The Gravity of the Situation: Crash Course Astronomy #7
Summary
TLDRThis script delves into the fundamental concept of gravity, explaining its perpetual yet distance-decreasing force and its impact on our perception of physics. It discusses mass, density, and how gravity influences motion, from everyday experiences on Earth to the complexities of space orbits. The episode covers the history of gravitational understanding, from Newton and Hooke to Kepler's laws, and explores the idea of escape velocity, highlighting the continuous yet weakening nature of gravity even as objects move further away.
Takeaways
- 🌍 We live on a planet, which is an unusual condition in the vast emptiness of the universe, often referred to as 'space'.
- 🚀 Gravity is a fundamental force that acts differently on a planet compared to deep space, where it weakens rapidly with distance.
- 📚 The concept of mass is crucial to understanding gravity; it's a measure of how much 'stuff' an object contains, irrespective of its size or density.
- 🔍 Historically, gravity was a fact of life until scientists like Robert Hooke and Isaac Newton began to mathematically investigate its principles.
- 🪐 Gravity is an attractive force that pulls objects together, with the force's strength depending on the masses involved and the distance between them.
- 📉 The force of gravity weakens with the square of the distance from the object exerting the force, making it a dominant factor in celestial mechanics.
- 💫 Gravity can cause objects to accelerate, as seen when a rock falls to the ground, gaining speed as it falls due to the continuous action of gravity.
- 🛰 Orbiting is a state where objects move under the influence of gravity, with the simplest form being a straight line, as in the case of a falling rock.
- 🌌 Kepler discovered that planets orbit the Sun in ellipses, not perfect circles, which was a significant advancement in our understanding of celestial motion.
- 🚀 The escape velocity is the minimum speed needed for an object to break free from the gravitational pull of another body, varying based on mass and size.
- 👨🚀 Astronauts in space appear 'weightless' not because gravity is absent, but because they are in a state of free fall, experiencing no resistance to their motion.
- 🌌 Even light, which has no mass, is affected by gravity, bending its path as it passes massive objects, demonstrating that gravity can warp the fabric of space.
Q & A
What is the primary difference between living on a planet and being in deep space in terms of physics?
-The primary difference is gravity. On a planet, gravity pulls objects toward its center, causing them to fall back down if thrown, whereas in deep space, objects would continue moving away at a constant speed if not influenced by other forces.
Why did Robert Hooke and Isaac Newton have a feud over the concept of gravity?
-They had a feud over who first conceived the mathematical understanding of gravity. Both were investigating the concept, and the dispute was about originality and priority in the discovery.
What is mass and how is it related to an object's resistance to changes in motion?
-Mass is a measure of how much 'stuff' makes up an object. It's related to an object's resistance to changes in motion because an object with more mass is harder to move or stop than one with less mass due to its greater inertia.
How does the force of gravity from an object affect another object?
-The force of gravity between two objects depends on their masses and the distance between them. The closer the objects are, the stronger the gravitational force, and the greater their masses, the stronger the attraction.
Why does the force of gravity weaken with the square of the distance?
-The force of gravity weakens with the square of the distance due to the inverse-square law, which states that the intensity of a physical quantity is inversely proportional to the square of the distance from the source.
What is the significance of an object accelerating when it falls towards the Earth?
-The acceleration signifies that the force of gravity is acting on the object, increasing its velocity as it falls. This acceleration is due to the continuous application of the gravitational force over time.
What is an orbit and what are the different types of orbits mentioned in the script?
-An orbit is a path followed by an object under the influence of a gravitational force. The script mentions straight lines, circles, ellipses, parabolae, and hyperbolae as different types of orbits.
How does the concept of escape velocity relate to the force of gravity?
-Escape velocity is the minimum speed needed for an object to break free from the gravitational influence of another body. It is related to the force of gravity because the object must overcome the gravitational pull to escape.
Why are astronauts on the International Space Station considered to be in a state of 'weightlessness'?
-Astronauts are in a state of 'weightlessness' because they are in free fall around the Earth. There is no surface pushing back against them, so they do not experience the sensation of weight, even though gravity is still acting on them.
