How can we describe the various motions in this situation?
Chapter 2 Describing Motion
I.
Speed
A.
70 miles per hour
1) In 1 hour, we would travel 70 miles
2) Speed has a number (70) and a unit (miles per hour)
3) Miles per hour = miles/hour = mph = miles  hours (distance/time)
B.
Average Speed = distance traveled divided by time of travel
distance
d
1)
speed 
2)
time
s
t
Average speed is a rate
x
miles
runs
cents
or
or
or
y
gal
game
dollar
C.
Converting units of speed: always distance/time
1) Common Speeds
m/h
Km/h
m/s
20
32
9
40
64
18
60
97
27
80
130
36
100
160
45
2)
Examples:
a. How fast in Km/h is 70 m/h?
-Conversion Factor: 1 Km = 0.621 m
 1Km   0.621m   1 


    1
 0.621m   1Km   1 
-Calculation:
 70m  1Km 


  112.7 Km/h
 h  0.621m 
b.
What is the conversion factor of Km/h to m/s?
 1Km  1000m  1h  1min  0.278m





s
 h  1Km  60min  60s 
-1Km/h = 0.278m/s
D.
Instantaneous Speed
1) A Speedometer measures instantaneous speed
2) When you use different “speeds”
a) Instantaneous speed tells you about the moment
b) Average speed tells you how long the trip will take
3) Definition of Instantaneous Speed = rate for a given instant = average
speed for a short time period where the speed changes little
4)
Animation02
II.
Velocity
A.
Velocity = speed in a defined direction
1) 20 mph = speed
2) 20 mph due North = velocity
B.
Changes in Velocity
1) Car on a curve: speed stays the
same, velocity is always changing
2) Any change in speed or direction
is a change in velocity
C.
Instantaneous Velocity = average velocity for a short time period where the
velocity changes little
D.
Changes in Velocity are caused by forces
1) Baseball: force of gravity, hand throwing and catching, wind, etc…
2) We will discuss forces more later
III. Vectors = quantities where size and direction are important
A.
Definitions
1) Magnitude = size of the vector, a number
2)
3)
4)
Direction = orientation of the vector relative to a fixed reference
Vectors are written as bold (v) or with a bar on top ( v )
There are many vector quantities in Physics: velocity, forces, acceleration,
momentum, etc… (Speed s is not a vector)
5)
B.
Vectors are represented by arrows (Velocity is always blue in the book)
Math with vectors: Appendix C
1) Specifying magnitude and
direction on a graph
2)
Vector Components = any 2 or more vectors that add up to the original
vector (usually components on the x and y axes are used)
3)
Adding Vectors
a) Plot first vector on a graph
b) Plot the second vector starting at the end (arrowhead) of the first
c) Draw the sum vector from the origin to the last arrowhead
a+b=c
4)
Subtracting Vectors
1) Plot first vector on a graph
c
2)
Find what the negative of the second vector is
a) Same magnitude as the positive vector
b) Opposite direction as the positive vector
3)
Add the negative of the second vector to the first
a
a–b=c
a
b
a
b
-b
c
-b
5)
More Examples