Glencoe Chapter 3
Describing Motion
Who Wins?????
Racer with the fastest speed?
 Racer with the shortest elapsed time?
 What is motion?
 What is speed?
 How is speed different from velocity?

Battery Buggy

After watching the
buggy, how is it’s
motion
different/similar to
a 100 meter
sprinter? What if
the batteries were
close to dead?
Motion is Relative


Perception of motion by
humans is always
related to nearby
objects.
Even if there is motion,
but no nearby reference,
motion is not perceived
by humans.
3.1 Picturing Motion
When an object moves, it’s position
changes.
 A MOTION DIAGRAM is a tool that
may be used to study the motion of
an object.

How do you know the answer is correct?
Motion Diagrams—you draw!
What kind of motion does the
diagram depict?
Operational Definition


An Operational Definition defines a
concept in terms of the procedure or
operation used.
Examples




Rest? Object does not change position with
relation to a reference point.
Constant speed? Object travels equal
distances in equal time periods.
Acceleration?
Deceleration
What’s Happening here?
Or here?
Particle Model

These types of Diagrams are a little
difficult to draw. Maybe we can
substitute something else for little
pictures of the moving object…
A ticker-tape timer
Ticker Tape: "slower" acceleration
** *
*
*
*
*
*
*
Ticker Tape: "Faster" acceleration
** *
*
*
*
*
Ticker Tape: "faster" deceleration
*
*
*
*
* **
Ticker Tape: "slower" deceleration
*
*
*
*
* **
Ticker-tape
Math
Speed = distance
time
From timer device rate
Time Taken = 5 X .025s = 0.125 s
Now you try it….

Page 46 of your text, #1-4
3.2 Where and When?
Coordinate Systems—Tell where zero
point of variable you are studying is
and the direction in which the value of
the variable increases.
 Origin—Point at which values for
variable are zero.

X-Y Coordinate System

Motion of many types may be shown on a
coordinate system, for example





a ball thrown upward
A cannon firing a cannonball
a high diver diving off a board
A high jumper
We can use an x-y system,


X is horizontal movement
Y is vertical movement
We can define upward as positive,
which is standard.
 We can define to the right as positive
 However, you can choose any
direction as either positive or
negative, just make sure the opposite
direction has an opposite sign.

Symbols in Physics
Symbols are frequently used to represent
quantities in Physics
m represents mass
SI unit kg
t represents time
s
s represents speed
m/s
d represents distance
m
a represents acceleration
m/s2
v represents velocity
m/s
Δd represents displacement
m
Vectors and Scalars


Another way of representing the position of an
object would be to use a vector. Vectors or
scalars are used to represent quantities in
physics.
A scalar is a line segment, whose length
represents only magnitude


Time is a scalar quantity
 A Time interval is the difference between two time
periods i.e 2 hours interval between 2:00 and 4:00
Mass is a scalar quantity, along with others
Vectors


A vector is a ray whose length represents
magnitude, and the ray points in the
direction.
Vectors are used to represent quantities
that have both magnitude as well as
direction.

Displacement is a vector quantity
 Is the distance and direction between two
positions
Examples






Speed………..
Velocity……...
Acceleration..
Time………….
Distance…….
Force………
Scalar quantity
Vector quantity
Vector
scalar
scalar
vector
Vector Quantities

Vector quantities
are represented
with arrows over
the symbol.

Velocity

acceleration
Examples

Equation for acceleration
A position vector
drawn from origin
to position of object
y
d0
x
Position Vectors
Another Position vector
Velocity Vectors
Displacement vs. Position

Position is the position where an object is
located.


Indicated as “d”, or sometimes d0, d1
Displacement is the change in position of an
object between two time periods.

Indicated as “Δd”

Found by subtracting, d1- d0
 Δd
= d1-d0
Time Interval

Time intervals (Δt) are defined as the time
between two time periods.
Δt = t1-t0
Displacement vectors drawn between
positions at two different time periods
Displacement (Δd) is equal to difference between two
position vectors (d1-d0)
Δd = d1-d0
Velocity vectors drawn for each time period, length
indicates magnitude (speed)
Distance vs. Displacement



You drive the path, and your odometer goes up by 8
miles (your distance).
Your displacement is the shorter directed distance
from start to stop (purple arrow).
What if you drove in a circle?
start
stop
3.3 Velocity and Acceleration


Motion diagrams can be used to show
objects moving with different speeds and
different kinds of motion.
Motion diagrams can show position and time,
but can we combine them to determine a rate
of motion?
3.3 Velocity and Acceleration
Velocity  displaceme nt
time interval
** Speed and velocity are similar except
speed is scalar and velocity vector.
Velocity
Velocity = {speed with a direction}
Examples:
70 mph is a speed.
70 mph North is a velocity.
average velocity = displacement
time interval
d
v
t
_

instantaneous velocity - the velocity
that something has at any one
instance

(The terms instantaneous speed and avg.
speed may also be used)
The average velocity for a
trip might be 53
miles/hour.
However, during this trip
your instantaneous speed
might have been 0
miles/hour at a stoplight
or 70 miles/hour on the
open road.
Instantaneous Velocity


Average Velocity, because it is calculated for
a time interval, can actually not be calculated
for a particular time period
Instantaneous Velocity—velocity at a
particular time period. Generally, however,
when used in this text, the term “velocity”
refers to an Instantaneous Velocity.
Displacement from Velocity
d  vt
Final Position : This equation can be used
to find “where” an object will be at some time
period.
Substitute
To get
d  d 1  d 0
Into
d  vt
d 1  d 0  vt
Then rearrange to get
d 1  d 0  vt
Acceleration

Acceleration - rate of change in
velocity due to change in speed or
direction
Acceleration = change of velocity
time interval
v
v1  v 0
a

t
t


Example:
 9.8
meters/second2 downward
Positive vs. Negative




Is it possible to have a negative velocity and
a positive acceleration?
Is it possible to have a positive velocity and a
negative acceleration?
Is it possible to have a positive velocity and a
positive acceleration?
Is it possible to have a negative velocity and
a negative acceleration?
Velocity & Acceleration Sign
Chart
VELOCITY
A
C
C
E
L
E
R
A
T
I
O
N
+
-
+
-
Moving forward;
Moving
backward;
Speeding up
Moving forward;
Slowing down
Slowing down
Moving
backward;
Speeding up
End of Chapter 3
☻ ☻ ☻ ☻ ☻ ☻ ☻ ☻
☻
☻
☻
☻
☻
☻
☻
☻
☻
☻
☻
☻