### Video Transcript

In this video, we’re talking about
the motion of planets, moons, and satellites. All of these objects move in what
are called orbits. An orbit is a cyclical path
typically around some other larger body. For example, the Earth orbits
around the larger body of the Sun. And there’s a smaller moon that
orbits around the Earth as well as all the satellites that orbit around Earth
too. The motion of these bodies through
space all comes down to the influence of one specific force, the force of
gravity.

Now, gravity is an interesting
force because any object that has any mass at all will tend to exert this force on
other objects with mass. Say, for example, we put another
object here. When we do, this object will be
affected by a gravitational pole created by this first object. If we drew in that force as a
vector, it would look like this. It would point from the second
object towards the center of the first one. And by the way, this attractive
force works the same in both directions. Just as the larger body exerts a
force on the smaller one, the smaller body exerts a force on a larger one which has
the same magnitude. But when it comes to motion, this
smaller body will tend to move more because it has less mass.

Now, as the force of gravity has
been investigated over time, it’s been discovered that distance has something to do
with this force. If we were to take a third mass
which had the same exact mass as this second one we placed here and position it
farther away from our original object. Then this mass would still
experience a gravitational force from the larger mass on the far right. But it would be smaller. It would be weaker. And if we put an identical mass
still farther away from our original object, the force on that mass from the object
would be weaker still. So far then, there are three things
we can say about this force of gravity.

First off, any pair of objects, so
long as those objects have mass, experiences a gravitational force. This means that two pencils sitting
on our desktop, two cars driving down the road, or a planet orbiting a star, all
have a gravitational force in between them. A second thing we can say about
this force is that the force of gravity always attracts, and it never repels. Notice the direction of the arrows
of the force acting on these three identical bodies from the larger one. The force of gravity always pulls
other objects in, and it never pushes them away. And thirdly, we can say that the
closer two objects are, the stronger the force of gravity between them. Another way of saying this is the
farther apart objects are, the weaker the forces. And again we saw this with our
three identical objects placed different distances away from our original mass. As the objects got farther away,
the force on them got weaker and weaker.

Keeping all this in mind, let’s say
that we’re now in outer space. And in this space, thanks to a
formation process that’s been going on for many years, we have a star. Now, things would be pretty
uninteresting if this was the only thing that existed in the whole universe. Because remember, for the force of
gravity to have an effect, we need to have at least two objects. Well, look at this. Here comes a second object moving
along past our first one. Now that we have two objects to
consider, we know that the force of gravity will be acting on them. If we were to sketch in a line that
goes from the center of our one object to the center of our other object, then along
this line there will be a force acting on the moving object and an equal but
opposite force acting on our star.

Now, depending on how strong this
force is and how fast our object in motion moves, a few different things can
happen. One possible outcome is that even
though there’s this attractive force which tends to pull this body in motion towards
the star. It could be that this object is
moving so fast that it escapes the pole of the star. In that case, the objects
trajectory might look like this. Yes, it is being pulled towards the
star, as we can see from the path slightly bending. But that pole isn’t strong enough
to bring this moving object in a regular, steady orbit around the star.

But let’s say that instead of the
object moving very fast compared to the force acting on it, it’s moving very
slow. In that case, the force of gravity
will be the dominating influence on this object’s motion. The fact that it was already moving
in a certain direction won’t have a strong effect. If that’s true, then the path of
this moving object might look like this. Following a curved line path that
we see here, it’s drawn into the star and eventually crashes into its surface. We can see that this path also
doesn’t lead to a steady regular orbit around the star. But if the initial conditions are
just right, if the velocity of this planet points in a certain direction, and if the
planet speed has a particular relationship to the force of gravity acting on it. When all these factors balance out
just right, this moving object is able to start moving around the star in a regular
path, an orbit.

Any time we see an object in orbit,
we’re witnessing a delicate balance between attractional forces of gravity and
object motion. And, in general, there are two
types of orbits that an object can have. If we clear a little bit of screen
space, we can see what these types are. The first kind of way that a
smaller body could orbit a larger one is by moving in a circular path, a circular
orbit, around it. This is how we often see orbits
drawn in sketches. But, actually, it’s the exception
rather than the rule when it comes to orbital pairs. It’s much more common for objects
to move in what are called elliptical orbits. An ellipse looks like a circle
that’s been squished or flattened. And ellipses themselves can have
different shapes. Some are more like a circle, and
some are less.

Now, when an object is moving in a
perfectly circular orbit around some other object. Then there’s a particular
relationship between the gravitational force acting on the moving object and its
velocity. For a circular orbit, an object’s
velocity is always perpendicular to the gravitational force acting on it. This isn’t always true, though, for
objects that have an elliptical orbit. In those cases, there’s usually not
a 90-degree angle between the object’s velocity and the force acting on it. Speaking of object velocity, let’s
consider this elliptical orbit in green. We saw from our three observations
about gravity that the closer two objects are together, the stronger the force is
between them.

This means that, for our object
that’s in an elliptical orbit around the star, when the object is here in its orbit,
the force of gravity acting on it is stronger than when it’s here. That’s because the distance between
the star and the object in this case is smaller than the distance between the star
and the object here. All of this has an influence on the
object’s velocity. When the object is closer into the
star, not only is the gravitational force on it stronger, but its velocity is
greater as well. This object in elliptical orbit
then will be moving fastest when it’s closest to the star it orbits. And it will be moving slowest when
it’s farthest away.

