Name: Simple Harmonic Motion
Uploaded: 2015-02-08T02:41:49.000Z
Duration: 8 min 5 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Hi. It's Mr. Andersen and this AP Physics essentials video 44. It is on simple harmonic

motion. Imagine we take an object that is in equilibrium and we push on it. But it does

Adverbs of degree

not just move away. There is a restoring force that returns it towards equilibrium and another

returning force that returns it towards equilibrium. And so what we get is this simple harmonic

motion. What would be an example of this? Pushing a child on a swing. And so there are

really two examples we will do in AP Physics. One is the mass spring oscillator. And so

it is going to bounce up and down like this. Or a pendulum that is going to swing back

and forth like this. So equilibrium is going to be right in the middle. But these restoring

forces continue to bring it back towards equilibrium. And so if you have an object moving towards

equilibrium based on restoring forces we call that simple harmonic motion. And so here are

the two types we will talk about in AP Physics. Now an important characteristic with each

of these is the period of that harmonic motion. Period is always going to be measured in seconds

and it its time. So it is the time it takes on this oscillating spring to move from here

to bounce up and then to bounce back again. Or for a pendulum to swing from here over

to here and then back again. And so in a mass spring oscillator how do we increase the period?

Well the easiest way to do that is simply increase the mass. As we increase the mass

hanging from the spring the period is going to get longer. How do we decrease it? Well

we can change the stiffness of the spring. As we make it stiffer then we are going to

make that period go down. What about a pendulum? How do we increase the period of a pendulum?

Well we do that by just simply changing the length of the pendulum. Now what is interesting

about pendulums is the mass does not have anything to do with it. And so how do we then

decrease the period of the pendulum? Well we have to change the gravitational field

strength. As we increase that field strength then we are going to decrease the period.

And so with both of these forms of simple harmonic motion you should be able to identify

the position of the object at any time, its relative velocity and acceleration. And figure

out when these are at a maximum and when they are at a minimum. And so let's look at some

simple harmonic motion. This one we have it horizontal with two springs. And so we can

see it is its motion. Equilibrium is going to be right in the middle. And so as I pull

it towards this side there is going to be tension in this spring right here. And it

is going to pull it towards the right. And so that restoring force is going to move it

towards equilibrium. Now it is going to move in that direction. Now this other spring is

going to move it back towards equilibrium like that. And so what we are going to get

is this simple harmonic motion. What is cool about this one is there is an accelerometer

with little LEDs so you can see the acceleration of the object. You can see right in the middle

there is no acceleration, right in the middle. And so what do you have to do in videos like

this, or in problems like this? You have to be able to identify the position of the object

and the velocity and the acceleration. When are they at a maximum? When are they going

to be at a minimum. So let's use this scenario. And so I am going to pull that object towards

the left. And so now I have let it go. And so at this point, right here, as it is starting

to move, where is our position going to be be? It is going to be maximum towards the

left. What about our velocity at this point? It is going to be minimum. It is going to

be zero. And what is going to be our acceleration? Our acceleration is going to be maximum towards

the right. We are going to have great acceleration at this point. Let's let it go for a second.

And I have paused it again here, right in the middle. What is our position? Again it

is minimum. It is at zero. What is our velocity? Well it is accelerated. And now our velocity

is at a maximum at this point. What is our acceleration? There is no acceleration right

when it is in the middle. Let's let it go again. It is going to move over to this side.

Now our position if maximum to the right. What about velocity? It is back to zero again.

Where is our acceleration? It is going to be maximum now towards the left. And so let's

do some little experimentation with a mass spring oscillator. We are using a PHET simulation.

And let's look at how the mass and that stiffness of the spring affect the period. And so I

am going to take a weight and put it on here. Then we will measure the period. Just qualitatively.

So that is going to be the period. So it is a 50 gram mass. Now let's put a 100 gram mass

on there. And let's listen to the period again. Okay. So you can see we are slowing down the

period as we increase mass. Let's put 250 grams on there. So you can see that as we

increase the mass on that spring then we are increasing the period. But now let's play

around with the stiffness of the spring. We call that the spring constant. So now I have

made it a stiffer spring. I put that 250 gram mass on it. And you can see the period is

now decreasing. And so by increasing the stiffness of the spring we are decreasing the period

of the spring. Now let's really make it a soft spring. What happens? You can see the

period get really really really large. And so in summary what have we learned just from

this experimentation? That if you increase mass you are going to make a longer period

for that harmonic motion. And if we increase the stiffness then we are going to make it

a shorter period. If we look at that quantitatively, so if we look at the actual period of a spring

like this, what is really affecting it are two things, the mass and the spring constant.

And so you can imagine just looking at this algebraically, if we increase mass what is

going to happen to the period? The period is going to increase because it is in the

numerator. What is going to happen if we increase the constant of the spring or the spring constant?

We make it stiffer, as this number gets larger in the denominator, then we see that the period

is going to become less. Now let's look at a pendulum. Again another PHET simulation.

So I am going to move this to the side. I am going to try to drop it at about a 30 degree

angle. And this one has a little photo gate timer so I can time how long it takes to get

back to that original position. So we can see here that we have a period of 2.862. Now

I am going to make the pendulum have a smaller length, dropping it from the same angle. And

you can see that the period now is less. It is 2.415. Now let me make it even less. I

am going to try to drop that weight again at a 30 degree angle. Set my photo gate timer

and you can see that the period is becoming less. So as we decrease the length, what are

we doing to it? We are speeding it up. Or the period is decreasing. Now watch what happens

as I change the mass now. As I change the mass, drop it from a 30 degree angle, start

my photo gate timer you can see that it does not affect it. So mass does not affect the

period. But what does remember is going to be the gravitational field strength. So now

let me change it to the moon. And you can see that they period is increasing. It is

slowing down. Let me change it to earth. Now it is speeding up. What happens if we make

it Jupiter? It is really speeding up. Or if we make it nothing, it just continues moving

like that. So if we look at that quantitatively as we, or rather qualitatively, as we increase

the length we have a longer period. And as we increase the gravitational field strength

then we have a shorter period. If we look at that algebraically this is the equation.

Length is going to be on the top remember. And so as we increase the length, we are going

to increase the period of the pendulum. What happens as we increase gravitational field

strength? As we increase that then we are going to decrease the overall period of the

pendulum. Okay. Did you learn to predict which properties affect harmonic motion? Again it

is a restoring force moving it back towards equilibrium. Could you design and plan and

collect data to ascertain what characteristics are affecting the period? Again in a spring

it is going to be the mass and the stiffness of the spring. If we are looking at a pendulum

it is going to be the length and the gravitational field strength. And finally could you figure

that out quantitatively? What happens as we increase length for example to our period,

or gravitational field strength? Well I hope so. And I hope that was helpful.