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 below. In each case, the position of something is changing.
Q: In each picture in the Figure above, 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 inchworm is slowly inching its way
along a branch. 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 below
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 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.
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.
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.
Review
- How is motion defined in science?
- Describe an original example that shows how frame of reference influences the perception of motion.
