—
In today’s episode, Phil looks at how gravity plays out across the universe.
—
—
Transcript Provided by YouTube:
00:03
We live — and stop me if I’m going too fast — on a planet.
00:06
I mean, sure, duh. But this isn’t the natural state of the Universe; or, at least, it’s
00:12
not the way things usually are. Most of the Universe is pretty empty — that’s why
00:16
we call it “space” — and if I were to magically transport you someplace randomly
00:21
in the cosmos, the chances are you’d be a million light years from the nearest substantial object.
00:26
Evolving on a planet has warped our sense of physics. If I throw an object away from
00:30
me, it comes back. That’s bizarre! It should just keep going, moving away from me at a
00:35
constant speed. Instead though it goes up, slows, stops, then falls back down toward me.
00:41
The difference between living on a planet and being in deep space is gravity. Gravity
00:46
from an object goes on forever, but it gets weaker rapidly with distance. A zillion light
00:51
years away, the Earth’s gravity is fantastically weak, but here on Earth it’s literally a force to be
00:56
reckoned with. And in some places it can be a lot stronger than what we experience right here.
01:11
For most of history, gravity was just a fact of life, neither understood nor examined terribly
01:17
closely. In the mid 1600s, scientists like Robert Hooke and Isaac Newton started investigating
01:21
it using math — in fact, the two men got into a bitter feud over who thought of what
01:26
first. But whoever it was who first got it right, now we have a much better understanding
01:30
of how gravity works.
01:31
One thing before we get to gravity. An important concept that comes up a lot is mass. It’s
01:36
a bit tricky to define, but you can think of it as how much stuff makes up an object.
01:40
I know, that’s not very scientific sounding, but it’s not a bad way to think about it.
01:44
Something with more mass has more stuff in it.
01:47
Size doesn’t really play into this; two objects can have the same mass but one can
01:51
be much larger than the other. In that case, the bigger object’s mass is more spread
01:55
out, so we say it has lower density, where density is how much mass is inside a given volume.
02:01
In science terms, mass tells us how much an object resists having its motion changed.
02:06
An object with more mass is harder to get moving than an object with less mass, which
02:10
is pretty obvious if you’ve ever tried pushing on a toy car versus a real truck. But mass
02:15
is also defined using gravity.
02:17
Everything that has mass also has gravity and can inflict this force on another object.
02:22
The amount of force you feel from the gravity of an object like a planet depends on three
02:26
things: How much mass it has, how much mass you have, and how far away you are from it.
02:32
In fact, distance dominates here; the force of gravity weakens with the square of the
02:37
distance. Double your distance from an object and the force of gravity drops by 2 x 2 = 4
02:42
times. Go 10 times farther away and the force drops by 10 x 10 = 100 times.
02:47
Gravity is also attractive: It can only draw things in, not repel them. But how it attracts
02:53
things is where it gets fun.
02:55
If I hold up a rock and let go of it, it falls to the ground. What might be hard to see is
02:59
that it gets faster the longer it drops. Forces accelerate objects, so the longer the force
03:04
acts, the more the object’s velocity changes – in this case getting faster. If I drop
03:08
a rock from higher up, it’ll move faster when it hits the ground. Other forces act
03:12
on moving objects, as well, like friction and air resistance, counteracting gravity,
03:16
making this acceleration hard to see. But in space, the force of gravity becomes very clear.
03:21
Two objects that have mass will attract each other. If there are no other forces acting
03:25
on them, they’ll accelerate toward each other until they meet. Remember, though, that
03:28
the force of gravity depends on those masses. If one is really massive, and the other not
03:33
so much, then in more practical terms the massive one will pull in the less massive
03:37
one. The more massive one does move, but much less than the other one.
03:41
When objects are free to move under the effects of gravity, we say they are in orbit. The
03:46
simplest kind of orbit may not be what you think: It’s actually just a line! When you
03:50
drop a rock, it’s very briefly in orbit. Ignoring things like the Earth’s rotation
03:54
(which adds a bit of sideways motion) it’s close enough to say the rock just falls straight
03:58
down, and is stopped because the Earth itself gets in the way.
04:01
That’s not a terribly interesting orbit! So what if, instead of dropping the rock,
04:05
we throw it? That gives it a little bit of sideways motion, so instead of hitting the
04:09
ground at my feet, it hits a bit farther away. If I throw it harder, it moves horizontally
04:14
even more before it hits.
04:15
What if I throw it really hard?
04:17
This is where Newton’s genius comes in. He realized that if you throw the ball hard
04:21
enough sideways, it will fall at the exact same rate the Earth would curve away underneath
04:26
it. As Douglas Adams said in “Hitchhiker’s Guide to the Galaxy,” flying is just falling
04:30
and missing the ground. It turns out, that’s exactly what orbiting is, too.
04:35
A rock thrown hard enough sideways will fall toward the Earth, but always miss it, going
04:40
instead into a circular path around it, guided only by gravity. It will orbit the Earth in
04:45
a circle, taking about 90 minutes to go around the planet once.
04:49
Circles are simple orbits. The speed at which the orbiting satellite travels depends on
04:53
the mass of the object it’s orbiting, and its distance from it. The farther it is, the
04:58
weaker gravity is, so it doesn’t have to travel as quickly to maintain the orbit.
