Measuring Motion

1
Measuring Motion

• Chapter 11

2
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)

3
How do things move?

• Vocabulary
• Frame of reference
• Distance
• Displacement
• Speed
• Velocity
• Slope
• Acceleration
• Gravity
• As we go through the section, create flashcards
  for these words.

4
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.

5
Distance vs. Displacement

• Distance is the total path an object travels.
• Measures how far an object moves along a path.
• Displacement is the line from the beginning to
  the end.
• Measures how far between the starting point and
  ending point.

End
Start
6
Distance vs. Displacement

• You start at your house (0). You go to the
  store. Then you go to your friends house.
• What is the distance that you travel?
• 8 houses
• What is your displacement from the beginning to
  the end?
• 2 houses

store
0
friend
7
Speed vs. Velocity

• Speed describes how fast an object is moving.
• Velocity is an objects speed in a particular
  direction.
• Ex. Does your cars speedometer read speed or
  velocity?
• Speed
• What could make this a velocity?
• If your car also has a compass in it.

8
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

9
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
10
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 t 90s
• 15m/s d ? 90s
• d 15m/s x 90sd 1350m

11
Problems for you

• p. 369 practice problems 1-3

12
Graphing Motion

• To investigate the relationship between distance
  and time, you can use
• Mathematical equations
• or
• Graphs of distance vs. time

Distance (m)
Time (s)
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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

14
What is slope?
Definition How steep a line is

• In math, to find slope, you have to
• Pick 2 points
• 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

B
A
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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.

16
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
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What else does this graph tell us?

• It tells us
• 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
18
What else does this graph tell us?

• What is Bs 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 As line

A
C
B
19

• What does a flat horizontal line mean?
• Time
• Increasing
• Distance
• Staying the same
• Speed
• None. The object is not moving

Distance (m)
Time (s)