CHAPTER 1 – DESCRIBING MOTION
LESSON 2 – SPEED AND VELOCITY
What is Speed?
Speed is the measure of the distance
that an object travels per unit of time.
 Speed of a running cheetah: 30 m/s
 This means 30 meters per second.
Units
For expressing speed, two units are
needed: distance divided time.
Example
Ways to express the speed of a car
going at a constant speed on a
highway:
27 m/s
97 km/h
 27 meters per second
 97 kilometers per hour
 60 miles per hour
60 mph
Constant Speed
With constant speed the rate of change
of position is the same for every second
(or hour).
Changing Speed
In this case an object moves faster and
faster or is slowing down.
 Question: How would the distance
that a car travels change while it is
slowing down?
You may want to know the speed at a
specific moment.
 Instantaneous speed is the speed at
a specific instant in time.
 A car’s speedometer shows the
instantaneous speed.
Average Speed
This is the total distance travelled
divided by the total time taken to go
that distance.
v= d
t
v is the average speed
d is distance t is time
 Be sure to include the units you use.
Example
A model rocket travelled 360meters in
4 seconds
v = d = 360 m = 90 m/s
t
4s
For practice, try the examples at the
bottom of p. 19.
Distance-Time Graphs
 This graph is for two horses running
at constant speed.
2.0 –-
Horse
A
distance (km)
1.0 –
60s
Horse
B
time (s)
120s
 Note how the graph makes a
comparison for the same time
period (60 seconds to 120 seconds).
 Which horse is going faster?
 What is the speed of each horse?
Comparing Speeds with a DistanceTime Graph
See previous example.
Using a Distance-Time Graph to
Calculate Speed
To calculate speed you need a distance
and a time.
Steps:





Choose two convenient points on
the line.
Using the points, draw a right
triangle under the line.
Use the vertical side of triangle to
find the distance, d, by subtraction.
Use the horizontal side to find the
time, t, by subtraction.
Then use v = d
Include the units!
t
Example (similar to the one on p. 21)
600m
distance
(meters)
200m
35s
time (seconds)
90s
 Distance is 600m – 200m = 400m
 Time is 90s – 35s = 55s
v = d = 400 m = 7.3 m/s
t
55 s
(Do not write 7.272727 m/s)
Distance-Time Graph and Changing
Speeds
The graph is a curve, not a line.
d
distance
(m)
b
a
c
0
time (min)
This graph is for a train.
 Between 0 and a the speed increases.
 Between a and b the speed decreases.
 Between b and c the train is stopped.
 Between c and d the speed increases.
You can calculate the average speed of
the train for any part of its trip
 Choose a starting point and an
ending point.
 Find the change in distance, d,
between those points.
 Find the change in time, t, between
those points.
 Use
v= d
t
 Be sure to include the units.
Velocity
 This gives a full description of the
motion of an object.
 Velocity gives the speed and
direction of a moving object.
Representing Velocity
 This is done with an arrow.
o (It is actually a vector.)
 The arrow shows the direction.
 Its length represents speed.
Examples
Two bugs start next to each other, then
crawl away.
Speed and direction change for the ball.
Are there any positions of the bouncing
ball where the velocity is the same?