- [Instructor] Imagine that I'm standing here
holding the end of a rope.
I'm over here on the left end, and while holding the rope,
I rapidly moved my hand up, down,
and back to the starting position.
If we were to take a snapshot of the rope
immediately after I finished my motion,
we're going to see something like this.
The rope has a squiggly disturbance
that mirrors the motion I made with my hand,
up, down, and back to the middle.
And the rest of the rope is still flat.
You might've seen something like this
if you've ever played with a jump rope
and wiggled it back and forth,
or a slinky and you've seen that oscillate
back and forth on the ground,
or if you've been in the gym
and seen somebody doing exercises with large battle ropes,
slamming them up and down repeatedly.
And we know that over time, this disturbance
is actually going to make its way through the rope.
If this is what we observe right after my hand motion,
at some later point in time,
we will observe that the beginning of the rope
is back to its original shape, the squiggly disturbance
has made its way further down the rope,
and it will keep traveling in this direction
until it reaches the end of the rope.
This is exactly what a wave is in physics.
A wave is a disturbance, in this case,
the squiggle in the rope caused by my hand motion,
and that disturbance can propagate,
it can travel or move in a particular direction.
So a wave is a disturbance that can propagate.
This particular example is called a mechanical wave.
It's called a mechanical wave because the disturbance
is traveling through a medium, in this case, the rope.
So mechanical waves travel through a medium.
One important point about waves
that is worth noting right now
is that waves transfer energy without transferring matter.
So what that means
is that the disturbance that is moving here,
this squiggly shape is moving through the rope,
but it isn't moving the rope to a different position.
Any part of the rope might go up and down
as the wave travels through that section,
but the rope itself is not going anywhere,
rather it's the kinetic energy
imparted to the rope by my hand
that is transferring from particle to particle
and making its way through the rope.
So waves transfer energy, but not matter.
So in my first example,
I only jerked my hand up and down once,
which created a single wave pulse
that moved through my rope.
If instead, I were to keep moving my hand
up and down consistently,
I would see a wave form that looks something like this.
And when we model a wave,
there are a few key characteristics that we need to know
about that wave.
First is the period.
Period is measured in seconds and it tells us
how long it takes for one wave cycle to complete.
Next is the wavelength.
Measured in units of distance like meters,
the wavelength is the distance between identical points
of adjacent waves.
And finally there's frequency.
So if the wave point that we've drawn here takes one second,
there are four cycles in that one second,
that means it has a frequency of 4hz
or four cycles per second.
So the frequency measured in cycles per second
tells us how many wave cycles there are every second.
Now using just these basic anatomical properties of a wave,
we can start to pick out
more interesting physical characteristics,
like speed or distance over time.
If we want to know how fast a wave is traveling,
we can take its wavelength,
which is the distance covered by a single cycle,
and multiply that by the frequency,
which is how many cycles are completed in a second,
a given amount of time.
The cycles cancel out
and we're left with units of distance over time,
the same as speed.
And that's our equation for the speed of the wave.
Wavelength times frequency.
The standard units for speed are meters per second.
There are a couple of factors
that can affect the speed of a wave.
The first is the wave type.
So different types of waves move at different speeds.
A relatable example of different waves
moving at different speeds is lightning.
Have you ever seen lightning strike
or been in a thunderstorm?
You know that the first thing you see
is the flash of lightning, and then you hear the thunder
associated with that lightning flash.
So the lightning comes first and the thunder comes second.
That's because those are two different waves
that are part of the same phenomenon.
When the lightning strike hits, you see the flash first
because that's an electromagnetic wave, light.
It travels much faster
than the sound associated with the lightning strike.
Electromagnetic waves are special
not only because they travel really fast,
but they also don't need a medium to travel through.
The thunder on the other hand is a sound wave
traveling slower than the light.
So you'll see the lightning before you hear the thunder.
Different wave types move at different speeds.
The second key factor that can affect the speed of a wave
is the medium through which the wave travels
and we'll consider sound as an example here.
So when someone is talking, right,
we have this talking head creating some vibrations
of the particles in front of their mouth,
that's the sound wave.
It's the vibration of those particles
propagating through the air.
When you speak, your vocal cords exert force
on the particles just in front of you.
They vibrate back and forth, creating a compression
that transfers to the surrounding particles.
As the vibrations continue to propagate, the sound travels.
You can imagine
that if these particles are packed closer together,
those vibrations are going to transfer a lot more quickly
because the particles are colliding much faster
than if they're further apart.
So sound travels much faster in water, a liquid,
than it does in air, for that exact reason.
The particles in the liquid are closer together.
Since they're closer compacted, they collide more,
and the propagation of the wave happens faster.
So different ways move at different speeds
and the medium through which a wave travels
can also affect the speed of a wave.
All right, so let's try to summarize all this information.
We have waves, a wave, a disturbance that can propagate,
and it has a few key characteristics.
There's the period
or how long it takes one cycle to complete,
there's the wavelength,
the distance between identical points
on two waves that are next to each other,
and the frequency,
which is how many waves cycles complete in one second.
In this case, we have two cycles in one second
for a frequency of 2hz.
Wave speed is found by multiplying wavelength and frequency,
and that wave speed is affected by the type of wave
and the medium through which the wave travels.
Mechanical waves are ways that travel through a medium,
so sound, a slinky or rope, ocean waves,
and electromagnetic waves like light are special
because they can travel through a vacuum.
They don't have to have a medium in order to propagate.
But all waves, no matter what type,
transfer energy, not matter.