Unit 8(Chapter 10 & 11)

Work, Energy and Power

Work• “Work” means many things in different

situations.

• When we talk about work in physics we are talking about applying a force over a certain distance.

• For work to occur a force must produce motion in the direction of the force.

• Work = Force x Distance

Power• How fast work gets done is called power• Power is measured in a unit called watts• A powerful machine can do the same amount of

work in less time than a less powerful machine• 1 watt = 1 joule/sec• Horsepower is the English unit for power. • 1 horsepower = 746 watts• Power = work

time

Energy • Energy is defined as the ability to do work– 6 Types of energy

• Kinetic Energy: associated with motion

• Heat: associated with the speed of internal particles

• Chemical: bonds atoms together• Electromagnetic: moving electric

charges• Nuclear: in the atomic nucleus• Potential Energy: energy of position

(energy at rest)

Energy

• The units of energy are joules• joules are also the same unit as work!

(How Confusing!)

• You can think of energy as stored work – that is ready to be released!

• Energy cannot be created or destroyed – only changed from one type of energy to another!

Kinetic Energy• Energy with motion• Energy an object has because it’s in motion• All moving objects that have mass have kinetic

energy• As mass increases, the KE increases• As speed increases, the KE increases• KE changes more with speed than with mass• KE = mass x speed2

2

Kinetic EnergyKinetic energy (joules) KE = ½ mv2

mm - - mass of the object inmass of the object in kgkgvv - - speed of the object inspeed of the object in m/sm/sKEKE - - the kinetic energy inthe kinetic energy in JJ

Kinetic Energy depends on two things: mass and speed

The amount of kinetic energy the cart has is equal to the amount of work you do to get the cart moving

A cart at resthas NO Kinetic energy

Applying a force can give the cart speed, andtherefore kinetic energy.

Applying a greater force increases the speed and therefore the kinetic energy

Increasing the mass also increases kinetic energy because it takes even more force to push.

Work-Energy TheoremWork-Energy Theoremthe net work done on an object is

equal to its change in kinetic energy

W KEnetThe amount of kinetic energy an object has is equal to the amount of work you do to get the object moving

Potential Energy• Is energy stored in an object

due to its position

• Potential Energy is also considered energy at rest

• GPE = mass x acceleration due to gravity x height

Potential EnergyPotential Energy (joules)

mm - - mass of object in kggg - - acceleration of gravity in m/s2

hh - - height of object, in mGPEGPE – – gravitational potential energy in J

GPE = mgh

Potential Energy

• Gravitational potential energy depends on an objects mass and height

• Objects with potential energy don’t use their energy until they start moving.

• The further the object is from the ground the more potential energy it has

• Chemical potential energy is energy stored in the form of chemical bonds.

Law of Conservation of Energy• Energy can never be created or destroyed just

turned into another form.

Law of Conservation of Energy

• When a ball flies through the air it does not lose energy even though it slows down and speeds up

• The energy in the ball changes from kinetic to potential and then back to kinetic energy