We can take this idea and apply it
to our own solar system. In this system, we have a star, our
sun, being orbited by eight planets. And these planets have varying
distances from the sun. Some are closer; some are farther
away. What we’ve seen here in this
example is that the closer an orbital object is to the object it orbits, the faster
it’s moving. And this principle holds true for
the planets in our solar system. The fastest moving planets are the
ones closest to the sun. With mercury, the closest planet of
all, being the fastest moving of all. And then, on the other end of
things way out here we have Neptune, the slowest moving planet.

Talking about faster or slower
moving objects in orbit brings up a term called orbital period. An object’s orbital period is the
amount of time it takes to go through one complete revolution of its orbit. So, for example, if we track the
Earth through one complete revolution of its motion, we know that this takes about
365 days. That’s its orbital period. Now, let’s think for a moment on a
bit of a smaller scale about objects that orbit Earth. So here’s our Earth. And besides the moon, we know that
there are also man-made objects which orbit our planet. Along with satellites used for
communication of various kinds, we’ve also constructed a satellite called the
International Space Station. The orbital period for the
International Space Station is only about 90 minutes. In other words, it goes through one
complete revolution of its orbit every hour and a half.

But imagine we wanted to make a
satellite which stayed over the exact same location on Earth’s surface as the Earth
rotated. In other words, as the Earth turns
on its axis, we would want a satellite to stay over one particular spot, say this
one right here, even as that goes on. In that case, we would send up a
satellite to be at that location, and its orbital period would need to be the same
amount of time that it takes the Earth to spin on its access. We know it takes 24 hours for that
to happen. So that would be this satellite’s
orbital period. Satellites like this that stay over
the same location on Earth’s surface at all times have a special name. They’re called geostationary
satellites. From the name, we can tell that
these satellites stay still. They’re stationary over the
Earth. In order for this to be true for a
satellite to stay over the same spot of the Earth at all times, it needs to be in
orbit above the Earth’s equator.

Knowing all this about orbiting
objects, planets, moons, and satellites, let’s get a bit of practice using an
example.

The diagram shows two different
possible orbits of an object around a star. Which of the following
correctly describes the shape of orbit a)? A) Elliptical, B) Circular, C)
Helical, D) Spiral, E) Highly elliptical.

All right, taking a look at our
diagram, we see these two orbits marked a) and b). And this question focuses on
orbit a). We want to know which of these
five paths correctly describes this particular orbit. Now, the first thing we can do
is recall that whenever one object is in orbit around another one, there are two
possible paths that that orbiting object can follow. Orbital paths are either
circular or they’re elliptical. These are the two allowed
shapes we could say for orbital motion. That eliminates two of our
possible answer choices, option C and D. Helical and spiral are not
allowed orbital paths.

At this point, we can recall
the difference between these two allowed orbits of circles and ellipses. We all know what a circle looks
like. And an ellipse looks like a
circle that’s been squished or compressed. So if this was a circular
orbital path, an elliptical path would look something like this. Now, we can see that our three
remaining answer choices are elliptical, circular, and something called highly
elliptical. For an orbit to be highly
elliptical, we could think of that as a very compressed circular orbit. The squishing, we could say,
has gone even farther to create something that looks like this. This would be an example of a
highly elliptical orbit. So which of these three —
elliptical, circular, or highly elliptical — is the orbit marked out in a)?

We can see that this orbit does
not look like a compressed circle, but rather it just looks like a circle
itself. It’s definitely not highly
elliptical then. We’ll cross off option E. And we also don’t think it’s
elliptical. The shape of orbit a) is
circular. Now, let’s consider the same
question, but about orbit b).

Our next question asks, which
of the following correctly describes the shape of orbit b)? A) Circular, B) Spherical, C)
Highly elliptical, D) Spiral, E) Helical.

Just like before, we can cancel
out any answer options which are neither circular nor elliptical. As we’ve seen, those were the
only two possible paths that an orbiting body can have. So this means that option B,
spherical, option D, spiral, and option E, helical, are off the table. So then, orbit b) is either
circular or it’s highly elliptical. If we revisit our sketch of a
circular orbit, an elliptical orbit, and a highly elliptical orbit marked out in
yellow, we can clearly see that orbit 𝑏 is not circular. This eliminates option A. And as we consider highly
elliptical as a description of this shape, we see that there’s a match. This orbit has a very
compressed or squished look compared to a circular orbit. And, therefore, it is highly
elliptical. This is the correct description
of the shape of orbit b).

Let’s take a moment now to
summarize what we’ve learned about the motion of planets, moons, and satellites. First off, we saw that all orbiting
objects, planets, moons, and satellites move in orbits due to the force of
gravity. We saw that gravity, first, exists
between all objects that have mass. Second, it always attracts and
never repels. And, third, that it gets stronger
the closer two objects are together. We learned further that when an
object is in orbit, it follows either a circular or an elliptical path. And, lastly, satellites that stay
over the same spot on Earth are called geostationary. This means that they have an
orbital period of 24 hours.