Bellaire High School Advanced Physics Chapter 5 - Energy

1. Lesson 5-1
Definition of Work

2. Definition of Work
 So far, many of the terms we have discussed
have had similar scientific and real world
definitions
 Usually when we say ‘work’, we think of doing
something that requires physical or mental
effort
 In Physics, work is very different

3. Definition of Work
 Consider the following:
 A student holds a book at arms length for several
minutes
 A student carries a bucket of water along a
horizontal path
 Even though work is required for both of
these actions, no work is done on the book or
the bucket

4. Definition of Work
 Only when a force displaces an object is work
done on the object
 Imagine your car runs out of gas
 If you push your car with a constant force to the
gas station, you are doing work on your car
 Work is equal to the applied force times the length
of distance the force is applied
W=Fd

5. Definition of Work
Work is not done on an object unless the
object is moved by a force
 That is why no work is done on the book in our
previous example
 No work is done because the book is stationary
 That is why no work is done on the chair in our
previous example
 Work is done within the body to move, but none on the
chair

6. Work
Work is done ONLY when components of a
force are parallel to a displacement
 When application of force and displacement are in
different directions, only the parallel component of
force to the displacement does work
 Perpendicular forces do no work

7. Parallel Forces Do Work
 Imagine pushing a crate across the floor
 If you get very low, almost laying on the ground,
and push exactly horizontally
 All of your force will go into moving the crate
 If you push at an angle, only your horizontal
component will help move the crate
 The vertical component ‘drives’ the crate into the
ground and does no work to help you move the crate
 Only forces parallel to the displacement do
work

8. Units of Work
 The SI unit of work is the Joule
 Joules = Force times length
 =Newton Meters
 Sample pg 169
 Practice pg 170

9. Sign on Work
Work is a scalar quantity and can be positive
or negative
 Work is positive when the component force is in
the same direction as the displacement
 Lifting a box, force and displacement in the same
direction
 Work is negative when the component force is in
the opposite direction as the displacement
 The force of friction between a sliding box and the
floor

10. Sign on Work
 If you carried a box into the next room, what
would be the sign on the work done on the
box?
 Since no work is done, sign does not matter, its
like asking “What is the sign on zero?”

11. Sign on Work
Work may result in a change in velocity
 If the work is in the same direction as the
displacement, how will the velocity change?
 Increase
 If work is in the opposite direction, how will the
velocity change?
 Decrease

12. Lesson 5-2
Energy

13. Kinetic Energy (KE)
 Kinetic Energy is energy associated with
motion
 Kinetic Energy depends on the speed of an
object
 As an object’s speed increases, the object’s KE
increases

14. KE
 If a bowling ball and a volley ball are rolling at
the same speed, which has more KE?
 You may think that they have the same amount
since they are traveling at the same speed
 KE depends on speed and mass
KE = 1 mv
2
2

15. KE
 KE is a scalar quantity
 The SI unit is the Joule, just like work
 As per the KE/Work theorem, work is a type of
energy
 Sample pg 173
 Practice pg 173

16. Potential Energy (PE)
 A perfect example of energy is the
‘Skycoaster’ at Kennywood.
When the riders are at the top, they are not
moving, so they have no KE.
 Recall, energy cannot be created or destroyed, so
the KE must go somewhere while the riders are
stationary at the top
We explain the lack of KE as Potential
Energy

17. PE
 Potential energy is concerned with the
position of the object, not the speed
 PE is stored energy
 Describes an object’s potential to move based on
its relationship to another location

18. Gravitational PE
 Gravitational PE depends on height from a zero
level
 The energy associated with an object due to the object’s
position relative to a gravitational source is Gravitational
PE
 If a ball falls off of a table, it gains speed.
 From where does the speed come?
PE mgh g =
 SI unit for PE is also the Joule

19. Gravitational PE
 This concept is valid only when free-fall
acceleration is constant, such as near the
Earth’s surface
 Gravitational PE depends on both height and
free fall acceleration, neither of which are
properties of an object
 For that reason, PE of an object is relative

20. Gravitational PE
 For instance, lets say a ball is dropped from a
second story roof and lands on a first story
roof
 If PE was measured from the ground, PE is NOT
now zero
 If PE was measured from the first story roof, PE
IS now zero
 Is it possible to have a negative PE?
 Is it possible for the same object to have both positive
and negative PE at the same time?

