What is Physics?

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Mr. Andersen explains the importance of physics as a science. History and virtual examples are used to give the discipline context.

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Transcript Provided by YouTube:

00:04
Hi. It’s Mr. Andersen and today I’m going to be talking about physics. Physics
00:10
comes from a greek word physis which just means nature. And so you could argue that
00:15
physics is the oldest science that we have. Because the first time somebody looked up
00:19
at the moon and wondered why the moon goes through its phases, they were doing physics.
00:25
And so it’s been around thousands of years. But it really didn’t become quantified until
00:30
we got to the scientific revolution. One of the big names is Galileo Galilei. Again he
00:35
really was good at using the telescope and made some amazing discoveries as far as space.
00:41
But he also did some studies in mechanics. Or in kinematics. Or how objects move. He
00:47
probably didn’t drop those objects from the Leaning Tower of Pisa. But he did do some
00:53
cool experiments on pendulums. And so let me show you an example of what he did. I’m
00:58
using software called phun. Phun is a physics simulator and it actually is a lot of fun.
01:05
What you can do is build objects. And then you just hit play. And then those objects
01:09
will start to move. And so that’s a simple pendulum. One similar to what Galileo was
01:14
actually studying. And so let me move back for a second. And let me make another pendulum
01:20
over here. And one of the first things he discovered is that when you have two pendulums,
01:26
we’ll say the one on the left and one on the right. And the one on the right has a longer
01:30
length. That one’s actually going to have a longer period. In other words the time it
01:34
takes to swing back and forth. He also found that no matter what the object’s weight is,
01:40
they have the same period. And so he was starting to figure out this idea, at least that all
01:45
objects seem to swing, if not fall at the same rate. So let me get rid of some of these.
01:53
And clear this off. And now we’ll go back and talk a little bit about the next scientist.
01:58
And his name was Isaac Newton. Isaac Newton came up with three theories. Or three laws.
02:06
And you’ve probably learned these before. First one is the idea of inertia. And you
02:10
have maybe heard it this way. That an object at rest tends to stay at rest. An object in
02:15
motion tends to stay in motion. So we call that his first law. His second law could be
02:20
summarized as this. Force equals mass times acceleration. And then the last one is this
02:26
idea that for every action there’s an opposite and equal reaction. And so we call those reaction
02:35
pairs is a good way to say that. As I make that. In other words when I push on a wall,
02:42
a wall is pushing back equally on me. And so let’s jump into the virtual world and take
02:48
a look at how those would work. First of all let me add a ground for just a second. And
02:55
so now I’m going to add, let’s see. Let’s go back. Actually, so let’s add a ground.
03:03
And now let me add an object. So let me rewind for just a second. So if I had a ball like
03:11
this and hit play, that object is going to fall like that. If I rewind it for a second
03:17
and I lose gravity. So there’s no gravity and I hit play. What’s going to happen? Well
03:21
if there’s no gravity and I hit play that object is now at rest and so it’s going to
03:27
stay at rest. Once I add gravity to it, it’s just going to fall. Now if I time it right,
03:35
again I had it bounce. And then I got rid of gravity and it just kept going in that
03:40
direction. If I add gravity again, it’s just going to fall all the way down. And so that’s
03:45
Newton’s First Law. An object in motion stays in motion. An object at rest is going to stay
03:49
at rest. And if you’re paying attention down here, this object is starting to move up because
03:53
I got rid of gravity. So we can make it stick down to the bottom. Okay. What was Newton’s
03:58
second law? Newton’s second law remember is that force is equal to mass times acceleration.
04:08
What does that mean? F=ma. Well it essentially means that if you apply a force to a small
04:15
mass you’re going to get a huge acceleration. Or if you apply a force to a huge mass you’re
04:21
going to get a small acceleration. So let me give you an example of that. Let’s bring
04:25
in a catapult. And so if I bring a catapult in like this, and hit play. Catapult is going
04:32
to throw that object. Oops. I got rid of gravity for a moment. So it’s going to throw that
04:38
object in the other direction like that. Let me rewind that for a second. And so what I
04:43
can do is I could grab this little object right here. And I can change what it’s made
04:49
