Standing Waves


Hi. It’s Mr. Andersen. And this AP Physics
essentials video 115. It is on standing waves. Standing waves are like they sound. They are
waves that appear to stand still. And so an example of a sound standing wave is the waves
found inside a vuvuzela. And so you blow on it. You get that sound. You blow it with a
higher frequency and then you could even try to pierce your lips a little bit more and
get an even higher frequency. But what is happening is you have these specific standing
waves that sit within the chamber, the chamber of the vuvuzela. Now a wine glass works the
same way. If you rub your finger with a little bit of water around the surface of it, what
you can do is generate these standing waves inside the wine glass. What you are doing
is vibrating the sides and as you do that you are vibrating the air. And since you have
a standing wave inside there it makes this pitch. Add water to it and you are going to
have a slightly different pitch. And so waves can either be traveling or standing waves.
Traveling waves will move from one place to another. And standing waves will appear to
stand still. Now when you look at it it is kind of an illusion. They are standing still
but the traveling waves are bouncing back and forth. In other words they are being reflected
off of the boundaries and then there is interference to create these standing waves. And so if
we look at this animation the dark wave is going to be the standing wave but in the background
you can see the blue and the red waves that are bouncing back and forth and back and forth.
And it is the summation of them that creates the standing wave. Now if you notice that
standing wave is going to have areas where it is not moving at all. And that is due to
destructive interference where the waves are canceling each other out. And we call those
in physics nodes, areas where it is not moving. And then we are going to have total constructive
interference in certain areas, and we call those antinodes. That is where it is moving
the maximum amount back and forth and back and forth. And so an example of this could
be found in instruments for example. So a string instrument is going to be fixed at
either end. And this animation is a good example of that. It is fixed on either end and as
the waves bounce back and forth and back and forth you create these standing waves. If
we shorten that, like on a guitar, then we can get different pitches. A tube is going
to be the same way. A tube could be open on both ends, like a flute or it could be closed
on one end like a vuvuzela. And that is going to create these standing waves as well. So
let’s starting with traveling versus standing waves. And so the PHET simulation, I am generating
some waves. Those waves, and this is just kind of theoretical, are moving out the door
and then they moved on forever. But if we add a boundary on the other side, in other
words if we add a fixed boundary you can see now there is reflection back. So those waves
did not just keep going. They will bounce back. And now they are going to bounce back
again. And as that wave goes back and forth and back and forth you can see that we are
starting to generate these standing waves. And you can even see those nodes and those
antinodes. In other words, those areas where it is not moving and those ares where it is
moving quite a bit. So this is an animation of standing waves. And so what we are doing
is as the blue wave moves to the right, becomes the red wave which moves back to the left
and it just keeps going and going and going, we are getting destructive interference, that
is causing these nodes where they cancel each other out. And then we have constructive interference
and that is creating these antinodes where those two waves add together to make a much
bigger wave. Now an example of this, if you have ever played baseball before, when you
hit the ball sometimes you will hit it and the bat will kind of buzz in your hands. In
other words it vibrates quite a bit. And the reason why is that you are hitting the ball
on antinode, an area where the wave is going up and down the bat are actually constructively
interfering to create a huge vibration in there. And we refer to it as the sweet spot,
the spot on a bat where the node is, is where you do not have much vibration, that you hit
the ball and you do not feel really any vibration at all. And so let’s go through experimentation.
So this is a simbucket simulation. What we are doing is looking at standing waves. The
first one we are going to look at is when you have two fixed ends. So an example of
this could be a guitar. And so what you do is you have a string. You hold it on one side
and then you pluck it and you are going to have vibration. So what do you get? Waves
going back and forth and back and forth. What is the standing wave then? The standing wave
is going to be that green wave in this simulation. It is that wave that is the addition of the
other waves. What is interesting in a guitar is that you do not only have one, what we
call harmonic or one standing wave, but you can have multiple standing waves. Now in all
of these standing waves what makes them the same is that we have a node on either side.
In other words there is not net movement on either side of that string. If we look at
an instrument like a tuba or a vuvuzela, what is the characteristic of that? Well, we are
vibrating it but you can see that on the left side we have a node. It is not vibrating.
But we have an antinode at the right side. So in each of these harmonics the left side
is going to be a node. It is not going to move at all, but the right side is going to
be an antinode. And so we still have those traveling waves going back and forth and back
and forth. It is just that one of those ends is free to create an antinode. And then we
just have reflection off of that end. These are all of the different harmonics. What keeps
them all in common is the node on the left, antinode on the right. If we then look at
something that has two open ends, like a flute. A flute you do not blow in one end, you are
blowing on the top to create a vibration or a pan flute would be an example of that. And
so what is going to create the standing wave here? Well in this first harmonic we have
the node in the middle and then we have antinodes on either side. Now we have two nodes near
the middle and antinodes on the side. So what is the characteristic of a standing wave in
a chamber that has two open ends? It is always going to be an antinode on either side. But
depending on the frequency we can get difference pitches as we blow through. Rubens tube is
a really cool example of that. And a Rubens tube what you do is you fill up a chamber
with gas, a gas that is flammable. And so this guy is lighting the chamber, so we have
flames coming out of the top. But on a Rubens tube you put a speaker on the side. And what
that speaker does is create these longitudinal waves. Those longitudinal waves, as they move
through will create areas where there is a lot of compression. And an area where there
is a lot of compression or antipodes, then the gas really cannot make its way out. And
so it will be really low there, but at the nodes it will actually come out quite a bit.
And so did you learn to predict the properties of standing waves. They result from reflection
of these traveling waves. Could you collect data? Again I used simbucket simulation to
do that. And then finally could you describe examples of when this occurs, standing waves.
Musical instruments are good. Baseball bats are great. And I hope that was helpful.

