Motion
Jean Brainard, Ph.D.
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Printed: April 15, 2014
AUTHOR
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Motion
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Motion
• Define motion.
• Explain how frame of reference is related to motion.
The wings of this hummingbird are moving so fast that they’re just a blur of motion. You can probably think of many
other examples of things in motion. If you can’t, just look around you. It’s likely that you’ll see something moving,
and if nothing else, your eyes will be moving. So you know from experience what motion is. No doubt it seems like
a fairly simple concept. However, when you read this article, you’ll find out that it’s not quite as simple as it seems.
Defining Motion
In science, motion is defined as a change in position. An object’s position is its location. Besides the wings of the
hummingbird in opening image, you can see other examples of motion in the Figure 1.1. In each case, the position
of something is changing.
Q: In each picture in the Figure 1.1, what is moving and how is its position changing?
A: The train and all its passengers are speeding straight down a track to the next station. The man and his bike
are racing along a curving highway. The geese are flying over their wetland environment. The meteor is shooting
through the atmosphere toward Earth, burning up as it goes.
Frame of Reference
There’s more to motion than objects simply changing position. You’ll see why when you consider the following
example. Assume that the school bus pictured in the Figure 1.2 passes by you as you stand on the sidewalk. It’s
obvious to you that the bus is moving, but what about to the children inside the bus? The bus isn’t moving relative
to them, and if they look at the other children sitting on the bus, they won’t appear to be moving either. If the ride
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FIGURE 1.1
FIGURE 1.2
is really smooth, the children may only be able to tell that the bus is moving by looking out the window and seeing
you and the trees whizzing by.
This example shows that how we perceive motion depends on our frame of reference. Frame of reference refers to
something that is not moving with respect to an observer that can be used to detect motion. For the children on the
bus, if they use other children riding the bus as their frame of reference, they do not appear to be moving. But if they
use objects outside the bus as their frame of reference, they can tell they are moving. The video at the URL below
illustrates other examples of how frame of reference is related to motion.
http://www.youtube.com/watch?v=7FYBG5GSklU
MEDIA
Click image to the left for more content.
Q: What is your frame of reference if you are standing on the sidewalk and see the bus go by? How can you tell that
the bus is moving?
A: Your frame of reference might be the trees and other stationary objects across the street. As the bus goes by, it
momentarily blocks your view of these objects, and this helps you detect the bus’ motion.
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Chapter 1. Motion
Summary
• Motion is defined as a change of position.
• How we perceive motion depends on our frame of reference. Frame of reference refers to something that is
not moving with respect to an observer that can be used to detect motion.
Vocabulary
• frame of reference: Something that is not moving with respect to an observer that can be used to detect
motion.
• motion: Change in position.
Practice
Do the frame of reference activity at the following URL. Watch the introduction and then do the nine trials. Repeat
any trial you answer incorrectly until you get the correct answer.
http://www.amnh.org/learn/pd/physical_science/week2/frame_reference.html
Review
1. How is motion defined in science?
2. Describe an original example that shows how frame of reference influences the perception of motion.
References
1. Train: John H. Gray; Bike: Flickr:DieselDemon; Geese: Don McCullough; Meteor: Ed Sweeney (Flickr:Navicore).
. CC BY 2.0
2. Bus: Flickr:torbakhopper; Children: Flickr:woodleywonderworks. . CC BY 2.0
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