Motion
Three Terms to know:
• Position. This is precisely where an object
is located.
• Speed. Precisely how fast an object is
moving.
• Acceleration. Precisely how fast an object's
speed is changing. (most challenging for
people)
What is Motion?
• Motion can be described
as a change in position.
• To know whether the
position of something
has changed, you need
a reference point.
(Train Example)
Motion and Time
• Descriptions of motion often include
speed
• If you think of motion as a change in
position, then speed is an expression of
how much time it takes for that change
in position to occur
Speed…more definitions
• Speed is the rate of change in position.
• Speed can also be described as simply
a “rate of motion”
Measuring Speed
• The speedometer in
a car shows
instantaneous speed
• Instantaneous speed
is the rate of motion
at any given instant.
Average vs. Instantaneous Speed
Speed Triangle/Pyramid
The Equations
• Distance = Speed x Time
• Speed = Distance / Time
• Time = Distance / Speed
Changing Speed
• Think about riding a bicycle to your friends house
2 km away
• You start from a stop. Speed = 0 km/hr
• You reach a speed of 20 km/hr
• As you pedal up a steep hill your speed drops to 12 km/hr
• As you go down the hill your speed increases to 35 km/hr,
and you pull into your friend’s driveway
• You look at your watch and see the trip has taken you
30 minutes
• How would you express your speed on such a trip?
Would you use your fastest time, your slowest
time, or some speed in between?
Average Speed
• In cases where the rate of motion (speed) varies a
great deal, the best way to describe speed is to use
average speed.
• Average Speed is the total distance traveled
divided by total time of travel
• So from the last slide: If the total distance was
2 km, and the total time was 30 minutes how fast
was the cyclist going?
Example Problem
• Your neighbor says she can skate at a speed
of 4 m/s. To see if you can skate faster, she
times you as you skate as fast as you can for
100 m.
• Your time is 20 seconds. Who skates faster?
Answer
1. What is known/given?
Distance, d = 100m
Time, t = 20s
2. What is unknown? (what are you trying to find?)
Average Speed = ?
3. Choose an equation with the appropriate variables
s = d / t
4. Input the values and solve the equation
s = 100m / 20 seconds = 5 m/s
Velocity
• Speed with a direction…
– 2 m/s SOUTH
– 5.6 km/s NORTHWEST
Graphing Speed
• Distance on the Y-axis
• Time on the X-axis
• Title your graphs correctly
• Label your axes correctly
Distance Vs. Time
12
Distance (meters)
10
8
6
4
2
0
T ime
D is tanc e
0
1
2
3
4
5
6
7
8
9
10
0
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
Time (seconds)
8
9
10
11