OF ENERGY AND

HOW ENERGY RELATES TO WORK.

Energy and Work

Terms and Definitions

Potential EnergyPotential Energy The energy that an object has because of position

Kinetic EnergyKinetic Energy The energy of an object due to the object’s motion

WorkWork Transfer of energy to an object by the application of a force

that causes the object to move in the direction of the forcePowerPower

A quantity that measures the rate at which work is done

Equations

Variable

Equation

MKS Units

English Units

Potential Energy

PE PE = mgh

Joules (J) ft x lbs

Kinetic Energy

KE KE=½mv2

J ft x lbs

Work W W = Fd J ft x lbs

Power P P = W/t Watts (W)

ft x lbs/s

ProblemProblem AnswerAnswer

An 0.125 kg apple is hanging on a tree 3 meters high.

How much potential energy does the apple have?

The apple falls. How much kinetic energy will it have when it hits the ground?

m = 0.125 kg g = -9.8 m/s2 d = -3 m PE = ? PE = mghPE = (.125)(-9.8)(-3)PE = 3.675 JKE = 3.675 J

Sample Problem

ProblemProblem AnswerAnswer

A crane uses an average force of 5,200 N to lift a girder 25 m.

How much work does the crane do on the girder?

How much power was needed if it took 50 seconds to lift the girder?

F = 5,200 N d = 25 m W = ? W = Fd W = (5,200)(25) W = 130,000 J t = 50 s P = W/t P = 130,000/50 P = 2600 W

Sample Problem