Speed and Velocity
• What is speed?
• How can you use a distance-time
graph to calculate average speed?
• What are ways velocity can change?
Speed and Velocity
• speed
• constant speed
• instantaneous speed
• average speed
• velocity
What is speed?
Speed is a measure of the distance an
object travels in a unit of time.
What is speed?
What is speed? (cont.)
• Calculate speed by dividing the distance
traveled by the time it takes to go that
distance.
• speed = distance/time (speed = d/t)
What is speed? (cont.)
•The SI unit for speed is meters/second;
kilometers/hour; miles/hour
What is speed? (cont.)
Constant speed: the rate of change of
position in which the same distance is
traveled each second.
Page 19
• What distance has the car traveled at 2 s?
• How long will it take the car to travel 132 m?
What is speed? (cont.)
When the car’s speed changes, it moves
a different distance each period of time.
What is speed? (cont.)
• Instantaneous speed: speed at a specific
instant in time.
Questions
• What is instantaneous speed?
• How would the distance the car travels
each second change if it were slowing
down?
Page 19
• What is the bottom car’s instantaneous
speed at 6 s?
What is speed? (cont.)
• Average speed: the total distance
traveled divided by the total time taken
to travel that distance.
Questions
• What is meant by total time in the
definition of average speed?
• How is a car’s average speed different
from its instantaneous speed?
• When might it be useful for a driver to
know the average speed of his or her car?
• When might it be useful to know the
instantaneous speed?
Distance-Time Graphs
• distance-time
graphs: graphs that
compare distance
and time
• Constant speed is
shown as a straight
line on a distancetime graph.
• How would the d-t
graph be different if
the horse’s speed
changed over time?
Questions
• What are distance-time graphs?
• How is constant speed shown on a d-t
graph?
• Do you think distance & time could also be
represented on a circle graph? Why or
why not?
Distance-Time Graphs (cont.)
• You can use d-t
graphs to compare
the motion of two
different objects.
• The steeper line
indicates a faster
speed.
Distance-Time Graphs (cont.)
• How far behind was
horse B when horse A
reached the finish line?
• If horse B’s speed
remains constant, at
what time will it cross
the finish line?
Distance-Time Graphs (cont.)
• You can use distance-time graphs to
calculate the average speed of an object.
• The difference in distance between two
points is divided by the difference in time
between the same points.
Distance-Time Graphs (cont.)
diff in distance/diff in time = avg speed
• What is the first step in calculating average
speed using a d-t graph?
• How does avg speed of the horse from 60 s to
120s compare to its avg speed from 120 s to
180 s?
Distance-Time Graphs (cont.)
How can you use a distancetime graph to calculate
average speed?
Distance-Time Graphs (cont.)
If the speed of an object changes instead
of being constant, its motion on a
distance-time
graph is a
curved line.
Distance-Time Graphs (cont.)
Changing Speed
• Slowing: the slope of a line on the d-t
graph decreases; meaning the speed of
the obj is decreasing
• Stopping: a horizontal line on the d-t
graph indicates no motion
• Speeding up: upward curve of a line on
the d-t graph
• Even when the speed of an object isn’t
constant, its avg speed can be calculated
from a d-t graph.
• How is this graph the same as the graph in
Fig 9?
• How is this graph different from the graph
in Fig 9?
• How far has the
train traveled
when it reaches
the ending
point? How
long does it take
the train to
make this trip?
•What is the avg speed of the train for the
trip show in Fig 10?
•What might account for the lack of motion
that occurs between 3 min and 5 min?
Velocity
Velocity is the speed and the direction of
a moving object.
• The velocity of an object can be
represented by an arrow.
• A greater speed is shown by a longer
arrow.
• The arrow points in the direction of the
object’s movement
Velocity (cont.)
Velocity changes when the speed of an
object changes, when the direction that
the object moves changes, or when both
the speed and the direction change.
• Which symbol is used to represent
velocity?
Figure 11 (page 23)
• Why are the two arrows show in Fig 11 the
same length?
• If, instead, the girl walks in the same
direction as the boy at a speed of s m/s,
how would the arrows be drawn? (Fig 11)
Velocity (cont.)
How can velocity change?
Figure 12 (page 23)
• Are there two positions of the bouncing ball in
which the velocity is the same? Explain.
• When does the ball have the greatest speed?
• Speed is a measure of the distance an
object travels in a unit of time. You can
describe an object’s
constant speed,
instantaneous
speed, or average
speed.
• A distance-time
graph shows the
speed of an object.
• Velocity includes both the speed and
the direction of motion.
The total distance traveled divided
by the total time taken to travel that
distance is known as which of
these?
A. speed
B. constant speed
C. instantaneous speed
D. average speed
Units of speed are units of
distance divided by which of
these?
A. units of speed
B. constant speed
C. units of time
D. average speed
How is constant speed
represented on a distance-time
graph?
A. a wavy line
B. a crooked line
C. a straight line
D. a curved line
Do you agree or disagree?
3. Constant speed is the same thing as
average speed.
4. Velocity is another name for speed.