Unit 07“Work, Power,

Energy and

Energy Conservation”

Introduction to Work, Power and

Energy

 Activity Pushing against the wall

 Pushing against the chair 

 Lifting a pailCarrying a pail 

 Picking up a Barbell 

Holding a Barbell

 Reading a book/HW

 My job at McDonaldsJust think about doing this one!

Activity #1:  Am I doing Work?Try each activity below and ask yourself…

“Am I doing work?”

 Activity  Do you think they are doing work?  

(The “everyday” definition)

Does science think they are doing work?(The “physics” definition)

 Pushing against the wall

Does the student do work on the wall? 

Pushing against the chair

Does the student do work on the chair?

Lifting a pail Does the student do work on the pail?

 Carrying a pail Does the student do work on the pail?

 Picking up a Barbell

Does the student do work on the barbell?

 Holding a Barbell 

Does the student do work on the barbell?

Reading a book/HW

Does the student do work on the book?

 My job at McDonalds 

  Does the student do work on McDonalds?

Yes!  

Yes!  

Yes!  

NO!  

NO!  

NO!  

NO!  

NO!  

Work is:

Equation for Work

W = FdWork = Force x distance

The 3 criteria that need to be met for work to be done on an object are:

•Force must be applied to the object

•The object must move (must be displaced)

•Force on the object and displacement of the object must be in same direction

The force applied to an object multiplied by the resulting displacement.

Units for Work

Joule (J)

Kg.m2/s2

Work is a vector:

Work includes amount and direction.

Work is –   (negative) If the force pushes west, south, to the left, down …

Work is +    (positive)If the force pushes east, north, to the right, up ….

Many times friction does (-) negative work on an object!You pull the box to the right: (+) work!

Friction pushes against the box to the left: (-) work!

A boy lifts a bucket of water vertically. Does he do work on the bucket?

A boy holds a bucket of water. Does he do work on the bucket.

Guide:

Is there a force on the bucket?

Does the bucket move?

Is the force and displacement in the same direction?

So does he do work on the bucket?

Yes, upYes, up

Yes, both upYes!!!!

No, the boy does not do work is done on the bucket.  He does put a force on the bucket (upwards)…But the bucket does not move, no displacement…So no work!

The boy holds a bucket of water and carries it water horizontally. Does he do work on the bucket?

The boy pushes a bucket of water horizontally across the floor. Does he do work on the bucket?

No, the boy does not do work is done on the bucket.  He does put a force on the bucket (upwards)…And the bucket does move (forward)…But the force and displacement are not in the same direction!So no work!

Yes, the boy does do work is done on the bucket.  He does put a force on the bucket forward)…And the bucket does move (forward)…Same direction!  Work is done!

 Activity Does science think they are doing work?(The “physics” definition)

 Pushing against the 

wallDoes the student do work on the wall?   

Pushing against the chair

Does the student do work on the chair?

Lifting a pail Does the student do work on the pail?

 Carrying a pail Does the student do work on the pail?

 Picking up a Barbell

Does the student do work on the barbell?

 Holding a Barbell  Does the student do work on the barbell?

Reading a book/HW Does the student do work on the book?

 My job at McDonalds  Does the student do work on McDonalds?

Yes! The force applied moved the barbell!  

Yes!  The force applied  moved the chair!   

Yes! The force applied moved the chair!  

NO!  The force and distance are not in the same direction!  

NO!  The wall doesn’t not move. 

NO!  The barbell does not move!  

NO!  No force applied, no distance either!  

NO!  No force applied, no distance either!  

Efficiency

Equation for EfficiencyEff = Wout x100 Win

A perfect or ideal machine means the “Work In” would be equal to the “Work Out” since you would be the only thing doing work on an object.

But, in real life, the work in is not equal to the work out. This is because friction also does work. Friction does negative work.

Machines are not 100% efficient because of the force of friction!

Activity #2What is Power?

• Two students are both given the task of lifting a barbell over their heads (the same distance) 10 times.

• Student “B” takes 10 seconds to complete the task.• Student “S” takes 5 seconds to complete the task.

    Who does more work?   Who is more powerful?They do the same amount of work!  

W=Fd Same force (same weight), same distance, same work!

Bobby is more powerful!!! WHY?

Equation for Power

Units for Power

Power is:

The rate at which work is done.“rate” means the time it takes.So power is how fast the work is done.

Power = Work time

P = W_ Δt

Watts (W) J/sEnglish Unit

Horsepower (hp)

746 W = 1 hp

Does power change the work done?Bobby and Sally both lift the barbell shown below. •They lift it up the same distance. •Bobby takes 10 seconds to lift the box. •Sally takes 5 seconds to lift the box.

Power ___________ change the work done!!!Work depends on

Power depends on

Who does more work? Bobby, Sally or neither?Neither!  Same distance, same force!  Same work!

Who has more power? Bobby, Sally or neither?Sally! Less timeMore power!

does not

Force and distance. Time does not change the work done. How fast does not change the work done.

Work and time. The faster the work is done, the more power.

• Energy: The ability to do work. • IF work is done, energy is used. If an object or person

has energy, it can do work!Units: Joule (J)

Energy has two categories:

Mechanical Energy

•Kinetic Energy•Potential Energy

Non-Mechanical Energy

•Chemical Energy•Thermal Energy•Electromagnetic Energy•Nuclear EnergyIn total there are 5 types of energy:

Mechanical, Thermal, Electromagnetic, Nuclear, Chemical

Energy

Notice the capital C0

Calories (that’s 1000 calories)

Notice that Energy is measured in both “kJ” (kilojoules) and kcal (kilocalories or Calories)

Energy measured in

kcal (kilocalories, Calories)

and also measured in

kJ (kiloJoules)

Mechanical EnergyKinetic Energy

The Energy of motionPotential Energy

The Energy of position

Electromagnetic EnergyWave energy due to magnetic and electric fields“Light waves”

MicrowavesUV wavesX-RaysVisible Light … and more!

Thermal EnergyEnergy stored in fast moving molecules which creates heatHeat Can be caused by friction

Chemical Energyenergy stored in chemical bondsFoodFossil Fuels (gas, oil)

Typesof

Energy

Nuclear EnergyEnergy stored in the nucleus of an atomFission (like the atomic bomb)Fusion (like the sun)

Chemical Energyenergy stored in chemical bondsFoodFossil Fuels (gas, oil)

Gas

Oil coal

Food

Mechanical EnergyKinetic Energy

The Energy of motionPotential Energy

The Energy of position

Thermal EnergyEnergy stored in fast moving molecules which creates heatHeat Can be caused by friction

Electromagnetic EnergyWave energy due to magnetic and electric fields“Light waves”

MicrowavesUV wavesX-RaysVisible Light … and more!

Nuclear EnergyEnergy stored in the nucleus of an atomFission (like the atomic bomb)Fusion (like the sun)

Fission Fusion

Kinetic Energy

Equation Units

KE = ½mv2

The Energy of Motion

KE = ½(mass)x(velocity)2

Joules (J)

kgm2/s2

If the object is moving it has Kinetic Energy 

KE = ½ mv2

Kinetic Energy = ½ (mass)x(velocity)2

Potential Energy

Equation

The Energy of Position

PEg = mghPEg=(mass)x(gravity)x(height)

Units

Joules (J)

kgm2/s2

If the object is above the ground it has Potential Energy 

PE = mghPotential Energy = (mass)(gravity)(height)