Work and EnergyWork and EnergyEnergyEnergy

Chapter 5: Section 2

Learning TargetsLearning TargetsIdentify several forms of energyCalculate kinetic energy for an

objectDistinguish between kinetic and

potential energyClassify different types of potential

energyCalculate the potential energy

associated with an object’s positionP4.1e, P4.3A, P4.3d, P4.3f

Kinetic EnergyKinetic EnergyKinetic energy is the energy of motion

◦An object in motion - whether it is vertical or horizontal - has kinetic energy

Kinetic energy depends on both velocity and mass◦Therefore, the kinetic energy of an object

with mass (m) and velocity (v) is :

KE = ½ mv2

KE = ½ mv2

According to the equation above, kinetic energy and mass are directly proportionalIf a bowling ball and a volleyball are

traveling with the same velocity, the bowling ball will have a greater kinetic energy because it has a greater mass

KE = ½ mv2

Kinetic energy of an object is also directly proportional to the square of its velocity.◦A 55 kg person running with a velocity

of 5 m/s would have four times the kinetic energy of a person running with a velocity of 2.5 m/s

Units of Kinetic EnergyUnits of Kinetic EnergyKinetic energy is a scalar quantity

because it does not have a direction

Like work, the SI unit of kinetic energy is the joule

Work-Kinetic Energy Work-Kinetic Energy TheoremTheoremThe net work done on a body

equals its change in kinetic energyAccording to the work-kinetic energy theorem:

Wnet = ∆KE

Work and Kinetic Work and Kinetic EnergyEnergy

An object’s speed increases if the net work is positive because the final KE is greater than the initial KE

The object’s speed decreases if the net work is negative because the final KE is less than the initial KE

Potential EnergyPotential EnergyPotential energy is the stored

energy of position possessed by an object

Potential energy is associated with an object that has the potential to move because of its position◦A bow that is drawn◦A wrecking ball hanging from a crane

Potential energy depends on an object’s interaction with its environment

Gravitational Potential Gravitational Potential EnergyEnergyGravitational potential energy

is the energy stored in an object as the result of its vertical position or height. ◦The energy is stored as the result of

the gravitational attraction of the Earth for the object.

Calculating Calculating Gravitational PEGravitational PE

Gravitational potential energy is dependent on both mass and height

Therefore, when free-fall acceleration is constant the formula for GPE is:

PEg = mgh

◦Both mass and height have a direct relationship with gravitational potential energy

◦The SI unit for PE is the joule

Height and GPEHeight and GPEGravitational potential energy is a

result of an object’s position so it must be measured relative to some zero level◦Typically, the ground is considered zero◦When working on a lab table, you

might assign that the position of zero height

Since the gravitational potential energy of an object is directly proportional to its height above the zero position, a doubling of the height will result in a doubling of the gravitational potential energy.

Suppose you drop a ball from a second-floor roof and it lands on a first-floor roof. Does the ball still have gravitational potential energy?◦If h is measured from the ground then

gravitational potential energy is not zero because the ball is still above the ground

◦If h is measured from the first floor roof, the PE is zero when the ball lands on the roof

Elastic Potential EnergyElastic Potential EnergyElastic potential energy is the

energy stored in elastic materials as the result of their stretching or compressing◦Example: A spring or the stretched

strings of a guitarThe amount of energy depends on

the distance the spring is compressed or stretched from relaxed length

Equation For Elastic PEEquation For Elastic PE

PEelastic = ½ kx2

k = spring constantx = distance compressed or stretched

For a flexible spring, the spring constant is small

For a stiff spring, the spring constant is large◦Spring constants have units of N/m

Chemical Potential Chemical Potential EnergyEnergy

Chemical potential energy is the energy stored in food or fuel that is transformed into work◦Energy is stored in the bonds between

atoms◦The stronger the bonds, the less

chemical energy