- Welcome everyone.
My name is Professor Rood.
I teach physical science
and my goal this semester is to get
the lectures available for
students to view online.
I'm going to start with chapter one,
which is entitled The Scientific Method.
We're gonna look at a
basic overview of the
basic physical science topics
that we'll be covering.
So let's start with the scientific method.
When we deal with the scientific method,
we must come up with a problem
that needs to be solved.
For instance, is Pluto a planet?
From there, you make observations
and perform many experiments.
With the data, you need
to interpret that data.
And initially, a scientific interpretation
may become a hypothesis
and upon further testing,
the hypothesis may be
elevated to a law or a theory.
This process is ongoing even though
you may have a theory or law.
As the scientific world changes
and new information becomes available,
theories can, occasionally, be overturned.
Here's a flow chart.
The scientific method.
Let's state the problem.
You experiment and observe,
collect data, interpret the data,
test the interpretation,
collect more data from experiments,
and this is an ongoing process.
Let's talk a little bit
about the laws of nature.
Everything in the universe behaves
in a certain regular way.
We're gonna call these the laws of nature.
And in order to be a law of nature,
there's a certain regularity
that must hold true
everywhere at all times.
And we use these laws to predict
things that are not yet discovered.
So the laws of nature
can give us an idea also
of what is going on in places
we cannot examine directly.
For example, the sun's interior,
or the interior of an atom,
or the interior of the Earth.
So your textbook likes to say
that laws tell us what,
while as theories tell us why.
I prefer to put it as theories
are usually more than one idea
and encompass several ideas,
whereas a law is often times
just simply stated as
a mathematical formula,
such as Newton's law of gravity.
So keep in mind science is successful
because laws and theories
are not necessarily the final word.
They are only valid as long
as nothing else comes to light
to indicate that they are no longer valid.
I wanna go over a few terms here
with regard to the solar system.
Polaris
is the North Star.
And its significance is
that it happens to lie
in the path of Earth's axis.
And in the northern hemisphere,
before compasses were available,
people traveled by using the North Star
because it appeared
never to move in the sky.
Constellations are the easily
recognizable groups of stars,
like the Big and Little Dipper.
And the planets that are
visible at various times
during the year to the naked
eye are Mercury, Venus,
Mars, Jupiter, and Saturn.
Here's a figure of some
common constellations.
The Little Dipper, the Big Dipper,
and notice that on the end here
the Pointer stars of the
Big Dipper points to Polaris
which is the last star on the
handle of the Little Dipper.
Little fast there, sorry.
Let's talk about some early ideas
of what the solar system looked like.
And we're gonna start with
Ptolemy and the Ptolemaic System.
Ptolemy was around
between 100 and 170 A.D.
and he had a geocentric view,
that is the Earth was the
center of the universe
and it stood in the center
of the universe motionless
with everything around it
moving in little circles
or combinations of circles.
Here's an image
of Ptolemy's solar system.
So we have the Earth here
in the center, moon, Venus,
Mercury, the sun out here.
But notice the planets
orbiting in these little
circular patterns.
The stars were close by
on this inner surface
of what they called the crystal sphere.
Jumping ahead to the Copernican system,
we're going to 1473, 1543.
And he thought the idea that
the Earth and all the planets
that were known at that
time rotated around the sun
and they rotated in a
circular path or orbit.
He indicated that the Earth
rotates daily on its axis,
the moon goes around the Earth,
and the stars are far away.
Copernicus is given credit
for the correct system.
He only had one thing
wrong with his system.
Pause the video for a moment
and see if you can figure out
what was wrong with the Copernican system.
Well, it's the fact that
the paths of the planet are not circular
but are in fact ellipses.
Here's the Copernican solar system.
About a hundred years after Copernicus,
Kepler, Johannes Kepler,
developed what are called
Kepler's laws of planetary motion
that explain how planets actually move.
Now Kepler worked with a very
famous scientist, Tycho Brahe,
and Tycho Braye had a
lot of data that he had
taken himself of the
positions of the stars.
Tycho ended up dying
and all of that data
fell into Kepler's hands.
Since Kepler had worked for Tycho,
he knew that the data was very good.
So he started working
with the data on Mars,
and he worked four years on that,
and he could not get the data from Tycho
to fit any of the models of
the solar system of that time.
So he decided to discard
the current theories
on the solar system and
look for a new cosmic design
that fit Tycho's observations better,
that's how confident
he was in Tycho's data.
So what developed from Kepler's work
are Kepler's three laws
of planetary motion.
The first one corrects the
orbit shape of the planets.
They are ellipses, not circles.
Now they are not very elliptical
but if you are calculating
where a planet is going to be in an orbit
and you are using a
circle versus an ellipse,
you will not get the right information.
And the sun is sitting at
one focus in the ellipse,
and in an ellipse there are two focuses.
This is just showing a
picture drawing an ellipse.
Here is the focus and
here is the other focus.
One of those focus represents
where the sun would be.
Kepler's second law is better demonstrated
in a figure but I'll state it here.
A planet moves so that its radius vector
sweeps out equal areas in equal times.
Lets take a look at that.
So here's an orbit.
Here's the radius vector.
So as the planet moves
in orbit around the sun,
it sweeps out equal areas in equal time.
So this means that area
A is equal to area B
which is equal to area C.
So what does that mean?
What do the planets have to be doing,
the planet have to be doing
as it's orbiting the sun?
What happens to its speed?
Pause the video and think
about it for a moment.
As the planet moves from A to B,
C to D,
E to F,
where in the orbit is the
planet moving the fastest?
Well the answer is E to F
because it's closest to the sun
and gravity is helping it move along.
The area, area C here shown in yellow,
is this equal, or equivalent, to B and A,
but what is happening is the planet
it speeding up or slowing down
depending on where in the orbit it is.
The closer to the sun,
the faster it is moving,
and the greater the distance
it is traveling here.
Little bit slower there and
slowest furthest from the sun.
Finally Kepler's third law says
the ratio between the square
of the time needed by a planet
to make a revolution around the sun
and the cube of its average
distance from the sun
is the same for all planets.
Let's look at this in formula.
So the period of the planet
squared, that's how much time
it takes for a planet to orbit the sun,
the Earth 365 days,
divided by the average orbit radius cubed
is the same for all planets.
In other words,
it's a constant.
Can I fit that on here? All right.
And it can be written in short form here,
T squared, with T being
the period of the planet,
divided by R cubed is
the same for all planets.
Here's an example of
proving Kepler's third law.
And we have Saturn's satellites,
Tethys, Dione, Rhea, and Iapetus,
and if you calculate the
period of the planet squared,
and this is for Tethys
here, 1.89 days squared,
divided by the average radius,
2.95 times 10 to the
fifth kilometers cubed,
you get this value right here,
1.4 times 10 to the
negative 16 days squared
per kilometer cubed.
And that ratio is the same
for all other satellites,
so we can say that Kepler's
third law in fact holds
for this satellite system.
I think this is a good stopping point
for our first video on chapter one
and we will continue on in
chapter one with video two.
