Describing and Measuring
Motion
Physical Science #1
Vocabulary
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Average Speed
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Motion
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Instantaneous Speed
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Reference Point
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Slope
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Speed
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Velocity
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International System of
Units
Meter
Describing Motion
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Deciding if an object is moving is not as
easy as you might think.
An object is in motion if its distance from
another object is changing.
Reference Point
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A reference point is a place or objet used for
comparison to determine if something is in
motion.
An object is in motion if it changes position
relative to a reference point.
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Objects that are stationary – such as a tree, sign or
building – make good reference points.
Relative Motion
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Are you moving right now?
The answer to that question depends on
your reference point.
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If you choose your chair, then no, you are not
moving.
If you choose the sun, then yes, you are
moving with the Earth.
Measuring Distance
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You can use units of measurement to
describe motion precisely.
Scientists use the same units of
measurement, called the International
System of Units (SI).
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The SI unit of length is the meter.
Measuring Distance
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Millimeter = 1000th of a meter
Centimeter = 100th of a meter
Decimeter = 10th of a meter
Meter
Decameter = 10 meters
Hectameter = 100 meters
Kilometer = 1000 meters
Calculating Speed
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A measurement of distance can tell you
how far an object travels.
If you know the distance an object travels
in a certain amount of time, you can
calculate the speed of an object.
Speed is a type of rate.
The Speed Equation
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Speed = Distance
Time
The speed equation consists of a unit of distance
divided by a unit of time.
If you measure distance in meters and time in
seconds, your speed would be in meters per
second (m/s).
Average Speed
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The speed of most moving objects is not
constant.
Although speed is not always constant,
you can find the average speed of a trip
by dividing the total distance traveled by
the total time.
Instantaneous Speed
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Instantaneous speed is the rate at which
an object is moving at a given instant in
time.
How does the average speed compare to
the instantaneous speed?
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Describing Velocity
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To describe an object’s motion completely,
you need to know the direction of its
motion.
When you know both the speed and
direction of an object’s motion, you know
the velocity of an object.
Graphing Motion
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You can show the motion of an object on
a line graph in which you plot distance vs.
time.
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Time is on the x axis
Distance is on the y axis
The steepness of a line on a graph is
called the slope.
Calculating Slope
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The slope tells you how fast one variable
changes in relation to the other variable in the
graph.
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The steeper the slope, the greater the speed.
Slope is calculated by dividing the rise by the
run.
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Slope = Rise
Run
Different Slopes
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Since moving objects
do not travel at a
constant speed,
sometimes lines will
have different slopes.
The steepness in each
segment tells you
how fast the person
went.
Homework Questions
a. How do you know if an object is
moving?
2. a. What is speed?
c. What is the difference between speed
and velocity?
3. a. What does the slope of a distance vs
time graph show you about the motion
of an object?
1.
Calculation Homework
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This week at practice, Jamie swam a total
of 1,500 meters, while Ellie swam 1.6
kilometers.
Convert Ellie’s distance to meters.
Who swam the greater distance?
How many kilometers did Jamie swim?