21. Gravitational PE
 The zero level is the level where PE = 0
 It can be chosen specific for each situation
 The zero level should be chosen carefully so
as to make the most sense for the specific
situation

22. Elastic PE
 Another type of PE is that of elasticity
 Depends on the compression or stretching of an
elastic object
 Examples?
 Imagine a pinball machine
 The plunger is pulled back, compressing a spring
 When released, the plunger flies forward and
propels the ball
 The ball travels because of the stored PE in the spring

23. Elastic PE
When a spring is not compressed or
stretched, it is said to be in a relaxed state or
relaxed length
When external forces compress or stretch the
spring, the spring stores PE
When the spring is released, the PE is
converted to KE
 The amount of PE is directly related to the
amount the spring was stretched or compressed

24. Hooke’s Law
 Named after British
Physicist Robert Hooke
 Mathematically
approximates the PEelastic
of a spring
PE kx elastic = 1
2
2

25. The Spring Constant
 The symbol k is called the spring constant
 For a flexible spring, k is small
 For a more rigid spring, k may be huge
 The spring constant is measured in N/m
 You will either be given k or asked to solve for k.
 You are not expected to just ‘know’ what k is.

26. Mechanical Energy
 Descriptions of motion of many objects
involves a more complete energy approach
 For example, think of a clock with a pendulum
 While the pendulum swings, it is constantly
converting PE into KE and KE into PE
 Also, there is elastic PE from the many springs
helping to power the clock

27. Mechanical Energy
 The expressions of these energies are
relatively simple
 Energies such as nuclear and chemical are
not so simple, but often they can be ignored
because they are not directly relevant to the
situation being analyzed

28. Mechanical Energy
Mechanical energy is the total sum of kinetic
and potential energies associated with an
object or group of objects
ME = KE +åPE
 Energy that is not mechanical is called non-mechanical
energy

29. Lesson 5-3
Conservation of
Energy

30. Conserved Quantities
When we say something is conserved we
mean that is remains constant
 That does not mean the quantity cannot change
forms during that time
 But if at any given time, if we consider all
forms of the quantity, we will have the same
amount at all times.
 An example of a conserved quantity is mass

31. Conservation of Energy
 Energy cannot be created nor destroyed
 That is to say, energy is always conserved
 But when we drop a ball, the ball does not
return the original height. Why not?
 Energy is lost through friction, sounds, heat

32. Conservation of Energy
 If we ignore these outside types of energy,
we see that mechanical energy is totally
conserved
ME ME i f =
ME = KE + PE
KE = 1 mv
PE = mgh 2 and
2

33. Conservation of Energy
 If we make the final equivalent substitutions,
we see that mechanical energy is
mathematically:
1
2
1
2
mv 2 mgh mv 2 mgh i i f f + = +

34. Conservation of Energy
 Notice that mass shows in every term
 Recall: all objects fall at the same rate no matter
their mass
 Do you need to know the mass to work this equation?
 Sample pg 181
 Practice pg 182

35. Lesson 5-4
Work, Energy, and
Power

36. The Work – KE Theorem
 Imagine sliding a hockey puck across the ice
 We know there exists a small amount of Fk
 The puck slows and eventually stops
We also know from our study of energy that
mechanical energy is not totally conserved
 There is a relationship between the energy
lost and the work done to an object

37. The Work – KE Theorem
 The Work – KE Theorem is defined as
W KE net = D
 Notice the type of force is not specified
because it could be any force working on any
object
 The theorem is universal for all objects

38. Extension of the Work – KE
Thm.
 The extension of the theorem is useful when
work is done by friction
W ME friction = D
 If there is no friction then:
 The equation can be simplified
DME = 0
ME ME i f =

39. Work – KE Theorem
 Notice the Work – KE Theorem in any form is
a method of transferring energy
 Recall that a force perpendicular to
displacement does no work
 The force must be parallel to the displacement for
work to be done
 If the force is perpendicular, and no work is
done
 No energy is transferred

40. Distinction Between Equations
W = Fd(cosq )
 Is the work done by an object on another
object
W KE net = D
 Relates net work done on an object to the
change in KE
 Sample 185
 Practice 186

41. Power
 The rate at which work is done is called
power
 Power is the rate of energy transferred by any
method
P W
=
D
t

42. Power
We may also rewrite the equation substituting
the definition of work
W = Fd P F d
 Therefore:
t
=
D
d
t
, and
v
D
=
P = Fv
, and recall

43. Unit of Power
 The SI unit of Power is the Watt
 Watts are most common in light bulbs
 A dim light bulb may require 40 W to power it
 A bright light bulb may require 500 W to power it
 Horsepower is also a unit of power
 1 HP = 746 W = 746 J/s
 Sample 188
 Sample 188 Explanation
 Practice 188