Chapter 5 - Physics

Work and Energy

Section 1 objectives

WorkWork – The product of the magnitudes

of the component of a force along the direction of displacement and the displacement. Work is not done unless the object is

moved. Work is only done when components of a

force are parallel to a displacementComponents of the force perpendicular

to a displacement do no work.

WorkW = Fd(cosθ)

Do sample problems 5A on page 169.Sign of work

Page 170; figure 5-3 Work is + when the force is in the

same direction of the displacement Work is – when the force is in the

opposite direction of the displacement

objectives Identify several forms of mechanical energy. Calculate kinetic energy for objects. Distinguish between kinetic and potential energy. Classify different types of potential energy. Calculate an object’s potential energy. Relate kinetic and all forms of potential energy to the

idea of mechanical energy.

Kinetic EnergyKinetic Energy-The energy of

an object due to its motion.Depends on both mass and velocity.

KE = ½mv2

Do practice problems 5B, page 173

Potential EnergyThe energy associated with an

object due to its position.Different types of potential energy:

Gravitational Potential Energy: The energy assoc. w/ an object due to its position relative to the Earth or some other gravitational source.PEg=mgh

Potential EnergyElastic Potential Energy: The energy

in a stretched or compressed springPeelastic=½kx2

k= spring constant x=distance compressed or stretched

Spring constant= A parameter that expresses how resistant a spring is to being compressed or stretched.

Do practice problems 5C; page 177

Mechanical EnergyThe sum of the kinetic energy and

all forms of potential energy Energy

Mechanical Nonmechanical

Kinetic PotentialGravitational Elastic

Nonmechanical Energy- other forms besides kinetic and potential

objectives Identify situations in which conservation

of mechanical energy is valid.Recognize the forms that conserved

energy can take.Solve problems using conservation of

mechanical energy.

Conservation of EnergyEnergy is conserved

See example pg 180; figure 5-1 In the absence of friction,

mechanical energy is conserved, but can change forms MEi=Mef

½mv2i + mghi = ½mv2

f + mghf

Do practice problems 5D; pg. 182When friction is present, mech. E is

not conserved – it changes to other forms of nonmech. energy.

objectivesObjectivesApply the work-kinetic energy theorem

to solve problems.Relate the concepts of energy, power,

and timeCalculate power in two different waysExplain the effect of machines on work

and power.

Work, Power, and EnergyWork-Kinetic Energy Theorem

The net work done on an object is equal to the change in the kinetic energy of the object.

Wnet=ΔKE Work is a method of energy transfer Do practice problems 5E, pg. 186

Work, Power, and Energy Power- the rate at which energy is

transferred. P=W/ΔT (Power = work/time) Remember W=Fd, so P =Fd/t, but d/t = v,

so this can be simplified to say that P=Fv. You can use any of these equations

depending on the given information. SI unit of power = Watt (W)

1 W = 1 J/s 1 hp = 746W (hp-horsepower is the

English unit) Do practice problems 5F, PG. 188

Chapter 5 problem setPg. 193-199#2, 3, 5, 6, 7, 10, 12, 13, 14, 16,

19, 23, 27, 31, 32, 35, 39, 40, 41, 48, 52.