Chapter 5

Work, Energy and Powerp. 79

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

Work• Work is measured in a unit called “Joules”• 1Joule = 1 N x m = 1 kg-m x m sec2

• If there NO movement there can be NO work

Work and Machines

• The work input for a machine should always be equal to the work output!

Work and Machines

• However we loose some work because of friction

• The amount of output work divided by input work measures the machine’s efficiency

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

Power Problem

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 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!

Potential Energy

• Is energy stored in an object due to its position

• Potential Energy is also considered energy at rest

• Ep = mass x gravity x height

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.

Kinetic Energy• Energy with motion• Energy an object has because it’s in motion• As mass increases, the KE increases• As velocity increases, the KE increases• KE changes more with velocity than with

mass• KE = mass x velocity 2

2

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.

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 loose 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