Measuring Motion
Chapter 11
What is Motion?
• Motion is changing position along a certain path.
• In the first part of physical science, we have 2 goals:
• To understand how things move (kinematics)
• To understand why things move (dynamics)
How do things move?
• Vocabulary
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Frame of reference
Distance
Displacement
Speed
Velocity
Slope
Acceleration
Gravity
• As we go through the section, create flashcards for these
words.
Frame of Reference
• A reference point is a starting point… a place that everyone
calls zero.
• When everyone knows where 0 is, then it is much easier to
describe the location or position of objects.
• Example: In races, the starting line is 0, or your reference
point. Everything is measured from the starting line.
Distance vs. Displacement
• Distance is the total path an
object travels.
• Measures how far an object
moves along a path.
End
• Displacement is the line
from the beginning to the
end.
• Measures how far between
the starting point and ending
point.
Start
Distance vs. Displacement
• You start at your house (0). You go to the store. Then you go to your
friend’s house.
• What is the distance that you travel?
• 8 houses
• What is your displacement from the beginning to the end?
• 2 houses
0
friend
store
Speed vs. Velocity
• Speed describes how fast an object is moving.
• Velocity is an object’s speed in a particular direction.
• Ex. Does your car’s speedometer read speed or velocity?
• Speed
• What could make this a velocity?
• If your car also has a compass in it.
Variables
• Time is shown as a little t.
• “t” is measured in seconds, usually.
• Sometimes minutes or hours
• Distance is shown as a little d or a little x.
• “d” or “x” both mean the same thing.
• Usually measured in meters
• Sometimes kilometers, miles, feet, or inches
• Velocity & speed are shown as little v.
• “v” is usually measured in m/s
• Sometimes mph, km/hr, ft/s
Finding speed and velocity
• To calculate speed, you need to know 2 things
• Distance that an object traveled (in meters)
• Time it took to travel that distance (in seconds)
Speed = distance  time
Or
Units for speed and velocity:
m/s or km/hr or miles/hour
Calculating
• Examples:
• Find the speed of a runner who runs 120m, south in 2 minutes.
d = 120m
t = 2 min = 120s
v = 120m  120s = 1m/s
• For velocity, we need to specify direction.
• 1m/s, SOUTH
• Find the distance a horse travels, who is galloping at 15m/s for 90
seconds.
V = 15 m/s
15m/s = d  90s
d = 15m/s x 90s
d = 1350m
t = 90s
Problems for you
p. 369: practice problems: #1-3
Graphing Motion
• To investigate the relationship between distance and time, you
can use
• Mathematical equations
Distance (m)
or
• Graphs of distance vs. time
Time (s)
Here is an example of a distance vs.
time graph.
• What can it tell us?
• Where the object was at
any time.
• How long it took the
object to travel a certain
distance.
• The speed of the object.
= SLOPE
What is slope?
Definition: How steep a line is
• In math, to find slope, you
have to
• Pick 2 points
B
• A = (5s, 15m)
• B = (15s, 45m)
• Calculate rise
• Change in y = rise
• 45m -15m = 30m
• Calculate run
• Change in x = run
• 15s – 5s = 10s
• Slope = rise  run
• 30m  10s = 3m/s
A
What does the slope mean?
• The slope of a distance vs. time graph gives us the
velocity.
• For the object graphed below, the velocity is 3m/s in
the positive direction.
Direction from a graph
• Line A is in the positive direction, because x is steadily
increasing as time increases.
• Line C is in the negative direction, because x is steadily
decreasing as time increases.
• What direction is the object in line B going?
• Positive direction
A
C
B
What else does this graph tell us?
• It tells us
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Starting position of each object
Ending position of each object
Speed and direction of each object
Which object is going faster or fastest
A
C
B
What else does this graph tell us?
• What is B’s speed?
1.33m/s
• Who is going faster, A or B?
A
• How could we tell that without doing the math?
because the line of B is not as steep as A’s line
A
C
B
• What does a flat horizontal line mean?
• Time
• Distance
• Staying the same
• Speed
• None. The object is not moving
Distance (m)
• Increasing
Time (s)