Physics ACT Review
Displacement vs. Distance
• DISTANCE
– the COMPLETE length of the PATH
traveled by a moving object
• DISPLACEMENT
– the length of the STRAIGHT LINE PATH
from a moving object’s ORIGIN to its
FINAL POSITION
Scalar vs. Vector
• SCALAR
– A measured quantity that has NO DIRECTION
– Examples
• Distance, Time, Mass, Volume
• VECTOR
– A measured quantity that HAS DIRECTION
– SIGN SHOWS DIRECTION
– Example
• Displacement
Examples
Sign of displacement refers to the direction
the object is moving .
A bird fliesAA5ball
cat
meters
rolls
runsnorth,
58 meters
meters
then
north.
west.
7 meters south
Distance = 512
8m
mm
Displacement = -2
+5m
-8
m
Distance vs. Time Graphs
Distance vs. Time
18
16
2 – 3 seconds
12
The interval
on the graph where
10
the distance remains constant!
Distance (m)
14
8
6
4
2
0
0
1
2
3
4
5
6
7
Time (s)
During what time interval was the object NOT MOVING?
Displacement vs. Time Graphs
Displacement vs. Time
6
5
Displacement (m)
4
3
2
1
0
-1 0
The object’s final position is
at +1 meter
(1 meter to the
When
the displacement
is
of the
origin)
the
object
has a
1 – 2 and negative,
4 – 5right
seconds
position to the
left that
of the origin
Constant displacement
means
the object doesn’t move
1
2
3
4
5
6
7
-2
-3
-4
Time (s)
During
During
At
what
what
what
time
distance
time
interval(s)
interval(s)
from was
thewas
origin
thethe
object
object
doestothe
NOT
theobject
left
MOVING?
ofstop?
the origin?
Example
• A man drives his car 3 miles north, then 4
miles east.
4 mi
East
3 mi
North
Distance
7 mi
Displacement
5 mi
Northeast
What distance did he travel?
What is his displacement from his point of origin?
Velocity vs. Speed
• VELOCITY
– change in DISPLACEMENT occuring over TIME
– MAGNITUDE and DIRECTION
• VECTOR
• SPEED
– change in DISTANCE occuring over TIME
– MAGNITUDE ONLY
• SCALAR
Average Velocity
vavg
d d f  d i


t
t f  ti
What does this remind you of?
SLOPE OF AWhat
GRAPH!
is happening in this
Position vs. Time
Position
Position
graph?
INCREASING
SLOPE
CONSTANT
CONSTANT
POSITIVE
ZERO
SLOPESLOPE
Time
Moving with
Motionless
INCREASING
CONSTANT
Object
positive
velocity
velocity
Using v-t Graphs
Velocity (m/s)
Velocity vs. Time
35
30
25
20
15
10
5
0
-5 0
-10
-15
-20
What can we DO
with a v-t graph?
Find average
velocity
1
2
3
4
5
6
7
8
9
10
11
Find distance
traveled
Time (s)
Area
under
under
the
graph to find
How doArea
you use
thethe
v-tgraph
graph
DISTANCE
AVERAGE
DISPLACEMENT?
TRAVELED?
VELOCITY?
Area
AreaononTOP
TOPand
= POSITIVE
BOTTOM
Area
both
onconsidered
BOTTOMPOSITIVE
= NEGATIVE
Find
displacement
Summary
Car
Bird
Distance = 100 m
Displacement = 70.7 m
Distance = 70.7 m
Displacement = 70.7 m
Avg Speed = 2 m/s
Avg Velocity = 1.4 m/s
Avg Speed = 1.4 m/s
Avg Velocity = 1.4 m/s
Note that the bird has the same average
SPEED and VELOCITY because its
DISTANCE and DISPLACEMENT
were EQUAL!
• ACCELERATION
– change in VELOCITY occuring over TIME
– units are METERS PER SECOND2
– VECTOR
Negative
Positive Velocity
Velocity
Negative
Positive Acceleration
Acceleration
Speeding
Speeding
Slowing
up
up in
indown
+- direction
direction
Eventually speeds up in +
– direction!