05:02
Roughly 400 years ago, the astronomer Johannes Kepler realized that there can be other shapes
05:07
of orbits as well. He discovered the planets orbit the Sun on ellipses, when previously
05:12
it was thought they orbited in perfect circles. An elliptical orbit happens when you throw
05:15
the rock sideways even harder than it takes for a circular orbit; it goes up higher on
05:20
one end of the orbit than on the other.
05:21
In fact, the harder you throw the rock, the more elongated the orbit gets. An orbit like
05:26
this is still closed; that is, the orbit repeats itself and the rock is still bound to the
05:30
Earth by gravity. At some point, though if you throw the rock hard enough, an amazing
05:34
thing happens: It can escape.
05:36
Remember, gravity gets weaker with distance. If you throw a rock hard enough, while gravity
05:41
can slow it down, the gravity gets weaker the farther away the rock is. If the rock
05:46
has enough velocity, gravity weakens too quickly to stop it. The rock can escape, moving away
05:51
forever, so we call this the escape velocity.
05:54
The escape velocity of an object like a planet or star depends on how much mass it has and
05:58
how big it is. For the Earth, that turns out to be about 11 kilometers per second — for
06:03
Jupiter, it’s about 58 kilometers per second, and for the Sun it’s a whopping 600 kilometers
06:08
per second. Whatever the particular escape velocity for your cosmic location is, if you
06:13
fling a rock away from it faster than that, I hope you kissed it goodbye first, ‘cause
06:18
it ain’t coming back. One way to think of it is that the rock is always slowing, getting
06:22
ever closer to stopping, but it never actually stops. If it could travel infinitely far away,
06:28
it would stop, but that’s kind of a long trip.
06:30
This works in reverse, too. If I go way far away from the Earth and drop a rock, it’ll
06:35
accelerate. When it hits the planet it’ll be moving at escape velocity, that same 11
06:39
kilometers per second. And if I give it a little sideways kick, it’ll miss the Earth
06:43
but still pass us at escape velocity. An escape orbit is open — it doesn’t come back — and
06:48
is shaped like a parabola.
06:50
What if you throw the rock even harder than that? The rock doesn’t come back, and moves
06:54
away even faster. The orbit is now a hyperbola, which is similar to a parabola, but is even
06:59
more open. The rock never stops, even at infinity. It just keeps movin’ on.
07:04
Like all forces, gravity gets weaker with distance. But its force never quite drops
07:08
to zero; it just gets smaller and smaller as you get farther and farther away.
07:12
So why then are astronauts on the space station “weightless”?
07:16
Gravity is still pulling on the astronauts! In fact, at the height of the station, Earth’s
07:20
gravity has only decreased by a little bit; it’s still about 90% as strong as it is
07:25
on the Earth’s surface. If they were in a tower 320 kilometers high they’d weigh
07:30
90% of what they do on the Earth’s surface. But the big difference is that the astronauts
07:34
are in orbit, falling around the Earth. Weight is actually not just the force of gravity
07:39
on a mass, but how hard a surface pushes back on that mass. For example, when you stand
07:44
on the ground, the ground pushes back. Otherwise you’d fall through! The force of the ground
07:49
back on you is what causes you to have weight.
07:52
In free fall, there’s nothing pushing back. You’re falling freely, and so you have no
07:56
weight. NASA likes to call this condition “microgravity,” since there are subtle
08:00
forces acting on you.
08:01
This actually highlights the difference between mass and weight. In space you have the same
08:05
mass as you do on Earth, but no weight. If another astronaut pushed on you they’d have
08:10
to exert a force, but if you stood on a scale in space it wouldn’t register anything.
08:14
Space is weird. Well, compared to Earth.
08:17
One more thing, and this is truly weird: Photons, particles of light, have no mass, yet they
08:23
can be affected by gravity, too, bending their direction of flight as they pass a massive
08:28
object! It turns out gravity can actually warp space! Light travels along the fabric
08:33
of space like a truck on the road, and if the road curves, so does the truck. I know
08:37
this is an odd concept, and we’ll be dealing with it later in more detail when we push
08:41
escape velocity to its limits… with black holes.
08:44
Today you learned that gravity is a force, and everything with mass has gravity. Gravity
08:49
accelerates object, changing their speed and/or direction. An object moving along a path controlled
08:55
by gravity is said to be in orbit, and there are many different kinds: straight lines,
08:59
circles, ellipses, parabolae, and hyperbolae. You can’t ever escape gravity, but if you
09:04
travel faster than escape velocity for an object you’ll get away from it without falling
09:08
back. And if you’re in orbit, in freefall, you have no weight, but you still have mass.
09:13
This episode is brought to you by Squarespace. The latest version of their platform, Squarespace
09:18
Seven, has a completely redesigned interface, integrations with Getty Images and Google
09:22
Apps, new templates, and a new feature called Cover Pages. Try Squarespace at Squarespace.com,
09:28
and enter the code Crash Course at checkout for a special offer. Squarespace.
09:32
Start Here. Go Anywhere.
09:34
Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head
09:38
over to their channel and find more awesome videos. This episode was written by me, Phil
09:42
Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
09:46
It was co-directed by Nicholas Jenkins, and Michael Aranda, edited by Nicole Sweeney,
09:50
and the graphics team is Thought Café.
—
This post was previously published on YouTube.
—
Photo credit: Screenshot from video.