Describing and Measuring Motion

1
Table of Contents

• Describing and Measuring Motion
• Slow Motion on Planet Earth
• Acceleration

2
Describing Motion
- Describing and Measuring Motion

• Whether or not an object is in motion depends on
  the reference point you choose.

3
Measuring Distance
- Describing and Measuring Motion

• You can measure distances shorter than 1 meter in
  centimeters. The wingspan of the butterfly is 7
  cm.

4
Converting Units
- Describing and Measuring Motion

• Use a conversion factor to convert one metric
  unit to another. A conversion factor is a
  fraction in which the numerator and denominator
  represent equal amounts in different units.
  Multiply the number you want to convert by the
  conversion factor.
• Suppose you want to know how many millimeters
  (mm) are in 15.5 meters (m). Since there are
  1,000 millimeters in 1 meter, the conversion
  factor is
• 1,000 mm/1 m
• Multiply 14.5 meters by the conversion factor to
  find millimeters.
• 14.5 m X 1,000 mm/1 m
• 14.5 X 1,000 mm
• 14,500 mm

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Converting Units
- Describing and Measuring Motion

• Practice Problem
• How many centimeters are in 22.5 meters?

• 22.5 m 2,250 cm

6
Calculating Speed
- Describing and Measuring Motion

• If you know the distance an object travels in a
  certain amount of time, you can calculate the
  speed of the object.

7
Graphing Motion
- Describing and Measuring Motion

• You can show the motion of an object on a line
  graph in which you plot distance versus time.

8
Using Prior Knowledge
- Describing and Measuring Motion

• Before you read, write what you know about motion
  in a graphic organizer like the one below. As you
  read, write what you learn.

What You Know

• A moving object changes position.
• Objects move at different speeds.

What You Learned

• Motion is compared to a reference point.
• The SI unit of length is the meter.

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Data Sharing Lab
- Describing and Measuring Motion

• Click the PHSchool.com button for an activity
  about sharing data for the Skills Lab Inclined to
  Roll.

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End of SectionDescribing and Measuring Motion
11
Earths Plates
- Slow Motion on Planet Earth

• According to the theory of plate tectonics,
  Earths landmasses have changed position over
  time
• because they are part of plates that are slowly
  moving.

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Plate Movement
- Slow Motion on Planet Earth

• Some plates move at a rate of several centimeters
  each year. Others move only a few millimeters per
  year.

13
Previewing Visuals
- Slow Motion on Planet Earth

• Before you read, preview Figure 8. Then write two
  questions that you have about the diagram in a
  graphic organizer like the one below. As you
  read, answer your questions.

Previewing Figure 8
Q. How have the positions of the continents
changed over time?
A. The distance between the continents has
increased.
Q. What causes Earths plates to move?
A. Slow-moving currents beneath Earths outer
layer cause the plates to move.
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End of SectionSlow Motion on Planet Earth
15
Calculating Acceleration
- Acceleration

• To determine the acceleration of an object moving
  in a straight line, you must calculate the change
  in speed per unit of time.

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Calculating Acceleration
- Acceleration

• As a roller-coaster car starts down a slope, its
  speed is 4 m/s. But 3 seconds later, at the
  bottom, its speed is 22 m/s. What is its average
  acceleration?
• Read and Understand
• What information have you been given?
• Initial speed 4 m/s
• Final Speed 22 m/s
• Time 3 s

17
Calculating Acceleration
- Acceleration

• As a roller-coaster car starts down a slope, its
  speed is 4 m/s. But 3 seconds later, at the
  bottom, its speed is 22 m/s. What is its average
  acceleration?
• Plan and Solve
• What quantity are you trying to calculate?
• The average acceleration of the roller-coaster
  car __
• What formula contains the given quantities and
  the unknown quantity?
• Acceleration (Final speed Initial
  speed)/Time
• Perform the calculation.
• Acceleration (22 m/s 4 m/s)/3 s 18 m/s/3 s
• Acceleration 6 m/s2
• The roller-coaster cars average acceleration is
  6 m/s2.

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Calculating Acceleration
- Acceleration

• As a roller-coaster car starts down a slope, its
  speed is 4 m/s. But 3 seconds later, at the
  bottom, its speed is 22 m/s. What is its average
  acceleration?
• Look Back and Check
• Does your answer make sense?
• The answer is reasonable. If the cars speed
  increases by 6 m/s each second, its speed will be
  10 m/s after 1 second, 16 m/s after 2 seconds,
  and 22 m/s after 3 seconds.

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Calculating Acceleration
- Acceleration

• Practice Problem
• A falling raindrop accelerates from 10 m/s to 30
  m/s in 2 seconds. What is the raindrops average
  acceleration?

• (30 m/s 10 m/s) 2 seconds 10 m/s2

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Calculating Acceleration
- Acceleration

• Practice Problem
• A certain car can accelerate from rest to 2.7 m/s
  in 9 seconds. Find the cars average acceleration.

• (27 m/s 0 m/s) 9 s 27 m/s 9 s 3 m/s2

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Graphing Acceleration
- Acceleration

• You can use both a speed-versus-time graph and a
  distance-versus-time graph to analyze the motion
  of an accelerating object.

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Identifying Main Ideas
- Acceleration

• As you read the section What is Acceleration?,
  write the main idea in a graphic organizer like
  the one below. Then write three supporting
  details that further explain the main idea.

Main Idea
In science, acceleration refers to...
Detail
Detail
Detail
Increasing speed
Decreasing speed
Changing direction
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Links on Acceleration
- Acceleration

• Click the SciLinks button for links on
  acceleration.

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End of SectionAcceleration
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Graphic Organizer

Motion
is described relative to a
is measured by
Reference point
Distance Time
in a given direction is called
equals
Speed
Velocity
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End of SectionGraphic Organizer