Equations
aavg
v

t
“Acceleration is a rate
of change in velocity”
“The slope of a v-t
graph tells what the
ACCELERATION IS
DOING!”
v f  vi  at
“An object’s velocity
at any point in time can
be found by considering:
- its starting velocity
- its acceleration
- the amount of time over
which it accelerates”
What’s the hurry?
The Kinematics of Freefall
What happens as objects fall?!?
• Physicists DO NOT KNOW WHY objects fall!
• But, we can describe HOW they fall
– As they fall, THEY GO FASTER
– This means that they ACCELERATE!
– They ACCELERATE at a CONSTANT RATE
“g” - The “Magic” Number
• “Little g” is a ‘shorthand’for
ACCELERATION DUE TO GRAVITY
• All LARGE OBJECTS have a “little g” value!
– Examples
• “g” is 1.67 m/s2 on the Moon
• “g” is 26 m/s2 on Jupiter
• “g” is 9.81 m/s2 on Earth
Over the Edge
Horizontal Projectiles
A red ball rolls off the edge of a table
What does its path look like as it falls?
Parabolic path
As the red ball rolls off the edge, a green
ball is dropped from rest from the same
height at the same time.
Which one will hit the ground first?
They will hit
at the SAME
TIME!!!
Push and Pull
Newton’s Laws
Newton’s First Law
An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity (that is, constant speed in a straight line)
unless it experiences a net external force.
Also known as the “Law of Inertia”
Inertia
Tendency of an object to maintain its STATE OF MOTION
Proportional to MASS
Do these guys have a lot of inertia?
MORE MASS
means
MORE INERTIA
LOTS OF INERTIA
hard to…
GET MOVING or
STOP
Force
• A push or pull on an object
• Changes STATE OF MOTION
• CONTACT FORCE
– Physical interaction between objects
– Normal, Tension, Friction
• FIELD FORCE
– “Action over a distance”
– Gravity (Weight)
A block of wood is sitting motionless on a table.
What forces are acting on it?
Normal
Weight
FN
Fg
Normal Force is a
REACTION
force that any
object exerts
when PUSHED ON
Weight is gravity
pulling toward
CENTER of the
EARTH
Net Force
• No NET FORCE if
– MOTIONLESS
– MOVING WITH CONSTANT VELOCITY
• Unbalanced force  CHANGE IN MOTION
• Changing motion  ACCELERATION
Force  Acceleration
• How much acceleration?
• Depends on:
– AMOUNT OF FORCE
• MORE FORCE = MORE ACCELERATION
– MASS OF OBJECT
• MORE MASS = LESS ACCELERATION
Newton’s Second Law
“The acceleration of an object is directly proportional to
the net external force acting on the object and inversely
proportional to the mass of the object.”
Fnet
a
m
F  ma
Unit of force is the NEWTON (N)
Free Body Diagrams
• VECTOR diagrams!
• Shows ALL FORCES acting
on an object
Motionless
Equilibrium
FN
• Must be properly LABELED
Fg
Newton’s Third Law
“For every action, there is an equal and
opposite reaction”
FN
FT
Fg
Fg
Third Law Examples
• A firefighter directs a stream of water
from a hose to the east. In what direction
is the force on the hose?
There will be a force on the hose to the WEST
• A man getting out of a rowboat jumps
north onto the dock. What happens to
the boat?
The boat will move to the SOUTH
Riding the Surf
Wave Properties
Definitions
MEDIUM – a continuous collection of particles
Examples:
AIR
WATER
METAL
PULSE – a single disturbance in a medium
WAVE – a regularly repeating pulse in a medium
that transmits energy without transmitting mass
Transverse Waves
Direction of wave travel is PERPENDICULAR
to the motion of the medium
Example:Ocean waves
Transverse Waves
WAVELENGTH
AMPLITUDE
the– amount
distance
that
from
one
wave
crest
rises
toor
another
falls
CREST
TROUGH
OR– PEAK
the
– the
lowest
highest
pointa
point
on
aon
wave
a wave
Tells how much ENERGY the wave contains
Transverse Waves
• The particles in a
transverse wave only move
UP and DOWN
• ENERGY is transferred
but the particles DO NOT
MOVE in the direction of
wave travel
• More ENERGY means
more AMPLITUDE
Longitudinal Waves
Direction of wave travel is PARALLEL to the
motion of the medium
Example: Sound waves
Longitudinal Waves
WAVELENGTH
AMPLITUDE
– the
–distance
the
size particles
from
of a compression
one
COMPRESSION
RAREFACTION
– area
where
in compression
the medium to
are
are sparsely
densely
populated
populated
another
The ENERGY contained in the wave
Longitudinal Waves
• The particles in a
longitudinal wave only
move SIDE to SIDE
• ENERGY is transferred and
particles MOVE BACK and
FORTH in the direction of
wave travel
• More ENERGY means
more AMPLITUDE
How does it do that?!?