How does the script explain the concept of mass affecting the motion of light?
-The script explains that even though photons, particles of light, have no mass, they can still be affected by gravity. This is because gravity can warp space, and light follows the curvature of space, changing its direction as it passes a massive object.
What is the role of Johannes Kepler in understanding orbits as described in the script?
-Johannes Kepler is credited with discovering that planets orbit the Sun in ellipses, not perfect circles as previously thought. This discovery expanded our understanding of orbits beyond just circular paths.
Outlines
🌌 Understanding Gravity and Mass
This paragraph delves into the concept of gravity and its omnipresence in shaping our everyday experiences on Earth, despite the universe being predominantly empty. It explains how gravity, which diminishes with distance, is a fundamental force that contrasts the emptiness of space. The discussion introduces mass as a measure of an object's 'stuff', affecting its motion resistance and interaction with gravity. The paragraph also clarifies that mass is not directly related to size or density, and that every object with mass exerts gravitational force on others. The force of gravity is influenced by the masses involved and the distance between them, with a specific focus on the square law of distance, which dictates how the force weakens as objects move apart. The paragraph concludes with an introduction to the concept of orbits, explaining how objects in freefall under gravity can be in a state of orbit, and how different forces can influence this motion, such as in the case of a rock thrown upwards that falls back due to gravity.
🚀 Orbits, Escape Velocity, and the Effects of Gravity
The second paragraph explores the complexities of orbits and the concept of escape velocity. It starts by discussing how the shapes of orbits can vary, from the simple circular orbits to elliptical ones, and how the force required to achieve these orbits depends on the velocity of the object and the gravitational pull of the body it is orbiting. The paragraph then introduces the idea of escape velocity, which is the minimum speed needed for an object to break free from the gravitational pull of another body. It provides examples of the escape velocities for Earth, Jupiter, and the Sun, illustrating how these values differ based on mass and size. The discussion continues with the notion of an escape orbit, which is an open trajectory that does not return, and contrasts it with the closed orbits that repeat themselves. The paragraph also touches on the astronauts' experience of 'weightlessness' in space, which is a result of being in a state of freefall around Earth, and how this differs from the absence of gravity. It concludes with a fascinating insight into how even light, which has no mass, can be affected by gravity, bending its path as it passes massive objects, hinting at the concept of space-time curvature that will be explored further in the context of black holes.
Mindmap
Keywords
💡Planet
💡Gravity
💡Mass
💡Density
💡Orbit
💡Escape Velocity
💡Inertia
💡Free Fall
💡Weightlessness
💡Spacetime
💡Photons
Highlights
We live on a planet in a universe that is mostly empty, which contrasts with our everyday experience of gravity.
Gravity is a fundamental force that acts differently on a planet compared to deep space.
The concept of mass is crucial for understanding gravity, defined as how much 'stuff' an object contains.
Density is introduced as a measure of mass spread out in a given volume.
Mass determines an object's resistance to changes in motion, affecting how easily it can be moved.
Gravity is defined by the mass of both the object exerting the force and the object experiencing it.
The force of gravity weakens with the square of the distance from the object.
Gravity is always attractive, never repulsive, and causes objects to accelerate towards each other.
Orbiting is described as a state of freefall where objects are in constant acceleration towards each other.
Newton's insight on the relationship between the sideways throw of an object and its orbit around Earth.
Kepler's discovery that planets orbit the Sun in ellipses, not perfect circles.
The concept of escape velocity and how it allows objects to break free from gravitational pull.
The difference between weight and mass, especially in the context of astronauts experiencing 'weightlessness' in space.
The subtle forces acting on astronauts in orbit, referred to as 'microgravity'.
Photons, despite having no mass, are affected by gravity and their paths can be bent by it.
Gravity's ability to warp space, demonstrated by the bending of light around massive objects.
A summary of the different kinds of orbits: straight lines, circles, ellipses, parabolae, and hyperbolae.
The episode's educational aim to explain the nature of gravity, orbits, and the experience of weightlessness.
Transcripts
We live — and stop me if I’m going too fast — on a planet.
