http://www.ck12.org Chapter 4. Motion and Forces
4.1 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,motionis 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 theFigure4.1. In each case, the position
of something is changing.
Q: In each picture in theFigure4.1, 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 theFigure4.2 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