Introduction to Energy, Work, and
Power
Where does FORCE come from?
Potential Energy (PE)
stored in a device or ‘field’
Gravitational PE
Spring PE
Electrical Potential
Chemical Energy
Nuclear Bonding Energy
WORK
(results
in force)
Kinetic Energy (KE)
energy of a
MOVING object
Thermal (Internal) Energy (Q)
“Waste Energy” or Heat
lost during any energy transfer
Definitions
• Energy
– the ability to do WORK
• Work
– Release of ENERGY in MOVING an object.
– FORCE exerted through a DISTANCE
• Power
W = F·d
– WORK done in a certain amount of TIME
P = W/t
Units
• Unit of energy  JOULE (J)
• Work is “change” in energy  JOULE (J)
• Power is ENERGY / TIME  WATT (W)
Law of Conservation of Energy
• The energy of a closed system will always remain
constant – energy cannot be created or destroyed.
• Seems to be violated by “waste” energy
• We must INCLUDE waste energy!
ETOT = KE + PE + Q
Electricity Comes Alive
Electrical Current
How can we manipulate energy in
electric fields?
Apply FORCE to push like charges TOGETHER
FA
+
+
FA
Apply FORCE to push unlike charges APART
+
FA FA
-
How much electrical PE?
• Electrical P.E. is also called VOLTAGE or
POTENTIAL DIFFERENCE
• Unit for electrical potential  VOLT
• 1 Volt = 1 Joule/Coulomb
W
V
q
Amount of work done in moving
a charge and the amount of charge
moved
Current
Current: is the rate at which charge flows
through a given point
Current can only be sustained if there is a POTENTIAL
DIFFERENCE or VOLTAGE between two points!
The unit of current is the AMPERE or AMP
1 ampere = 1 COULOMB PER SECOND
1A = 1C/s
Resistance
Resistance: is a measurement of how strongly
an object will oppose current
An object’s resistance depends on FOUR factors:
Resistivity
Is specific to a material – the
higher it is, the more natural
resistance the material has.
Cross-sectional Area
How big is the object across?
The wider it is, the more current
it will allow to pass.
Length
How long is the object?
The longer it is, the more it will
oppose current flow.
Temperature
If the object is warm, the
molecules inside will be bouncing
around more – opposing current.
Magnetism
• Magnetism is a FIELD FORCE
– Acts over a DISTANCE without CONTACT
• Magnets produce FIELDS around them that
influence some types of metal
– IRON, NICKEL, and COBALT
• Closely related to ELECTRICITY
Light is a WAVE
Electromagnetic Spectrum and
EM Waves
EM Waves
• Light is part of the ELECTROMAGNETIC
SPECTRUM
• EM WAVES are radiated by ALL objects at the
SPEED OF LIGHT
• c = 3 x 108 m/s (in vacuum)
Example
v = fλ #1
What is the wavelength
of a radio
wave traveling
8
4
3 x 10 m/s = (5 x 10 Hz) λ
through outer space with a frequency of 5 x 104 Hz?
λ = 600 m
EM Wave Speed
• Depends on theExample
MEDIUM#2
OF TRAVEL
What is the speed of light in lucite?
c
 depends on its
• The “speed” of an
medium
density and is described by
its INDEX OF
v
REFRACTION (n)n = 1.50
Index of Refraction = speed of light in vacuum
n=c/v
speed
• HIGH index = SLOW
medium
8 of light in medium
1.50 = (3 x 10 m/s) / v
v = 2 x 108 m/s
Wein’s Law
EM Waves
ALL objects emit EM radiation with a frequency related
to their temperature
Outer space  3K
Humans  310K
Sun  5630K