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Hi. It’s Mr. Andersen. And this is AP Physics essentials video 104. It is on wave period
and frequency. Period is the time between oscillations in a wave. And frequency is going
to be how many oscillations or waves do we have per unit time. Now it is sometimes easy
to confuse frequency with the velocity of the wave. So we are not even going to start
with waves. We are just going to start with some flashing lights. And so the light on
the left is flashing. Try to figure out the period. How much time is there between each
flash? And the right answer is there is 1 second. And so the period or the capital T
is 1 second. There is a flash every one second. That is the period. What is the frequency
then? Well, we are having the flashes, and so it is how many flashes we have per second.
And since we have 1 flash every second, our frequency is 1 flash per 1 second. Now the
units for that is going to be in hertz. And so our frequency is 1 hertz. Now let’s look
at the other light. So it is flashing as well. Try to just figure out what the period is.
You can see it is slower. And so the period in this case is going to be 2 seconds. So
it is taking twice as long between each of the flashes. So what is going to happen to
the frequency? So now we have 1 flash per 2 seconds. And so what is going to be the
frequency? It is going to be 0.5 hertz. And so period and frequency will always be the
reciprocal of each other. Once you know period, 1 divided by period is going to tell you the
frequency and vice versa. And so waves transfer energy through oscillations. The wave period
is going to be the time between the waves. And then the wave frequency is going to be
a wave, or a set number of waves per unit time. Now we measure period in seconds. And
then we are going to measure wave frequency in hertz, which is the number of waves per
unit time. Now if you are ever looking at a wave diagram like this, the first thing
you want to do is figure out what are my axes labelled. And so the position is changing
here. But this is over time. Now you could also have wave diagrams where its position
here and position here. But since we have time on the x-axis, if I want to figure out
what the period is, I simply measure the distance between waves. And so I find one wavelength,
so one oscillation. And then the distance between the two is going to be the period
or it is going to be the time. If I wanted to figure out the frequency then it is simply
the reciprocal of that. And so let’s do a little simulation. This is a phet simulation.
What I am going to do is generate some waves. And so those waves are moving from left to
right. So this is going to show us the position. But it is going to have a certain frequency.
And therefore a certain period. Now a way to see that is to slow it down, since we are
looking at a position of the wave over here, I have also added a clock. And so we can keep
track of that time. And so we are going to create one wave. And so I am going to stop
it after we go through one oscillation. And so what is going to be our period? Well we
have the one wave here. But what is the period? It is the time. It is simply 0.67 seconds.
And so that is the time distance between waves. We are just about to start another wave. Now
if I wanted to figure out frequency using this simulation, what I could do is just let
that clock run until it gets to 1 second. So I have stopped it here. So that is 1 second.
And so how many waves do you see on the screen? Well I see one and a half waves on the screen.
And so our frequency is going to be 1.5 hertz. Now we could have done it just by looking
at the simulation, doing some experimentation. Or we could just do it mathematically. If
we want to convert from 0.67 seconds into frequency all you do is we would take that
period that we calculated, 0.67 seconds. If we take 1 divided by 0.67 seconds and we will
get 1.5 hertz. Let’s say you are given a problem and you want to solve what the period
and the frequency are. The first thing you want to do is look at your axes. And so we
have meters here. And then we have time. So if I want to figure out the period, remember
I just find one wavelength. And so I could say right here is the top of one. And we go
back to another top. So this one is at 25 seconds. This one is at 45 seconds. So what
is my period going to be? It is simply going to be 20 seconds. If I want to figure out
what the frequency is then, what is the easiest way to do that? Well once I have my period
at 20 seconds I simply take 1 divided by the period and I am going to get a frequency of
0.05 hertz. And so did you learn to use the graphical representation of position versus
time to calculate the period and frequency? I hope so. And I hope that was helpful.
