2. Work…
In everyday speech work has a very general meaning. In describing motion in physics, work has a very specific meaning.

3. No work is done on the chair
Chair Example
Standing
Walking
No work is done on the chair

4. Work is defined as the product of the force applied to cause motion and the distance the object moves in the direction of the force.
Work is done only when components of a force are parallel to a displacement

5. FORMULA W = fd IN DIRECTION OF MOTION The symbol for work is W
Work has 2 acceptable units Nm
Joules (J)

6. JOULE Lifting an apple about 2ft is a Joule
3 good push-ups is about 1000J

7. Situations that affect the sign of work
Force is in the direction of motion
Positive Work
Force opposes motion
Negative Work
Force is 90° to motion
No Work
Object is not in motion
No Work

8. Notice direction of motion is the same as the applied force
W = fd
W = 20J of Work
10N
Moves 2m
Notice direction of motion is the same as the applied force

9. How would you solve this
How would you solve this? Force applied is NOT in the same direction as the objects motion.
10N Y
60° X 2m
Think back to vectors and use the component of the force applied in the direction the object moves.

10. COS 60° = force parallel to motion
COS θ = adj/ hyp
COS 60° = force parallel to motion
10N
10N
force para. =
COS 60° (10N)
force para. = 5N Y
60° X 2m
w = F(parallel) D
5N (2m) = 10Nm

11. W = Fd (COS θ) θ = 0° θ = 90° Always measure angle with horizontal!
The above formula works in every case
θ = 0°
No work because no motion in direction of force
θ = 90°

12. Pg. 170
Problems
1-4

13. Section Review p. 171
With your Neighbor, answer questions
2, 3, 4

14. Section Review Answers
2 the neighbor, twice as much
3 a-negative
B-positive
C- negative
4 a-yes
B- no
C-yes

15. The Stuff that makes things move
Energy
The Stuff that makes things move
The ability to do work
Has the units of Joules (J)
There are 2 kinds of mechanical energy

16. Kinetic Energy KE = ½ mV2 manipulated V = 2KE/m M = 2KE/V2
This is the energy associated with an objects motion.
KE depends on mass and velocity
When the object is treated as a particle, the formula for KE is…
KE = ½ mV2
manipulated
V = 2KE/m M = 2KE/V2

17. KE is a scalar quantity
The SI unit for KE is the Joule, yes the same as for work
Look at sample prob. 5B
Page 173
DO practice problems
5B 1-5 on page 174

18. Work- Kinetic Energy Theorem
The net work done on an object is equal to the change in the kinetic energy of the object
Wnet = ΔKE
Wnet = KEfinal – KEinitial
fd(cos θ) = ½ mV2
The KE of an object is equal to the work that moving object can do

19. This theorem allows us to think of KE as the work an object can do as it comes to rest, or the amount of energy contained in the moving object
The KE of the moving hammer can do work
KE = Work done (net)
fd = ½ mv2
some of the energy is sound, heat and light (if spark)

20. Practice Problems 5C p.176 #1 and 4 only

21. Potential Energy
This is the energy associated with an object due to the position of the object.
STORED ENERGY
There are two kinds of potential energy
GRAVITATIONAL POTENTIAL ENERGY
ELASTIC POTENTIAL ENERGY

22. Gravitational Potential Energy (PEg)
The energy associated with an object due to the objects position relative to a gravitational reference
Wh = PEg = mgh
= mass x gravity x height acceleration
gm = w
Has the unit of joules

23. Elastic Potential Energy (PEelastic)
The energy associated with a stretched or compressed elastic object
Spring, bungee cord, rubber band

24. Elastic Potential Energy

26. Practice Problems 5D 1-3 pg. 180
Overhead (springs)
In both the compressed and stretched example, energy is stored
PEelastic = ½ KX2
K = spring constant
X = distance stretched or compressed
Practice Problems 5D pg. 180

27. Conservation of Energy
To say something is conserved is to say it remains constant. Something can change form and still be conserved.
burning log: matter and energy are conserved.
smoke
water vapor
gas
ASH
5Kg
5Kg

28. Pendulum Energy is transferred from one form to another
As the pendulum swings, PE is transferred to KE. As the bob swings upwards KE is stored as PE
PE = max
KE = min
PE = max
KE = min
PE = min
KE = max

29. A falling egg PE = 10 J KE = 0 J PE = 5 J KE = 5 J PE = 0 J KE = 10 J
Mass = .1kg Height = 10m
PE = 10 J
KE = 0 J
PE = mgh
PE = 5 J
KE = 5 J
10M
PE = 0 J
KE = 10 J

30. Mechanical Energy
The sum of Kinetic Energy and ALL forms of Potential energy associated with an object or group of objects
ME is not a unique form of energy. Its merely a way of classifying energy
ME includes KE and PE

31. Mechanical Energy Σ SPN
ME is different from non mechanical energy (nuclear, chemical, thermal, internal, electrical)
ME = Σ KE + Σ PE
ME = ½ mv2 + mgh (if PE is NOT present, elastic)
Σ SPN

32. Conservation Of Mechanical Energy
Conservation of Mechanical Energy can also be written as…
MEi = MEf
½ mvi2 + mghi = ½ mvf2 + mghf
True when friction can be ignored

33. The Law of Conservation of Energy: The total energy of a closed system is constant.
Notice: The total energy of the system remains constant.
Energy just simply changes from KE to GPE and back and forth again.
GPE-relates to height
KE- relates to speed 33

34. Often is the case that KEi or KEf or PEi or PEf will be zero
Often is the case that KEi or KEf or PEi or PEf will be zero. When that is the case…
mgh = ½ mv2
2mgh = v2 m
V = 2gh
h = V2 2g

35. 28-31, 33, 34a Look at sample problem 5E Practice problems 5E #s 5,2,1
Problems p.185
Look at sample problem 5E
Practice problems 5E #s 5,2,1
HOMEWORK!!
28-31, 33, 34a
All on page 195 of packet

36. Power
This quantity also has a very specific meaning in science that can be confused by common English usage
Power is the rate of doing work
That is to say that power is the rate at which energy is transferred

37. Power Power is work done divided by the time taken to do the work
Power = Work = fd P = w
Time t t
Power is measured in watts (W) J/s
A watt is a small unit, 1 watt is about what is needed to lift a 2N glass of water .5m to your mouth in 1 second.

38. Watts Since watts are so small, we sometimes use Kilowatts
1 KW = 1000W
Watts are metric
Horse power is traditional
1 Horse power = 746 Watts

39. Watts
Watts are named after James Watt, the inventor of the steam engine

40. Practice Problem
An electric motor lifts an elevator that weighs 12000N a distance of 9m in 15sec
What is the motors power in watts?
What is the motors power in kilowatts?
Given
f = 12000N
d = 9m
t = 15s
P = ?
Formula
P = fd/t
Solution
P = 12000(9) 15
P = 7200 W
7.2 KW

41. Sample Problem
p. 188
Pg. 189 5F
5, 4, 3, 2