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Work…

In everyday speech work
has a very general meaning.
In describing motion in
physics, work has a very
specific meaning.
Chair Example
Standing
Walking
No work is done on the chair


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
FORMULA
W = fd
IN DIRECTION OF MOTION
 The symbol for work is W
 Work has 2 acceptable units
 Nm
 Joules (J)
JOULE
Lifting an apple about
2ft is a Joule
 3 good push-ups is
about 1000J

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
10N
W = fd
W = 20J of Work
Moves 2m
Notice direction of
motion is the same
as the applied force
Y
10N
How would you
solve this? Force
applied is NOT in
the same direction
as the objects
motion.
60°
X
2m
Think back to vectors and use the
component of the force applied in the
direction the object moves.
COS θ = adj/ hyp
COS 60° = force parallel to motion
10N
Y
10N
force para. =
COS 60° (10N)
60°
X
force para. = 5N
2m
w = F(parallel) D
5N (2m) = 10Nm
W = Fd (COS θ)
Always measure angle with horizontal!
The above formula works in every case
θ = 0°
θ = 90°
No work because
no motion in
direction of force
Pg. 170
Problems
1-4
Section Review p. 171
With your Neighbor, answer questions
2, 3, 4
Section Review Answers
2 the neighbor, twice as much
3 a-negative




B-positive
C- negative
4 a-yes


B- no
C-yes
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
Kinetic Energy



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
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

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


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)
Practice Problems 5C
p.176
#1 and 4 only
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
1. GRAVITATIONAL POTENTIAL
ENERGY
2. ELASTIC POTENTIAL ENERGY
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
Elastic Potential Energy
(PEelastic)

The energy associated with
a stretched or compressed
elastic object
 Spring, bungee cord,
rubber band
Elastic Potential Energy
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 1-3
pg. 180
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.
5Kg
5Kg
ASH
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
PE = min
KE = max
KE = min
A falling egg
Mass = .1kg
PE = mgh
Height = 10m
PE = 10 J
KE = 0 J
10M
PE = 5 J
KE = 5 J
PE = 0 J
KE = 10 J
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

Mechanical Energy
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)
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
The Law of Conservation of Energy:
The total energy of a closed system
is constant.

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
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
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
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.

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

Watts

Watts are named after
James Watt, the inventor of
the steam engine
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
Formula
f = 12000N
Solution
P = fd/t
P = 12000(9)
d = 9m
t = 15s
P=?
A. P = 7200 W
B. 7.2 KW
15
Sample Problem
p. 188
Pg. 189
5F
5, 4, 3, 2