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1. Motion
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Date:
MOTION AND SPEED
An object is said to be in motion when there is a change in its position compared to a reference
point. However, whether an object is in motion or not depends on your point of view. For example, if
you are sitting across from a woman on a bus, she is not moving relative to the seat she is sitting on, but
she is moving relative to the buildings that the bus passes. A reference point is needed to determine if
something is in motion. A reference point is a place or an object used for comparison to determine if
something is in motion. It is assumed that a reference point is stationary, or not moving. An object is
in motion if it changes position compared to a reference point.
For example, suppose you are inside your house when you see a mail truck stopped next to your mailbox
through the window. You look away but a few moments later you see through the window that the mail
truck is farther down the street by a tree. You did not see the mail truck move, but you know motion has
taken place because of the truck’s new position relative to your mailbox, which is your reference point.
2. Measuring Distance
Units of measurement are needed to describe an object’s motion. The system of measurement used by
scientists all over the world is called the International System of Units, or in French, Systeme
International (SI). The SI system is based on the number 10.
The basic SI unit for length or distance is the meter (m). To measure distances smaller than a meter,
scientists use the centimeter (cm). There are 100 cm in a meter. To measure distances larger than a
meter, scientists use the kilometer (km). There are 1000 m in a kilometer.
3. What is Speed
So far, motion has been described by the distance an object has moved from a reference point. You may
also want to tell how fast an object is moving, or the rate at which an object is moving. A rate is a
measurement of a change in something in a certain unit of time. Speed is a type of rate.
Speed is defined as the distance an object travels in a specific unit of time.
4. Calculating Speed
As described earlier, the SI unit of distance is the meter (m). The SI unit of time is the second (s).
Therefore, in the SI system, speed is measured in meters per second (m/s).
To calculate the speed of an object, divide the distance traveled (m) by the time it took to travel that
distance (s). The formula for calculating speed is:
𝒔𝒑𝒆𝒆𝒅 (𝒎/𝒔) =
𝒅𝒊𝒔𝒕𝒂𝒏𝒄𝒆 (𝒎)
,
𝒕𝒊𝒎𝒆 (𝒔)
𝑶𝑹
𝒔=
𝒅
𝒕
Some objects move very quickly, such as a rocket. Very high speeds can be measured in kilometers per
second (km/s). Some objects move very slowly, such as the movement of tectonic plates below Earth’s
surface. Very low speeds can be measured in centimeters per year (cm/y).
5. Constant Speed vs. Average Speed vs. Instantaneous Speed
Constant speed is when an object moves at the same speed; the speed at any moment is always
exactly the same. Most objects do not move at constant speeds; sometimes they move faster and
sometimes they move slower.
Average speed describes motion when an object’s speed is changing all the time. Average speed is
calculated by dividing the total distance traveled (m) by the total time of travel (s).
𝑎𝒗𝒆𝒓𝒂𝒈𝒆 𝒔𝒑𝒆𝒆𝒅 (𝒎/𝒔) =
𝒕𝒐𝒕𝒂𝒍 𝒅𝒊𝒔𝒕𝒂𝒏𝒄𝒆 (𝒎)
𝒕𝒐𝒕𝒂𝒍 𝒕𝒊𝒎𝒆 (𝒔)
Instantaneous speed is the speed an object at one exact point in time. If an object is moving with
constant speed, the instantaneous speed is the same at every point in time. However, when an object
speeds up or slows down, its instantaneous speed is changing.
The graph below shows the speed of a bicyclist during a 5-km ride. Follow the graph as the ride is
described. As the bicyclist starts off, his speed increased from 0 km/h to 20 km/h. Then he rides up a
steep hill and his speed slows down to 10 km/h. He then speeds up to 30 km/h as he goes down the
other side of the hill. At the bottom he stops at a red light. He speeds up when the light turns green. At
the end of the ride, he slows down and then fully stops. The ride took 15 min in total.
6. Distance-Time Graphs
You can display the motion of an object on a line graph. The time is plotted on the X-axis and the
distance is plotted on the Y-axis. This is called a distance-time graph (also called a D-T graph).
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A straight line on a D-T graph represents an object traveling at a constant speed.
A line that is rising means that the object is traveling away from the reference point.
A line that is falling means that the object is traveling towards the reference point.
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The slope of the line on a D-T graph is the speed.
A steep slope means a faster speed.
A less-steep slope means a slower speed.
A perfectly horizontal line represents an object that is not moving at all.
The sample graph below shows the motion of three swimmers during a 30-minute workout. The practice
was 30 min long, so the horizontal axis must go to at least 30. Mary swam the farthest during the
practice, 2,400 m. So, the vertical axis must go to at least 2,400.
The line for Mary is rising, but it is straight. This means that she swam at a constant speed the entire
time. Her speed was 80 m/min for the whole 30 min.
The line for Kathy is also straight since Kathy swam at a constant speed. However, the line for Kathy is
not as steep as Mary’s. Kathy’s speed was 60 m/min for the whole 30 min. Since Mary was swimming
faster than Kathy, she swam a farther distance in the same amount of time.
The line for Julie is not straight throughout. This means that Julie did not swim at a constant speed for
the entire 30 min. Examining Julie’s line, it shows that Julie swam at a constant speed for the first 10
min. Then she rested for 10 min. For this part of the practice, Julie’s line is horizontal, since her speed
was 0.0 m/min. She then swam 800 m for the last 10 min. During this part of the practice, she swam as
fast as Mary, so that part of the line is the same slope as Mary’s line.
REVIEW: MOTION AND SPEED
1. Use the text to define the words below.
Vocabulary
Definition
a. motion
b. speed
2. Using the example of a runner in a race, contrast the idea of average speed and instantaneous speed.
3. Complete the tale below on SI.
Question
Answer
a. What is the basic SI unit for distance/length?
b. What is the basic SI unit for time?
c.
What is the SI unit for speed?
4. Write down the formula used to calculate the speed.
5. Describe how you would calculate the average speed of a cyclist through an entire race, even if the
cyclist went slower and faster at different stages of the race.
6. In the 2008 Beijing Olympics, Eamon Sullivan of Australia swam 100 m in 47 seconds to win the gold
medal and set a record. Calculate his average speed and include proper SI units.
7. The slant on a distance-time graph is called its ___________________.
8. Complete the tale below on how to interpret the slant of a line on a distance-time graph.
Slant
a. The line for Car X is perfectly flat.
b. The line for Car Y is rising up and is steep.
c.
The line for Car Z is rising up and is shallow.
What does it mean about the speed?