up of. And so let’s change its mass. And so now let’s not have it be a mass of 60 grams.
04:54
Let’s make its mass be much bigger. Like 1.5 kilograms. So if I throw it again. Uh . . . It’s
05:02
not able to throw it. So again what I’ve done is I’ve increased its mass and so its acceleration
05:08
got slower. Let’s try and find something in the middle. Let’s find a mass that maybe works.
05:13
So something like that. And so that would be Newton’s Second Law. In other words as
05:19
you increase the mass of an object, that object is going to, its acceleration is going to
05:24
drop off. Now one thing you should have seen when we did that is don’t take a look at the
05:29
object this time. But let’s take a look at the catapult. What happens when it . . . did
05:32
you see that? So when the object goes to the right, in this case, then the catapult goes
05:38
in the left. And so that’s Newton’s Third Law of Motion. For every action there’s an
05:42
opposite and equal reaction. Now why did the object go farther than the catapult? That
05:47
goes back to Newton’s Second Law. Well I’m getting into a lot of physics and physics
05:51
that we’ll actually deal with later. But the reason I’m doing all of that is that I want
05:56
you to understand that physics, once we started quantifying it and we came up with these laws,
06:01
it never changed. In other words for the next two hundred years, once we had Newton’s Laws
06:05
of Physics, that was it. And in fact in our book the physics that we cover is just Newtonian
06:12
physics. And so what do I mean by that? Well mechanics. So we’re just talking about one
06:18
specific type or one part of physics. Mechanics and all we understood about mechanics dealt
06:24
with just this box down here. And so if we look at the size of objects. And so right
06:30
here I’m going from size of objects down at the level of an atom. So this would be an
06:35
atom down here, up to the size of a planet up here. So if we go from the very small to
06:43
the very big classical mechanics or Newtonian physics only works if you’re dealing with
06:49
big objects. Like objects the size of me. Or the objects the size of a catapult. But
06:54
when you get down to the level of an atom, it really didn’t make sense. Speed is another
06:59
thing. So if we go from speed where we’re not moving, or moving as fast as a bicycle.
07:03
And then we move up to the speed of light, the rules tend to change as well. And so physics
07:08
is a really broad science. But we didn’t start to understand this until we started to get
07:15
some people. And so Einstein was the first person to start to explain what happens to
07:21
objects, not just swinging on a pendulum but as they start to move really fast. And as
07:25
they move towards the speed of light. And he found some really crazy stuff. Like as
07:29
you start to approach the speed of light, time will actually slow down for you. In other
07:33
words you could travel away from here at the speed of light and come back. And you and
07:37
I are going to be a different age. We also had, this would be Max Plank. But a lot of
07:43
scientists came up with this idea of quantum mechanics. Quantum mechanics is kind of explaining
07:48
what happens at that atomic level. Where we start to get this duality. Where it’s not
07:53
only a particle, these small little objects. But they have properties of waves as well.
07:58
And then finally we have in this last century the arrival of what’s called quantum field
08:03
theory. Again I could put tons of scientists with each of these, but this is Richard Feynman
08:08
who actually is, it’s just fascinating. If you want to be fascinated by a scientist,
08:14
type that into YouTube and you’re going to find some wonderful interviews. But he came
08:18
up with these Feynman diagrams to explain what’s going to happen, not only at a high
08:22
speed, but at the atomic level. And so what I want you to understand is that classical
08:27
mechanics or Newtonian physics are really what we’re going to talk about for a lot of
08:31
the year in physics. But it’s just one thin slice of the pie that is physics. And even
08:37
with that, mechanics is one thin slice of physics. So we’re going to deal a lot with
08:43
kinematics, like acceleration, motion, graphing. But there’s a whole other groups of physics
08:50
in physics called thermodynamics where we deal with things like heat. There’s another
08:55
group where we deal with electromagnetism. So we deal with things like electricity, magnetism
09:00
and light and optics. And so I’m leaving some things off here. Physics also is made up of
09:05
a lot of the idea of this quantum theory and how we move at that small level. And so there’s
09:13
a lot to physics. But what I want you to understand is that physics is simply a way to understand
09:18
our world. And if you understand that, you’ve taken a first step. And so I hope that’s helpful.