Student Responsibilities

•Be willing and ready to learn

•When I am talking, you are not

•Do not hinder the learning of your classmates

EnergyEnergy

What is Energy?What is Energy? Energy is the ability to do work.Energy is the ability to do work.

What is Work?• Work is done when a force causes a change in distance

WorkWork Work can be expressed as an Work can be expressed as an

equationequation

W = F dW = F d

W – Work (Joules, J W – Work (Joules, J oror Newton-meter, Newton-meter,

N m)N m)

F – Force (Newtons, N)F – Force (Newtons, N)

d – Distance (meters, m)d – Distance (meters, m)

Sample ProblemSample Problem A man pushes a box across a floor A man pushes a box across a floor

with a force of 20 N for 10 m. How with a force of 20 N for 10 m. How much work does the man exert?much work does the man exert?

Work vs. ImpulseWork vs. Impulse Impulse is a force exerted over an Impulse is a force exerted over an

amount of timeamount of time Work is a force exerted over a Work is a force exerted over a

distancedistance

What about how much work is What about how much work is done over time?done over time?

PowerPower Power is the rate at which work is donePower is the rate at which work is done

(how fast work is done)(how fast work is done) EquationEquation

P = WP = W tt

PP – Power (Watts, W – Power (Watts, W or Jor Joules/secondoules/second))WW - Work (Joules, J - Work (Joules, J oror Newton-meter, N Newton-meter, N

m)m)tt – time (seconds) – time (seconds)

Horsepower

When the steam engine first came out, people wanted to know how they compared to horses.

James Watt expressed the power of his steam engines in horsepower, or the rate at which horses could do work.

745 Watts = 1 Horsepower

Sample ProblemsSample Problems

If a machine does 5000 J of work in If a machine does 5000 J of work in 10 sec, what is the power?10 sec, what is the power?

EnergyEnergy Energy is the ability to do workEnergy is the ability to do work Energy is measured in JoulesEnergy is measured in Joules When work is done, energy is When work is done, energy is

used. used.

Work, Power and Energy are Work, Power and Energy are scalarscalar quantities quantities

Energy Comes in Energy Comes in Different Forms!Different Forms!

NNot ot

EEveryvery

CCatat

HHatesates

MMiceice

NuclearNuclear

ElectricalElectrical

ChemicalChemical

HeatHeat

MechanicalMechanical

Potential EnergyPotential Energy Energy stored by an object Energy stored by an object

because of its position or its because of its position or its conditioncondition

Gravitational Potential Gravitational Potential EnergyEnergyEnergy stored as a result of its Energy stored as a result of its

height. height.

Gravitational Gravitational Potential EnergyPotential Energy

PEPEgravity gravity = mgh = mgh

m – mass (kilograms)m – mass (kilograms)

g – acceleration due to gravity (m/sg – acceleration due to gravity (m/s22 ) ) h – height (meters)h – height (meters)

Note – Relationship between Work and Note – Relationship between Work and PEPE

Which has more PE?Which has more PE?

Sample ProblemSample Problem

What is the PE of a 200 kg piano What is the PE of a 200 kg piano hoisted 10 meters above the ground?hoisted 10 meters above the ground?

Elastic Potential EnergyElastic Potential Energy

Elastic PE is found in a Elastic PE is found in a stretched or compressed elastic stretched or compressed elastic object (springs)object (springs)

If a spring is not stretched or If a spring is not stretched or compressed, then there is no compressed, then there is no elastic potential energyelastic potential energy

Kinetic EnergyKinetic Energy

Kinetic Energy is the energy of motionKinetic Energy is the energy of motion Tells how much work is done to bring Tells how much work is done to bring

the object from rest to its present the object from rest to its present speedspeed

KE = ½ mvKE = ½ mv22 m – mass (kg)m – mass (kg)

v – velocity (m/s)v – velocity (m/s)

KE – Kinetic Energy (Joules)KE – Kinetic Energy (Joules)

Sample ProblemSample Problem

Find the K.E. of a 4.0 kg cart moving Find the K.E. of a 4.0 kg cart moving at 50 m/s. at 50 m/s.

Conservation of Energy

Energy cannot be created or destroyed. It can be transformed from one form into another, but the total amount of energy never changes.