43 thoughts on “Standing Waves

  1. This is really funny, I made a rubens' tube and did a presentation on it today in my AP physics class.

  2. Thanks Paul! These videos on waves are great and just in time for my physics exam tomorrow! Your biology ones also saved me so hopefully ive done well!
    Keep them up 🙂

  3. Personal Notes:
    3:55
    Carefully observe the three different types of standing waves …..
    Also refer : http://www.physicsclassroom.com/Physics-Interactives/Waves-and-Sound/Standing-Wave-Patterns/Standing-Wave-Patterns-Interactive
    Set on pre set waves …. amazing

  4. If I can hear the standing waves they must be traveling to my ears, no? So is it correct to call them standing waves if just like regular waves they travel to my ears? Can you help clear this up, I'm confused. Thank you.

  5. In a few days I have to do a presentation on this, I made a Rubens' Tube, but the theory is somewhat hard because, we haven't learned that in school yet (we'll learn that in 2 years I think)

  6. What would have happened if Mr. Anderson did exist in this world???

    I'd have got lots of eggs from my test papers ; sold it and would have become a millionaire

  7. Hey I am from India , my question to all of you out there is that , do y’all also have to DERIVE EXPRESSIONS like deriving the formation of a standing wave in your countries…. or is it just us ?

  8. I was watching your video while signed out but I signed in just to like your video.
    I'm sure your pictorial explanation will help me to remember this for a long time.
    Thanks for the video!

  9. HI, for a standing wave i don't get how you created two travelling waves(Blue and red waves) in your video. Do you pluck the string twice or once?
    Thanks for the help!

  10. 4:42 It would be helpful to to elaborate on which end is the Node and Which end is the anti node.

    Im assuming the node is open "trumpet" end where sound is met with equilibrium end of open air, a "barrier" of sorts that won't allow compression; and the Lip end is the antinode since thats were most of the pressure is being generated, and acts as a hard barrier, an ideal place where compressions can be built upon itself.

  11. So are there standing waves inside the Reubens tube? If so would the Reuben tube have one fixed end and one open end? Or both fixed ends?

Leave a Reply

Your email address will not be published. Required fields are marked *