Energy, Work & Power

Regents PhysicsMr. Rockensies

Introduction to EnergyHow does energy manifest itself physically?

What is energy?

Energy is the ability to do Work.

Energy exists in several different forms. Almost all energy on Earth can be traced back to the Sun.

Examples:Mechanical, Thermal (Heat), Chemical, Nuclear, Electromagnetic, Kinetic, Potential

What is Work?

Work is a measure of the amount of energy used to move an object.

Mathematically: Work = Force • displacementBecause we are multiplying two vectors, we end up with a scalar quantity – Work is a scalar quantity.

Work is only done when the Force and the displacement are in the SAME direction.

Unit for work:

Work = F • d = Newton•m

To simplify this, we use Joules (J).

1 J = 1 N•m

All energy quantities will be measured in Joules.

Work and Friction

When friction is present, energy in a system is lost because of it.When two objects come into contact and move past each other, energy is given off in the form of HEAT – this is due to FRICTION.

What is Power?

Power is the rate at which energy is used, transformed, or transferred.

Power = Work/time AND

Power = Velocity • Force Power = Force • Distance

timeUnits: Watts = Joules/second = N •m = kg • m2

s s3

We measure Power in Watts to simplify the units

What is horsepower then?

A measure of the amount of energy expended by the average horse over a given period of time. One horsepower is equal to 746 Watts, but depending on how you are using horsepower, it is up for debate.

Practice Problems

Practice More

More Practice

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Law of Conservation of Energy

Energy cannot be created or destroyed; it can only change forms.

Conservation in Free Fall

Ei = Ef

PEi = PEf + Kef

mghi = mghf + 1/2mvf

2

ghi – ghf = v2/2Δhi

Δhf

m

m

initial

final

vf

If an object starts from some initial height and enters free fall, its initial Potential Energy will equal the sum of its final Potential Energy and Kinetic Energy

Energy is a scalar quantityBecause energy is a scalar , direction doesn’t matter, and this works for curved motion as well

hi

m

m

hf

vf

Hill slides and roller coasters are the same

A

B

C

EA = EB = EC

PEA + KEA = PEB + KEB = …

hA

hC

Pendulum

vi = 0Kei = 0All PE

vf is maxKE is maxNo PE

Etop = Ebottom

PE = KE

Springs

Ei = Ef

Pes = Kecart

1/2kx2 = 1/2mv2

PE = 1/2kx2

KE = 1/2mv2

Systems with Friction

Initial Mechanical energy (PE + KE) becomes final mechanical energy plus some internal energy, converted by friction. (PE + KE + Q)

height reached with no friction

height reached with friction

Q = internal energy, usually heat, caused by friction

Summary

In a closed isolated system, what happens to the gravitational potential energy of an object as it falls? the kinetic energy? the total energy?

Does the amount of energy an object has change if the object changes direction?

How does Potential Energy change if a spring is compressed more?

When is Kinetic Energy the greatest for a swinging pendulum?

Energy Conservation Practice

Energy Conservation Practice

Energy Conservation Practice

Energy Conservation Practice

Energy Conservation Practice

Energy Conservation Practice