1. Introduction to Work, Power and Energy
Unit 07
“Work, Power, Energy
and
Energy Conservation”
Introduction to Work, Power and Energy

2. Pushing against the wall Pushing against the chair
Activity #1: Am I doing Work? Try each activity below and ask yourself…
“Am I doing work?”
 Activity
 Pushing against the wall
 Pushing against the chair 
 Lifting a pail
Carrying a pail 
 Picking up a
Barbell 
Holding a Barbell
 Reading a book/HW
 My job at McDonalds
Just think about doing this one!

3. NO! Yes! Yes! NO! Yes! NO! NO! NO! 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
 Picking up a
Barbell
Does the student do work on the barbell?
 Holding a Barbell 
Reading a book/HW
Does the student do work on the book?
 My job at McDonalds 
Does the student do work on McDonalds?
NO!
Yes!
Yes!
NO!
Yes!
NO!
NO!
NO!

4. Force must be applied to the object
Work is:
The force applied to an object multiplied by the resulting displacement.
Equation for Work
W = Fd
Work = Force x distance
Units for Work
Joule (J)
Kg.m2/s2
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

5. Work includes amount and direction.
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!

6. A boy holds a bucket of water. Does he do work on the bucket.
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, up
Yes, up
Yes, both up
Yes!!!!
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!

7. The boy holds a bucket of water and carries it water horizontally
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!

8. NO! The wall doesn’t not move.
 Activity
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
 Picking up a
Barbell
Does the student do work on the barbell?
 Holding a Barbell 
Reading a book/HW
Does the student do work on the book?
 My job at McDonalds 
Does the student do work on McDonalds?
NO! The wall doesn’t not move.
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!
Yes! The force applied moved the barbell!
NO! The barbell does not move!
NO! No force applied, no distance either!
NO! No force applied, no distance either!

9. Equation for Efficiency
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!
Equation for Efficiency
Eff = Wout x100
Win

10. Activity #2 What 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?

11. The rate at which work is done. “rate” means the time it takes.
Power is:
The rate at which work is done.
“rate” means the time it takes.
So power is how fast the work is done.
Units for Power
Watts (W)
J/s
Equation for Power
P = W_ Δt
English Unit
Horsepower (hp)
746 W = 1 hp
Power = Work
time

12. 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.
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 time
More power!
does not
Power ___________ change the work done!!!
Work depends on
Power depends on
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.

13. Non-Mechanical Energy
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 Energy
In total there are 5 types of energy:
Mechanical, Thermal, Electromagnetic, Nuclear, Chemical

14. Calories (that’s 1000 calories)
Notice the capital C
Calories (that’s 1000 calories)

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

16. kcal (kilocalories, Calories)
Energy measured in
kcal (kilocalories, Calories)
and also measured in
kJ (kiloJoules)

17. Types of Energy Mechanical Energy Chemical Energy Thermal Energy
Kinetic Energy
The Energy of motion
Potential Energy
The Energy of position
Chemical Energy
energy stored in chemical bonds
Food
Fossil Fuels (gas, oil)
Types of
Energy
Thermal Energy
Energy stored in fast moving molecules which creates heat
Heat
Can be caused by friction
Electromagnetic Energy
Wave energy due to magnetic and electric fields
“Light waves”
Microwaves
UV waves
X-Rays
Visible Light
… and more!
Nuclear Energy
Energy stored in the nucleus of an atom
Fission (like the atomic bomb)
Fusion (like the sun)

18. Chemical Energy energy stored in chemical bonds Gas Oil coal Food Food
Fossil Fuels (gas, oil)
Gas
Oil
coal
Food

19. Mechanical Energy Kinetic Energy The Energy of motion Potential Energy
The Energy of position

20. Thermal Energy
Energy stored in fast moving molecules which creates heat
Heat
Can be caused by friction

21. Electromagnetic Energy
Wave energy due to magnetic and electric fields
“Light waves”
Microwaves
UV waves
X-Rays
Visible Light
… and more!

22. Nuclear Energy Energy stored in the nucleus of an atom Fission Fusion
Fission (like the atomic bomb)
Fusion (like the sun)
Fission
Fusion

23. Kinetic Energy KE = ½ mv2 Kinetic Energy = ½ (mass)x(velocity)2
The Energy of Motion
Equation
Units
KE = ½mv2
Joules (J)
KE = ½(mass)x(velocity)2
kgm2/s2
If the object is moving it has Kinetic Energy
KE = ½ mv2
Kinetic Energy = ½ (mass)x(velocity)2

24. PE = mgh Potential Energy Potential Energy = (mass)(gravity)(height)
The Energy of Position
Equation
Units
PEg = mgh
Joules (J)
PEg=(mass)x(gravity)x(height)
kgm2/s2
If the object is above the ground it has Potential Energy
PE = mgh
Potential Energy = (mass)(gravity)(height)