Now that we've discussed the basic properties of electromagnetism.
We're ready to see how to describe a very common phenomenon
which is known as periodic motion which is a way of describing anything
that is repeating its motion periodically with some periodicity.
So we'll see how to describe that here.
As an overview one more time, what we've done is describe the mechanics.
We talked about how to apply it to fluids and gases
and then just recently electromagnetism.
And now with this periodic motion we'll be able to introduce waves
as well as sound and light which are both examples of these waves
before we move in to the more intricate
and a very zoomed in phenomena of atoms and thermodynamics.
So first let's start with just waves and how to describe them.
We'll start with just the wave properties
and how we discussed those and then bring up some examples of periodic motion
which will hark back to many of the Newton's laws examples
that we discussed previously.
But first, let's talk about how we can describe waves
and what are the basic wave properties that we should be familiar with.
We can ask ourselves a question in defining a wave
which is how could I if I wanted to transfer energy maybe for me to you,
how could I give you some sort of energy if you wanted to be able to use it?
One way would be, I could just take some object and throw it to you
and that object would have energy and then that object would go from me to you
and I would have transferred energy to you.
A wave is a way of transferring energy without having to actually move objects themselves.
What we do is say that the energy transfers through some medium.
So for example in an ocean wave the energy
from deep out in the ocean is moving in towards the shore
without the water from deep out into the ocean
having to actually move itself to the shore and so this is the common definition
or idea waves that we'll be using and thinking about as we go.
It's a transfer of energy through some medium without having to transfer
any actual objects through that medium.
Let's take a particular example of something that is a wave phenomenon
that you might be familiar with which is a guitar string,
so just imagine for yourself a guitar string like this.
And we're just zooming into one part of it
and maybe after you pluck the string it is vibrating up and down
and you can see some sort of profile to the motion.
We describe two different kinds of waves,
the guitar string as we've just introduced here is what we would call a transverse wave
and we call it transverse because the direction of the oscillations are up and down
whereas the wave itself if you were watching it on the guitar string
would be moving from left to right if you picked one crust of the wave,
one high point and followed it across,
you would see it moving left to right whereas any point on the wave is moving up and down
and ocean waves would be a good example of this as well.
You see the wave itself is moving from right to left or from out towards into the shore.
Whereas any point on that wave,
if you had maybe a buoy or a boat out on the water.
You would see it just moving up and down.
And so the direction of the motion of any point on the wave
is perpendicular to the motion of the wave itself, so we call this transverse waves.
And again the oscillations will be perpendicular to the direction of motion
which is something to keep in mind as we look through waves
to always identify this particular kind of behavior
and this is just the opposite of what we would call a longitudinal wave.
A longitudinal wave is a wave where the directions of the oscillations
are in the same direction as the motion of the wave.
For example if I had a spring or a slinky as a toy, we can see here,
I could push my hand a long it and that would send a pulse
or a wave through the slinky and you can see that the direction
of what we would call the displacement,
the motion of the actual material is moving left to right
which is the same direction at the waves are moving.
In other words, any point on the slinky is just moving left to right like this
and that's also the direction left to right in the same direction
at the waves themselves are moving as they go through the spring.
This oscillations are happening on a small scale
so I'm talking about the overall motion of the wave through an object like the slinky.
I'm comparing that to the motion of a particular point in the slinky or the spring.
And those are the two things you should always compare
when you're thinking about whether a wave is longitudinal or transverse.
So again think about one point on the spring,
it will move left to right and the entire wave as it's going through the spring
is also moving left to right and this defines a longitudinal wave.