I mean, sure, duh. But this isn’t the natural state of the Universe; or, at least, it’s
not the way things usually are. Most of the Universe is pretty empty — that’s why
we call it “space” — and if I were to magically transport you someplace randomly
in the cosmos, the chances are you’d be a million light years from the nearest substantial object.
Evolving on a planet has warped our sense of physics. If I throw an object away from
me, it comes back. That’s bizarre! It should just keep going, moving away from me at a
constant speed. Instead though it goes up, slows, stops, then falls back down toward me.
The difference between living on a planet and being in deep space is gravity. Gravity
from an object goes on forever, but it gets weaker rapidly with distance. A zillion light
years away, the Earth’s gravity is fantastically weak, but here on Earth it’s literally a force to be
reckoned with. And in some places it can be a lot stronger than what we experience right here.
For most of history, gravity was just a fact of life, neither understood nor examined terribly
closely. In the mid 1600s, scientists like Robert Hooke and Isaac Newton started investigating
it using math — in fact, the two men got into a bitter feud over who thought of what
first. But whoever it was who first got it right, now we have a much better understanding
of how gravity works.
One thing before we get to gravity. An important concept that comes up a lot is mass. It’s
a bit tricky to define, but you can think of it as how much stuff makes up an object.
I know, that’s not very scientific sounding, but it’s not a bad way to think about it.
Something with more mass has more stuff in it.
Size doesn’t really play into this; two objects can have the same mass but one can
be much larger than the other. In that case, the bigger object’s mass is more spread
out, so we say it has lower density, where density is how much mass is inside a given volume.
In science terms, mass tells us how much an object resists having its motion changed.
An object with more mass is harder to get moving than an object with less mass, which
is pretty obvious if you’ve ever tried pushing on a toy car versus a real truck. But mass
is also defined using gravity.
Everything that has mass also has gravity and can inflict this force on another object.
The amount of force you feel from the gravity of an object like a planet depends on three
things: How much mass it has, how much mass you have, and how far away you are from it.
In fact, distance dominates here; the force of gravity weakens with the square of the
distance. Double your distance from an object and the force of gravity drops by 2 x 2 = 4
times. Go 10 times farther away and the force drops by 10 x 10 = 100 times.
Gravity is also attractive: It can only draw things in, not repel them. But how it attracts
things is where it gets fun.
If I hold up a rock and let go of it, it falls to the ground. What might be hard to see is
that it gets faster the longer it drops. Forces accelerate objects, so the longer the force
acts, the more the object’s velocity changes – in this case getting faster. If I drop
a rock from higher up, it’ll move faster when it hits the ground. Other forces act
on moving objects, as well, like friction and air resistance, counteracting gravity,
making this acceleration hard to see. But in space, the force of gravity becomes very clear.
Two objects that have mass will attract each other. If there are no other forces acting
on them, they’ll accelerate toward each other until they meet. Remember, though, that
the force of gravity depends on those masses. If one is really massive, and the other not
so much, then in more practical terms the massive one will pull in the less massive
one. The more massive one does move, but much less than the other one.
When objects are free to move under the effects of gravity, we say they are in orbit. The
simplest kind of orbit may not be what you think: It’s actually just a line! When you
drop a rock, it’s very briefly in orbit. Ignoring things like the Earth’s rotation
(which adds a bit of sideways motion) it’s close enough to say the rock just falls straight
down, and is stopped because the Earth itself gets in the way.
That’s not a terribly interesting orbit! So what if, instead of dropping the rock,
we throw it? That gives it a little bit of sideways motion, so instead of hitting the
ground at my feet, it hits a bit farther away. If I throw it harder, it moves horizontally
even more before it hits.
What if I throw it really hard?
This is where Newton’s genius comes in. He realized that if you throw the ball hard
enough sideways, it will fall at the exact same rate the Earth would curve away underneath
it. As Douglas Adams said in “Hitchhiker's Guide to the Galaxy,” flying is just falling
and missing the ground. It turns out, that’s exactly what orbiting is, too.
A rock thrown hard enough sideways will fall toward the Earth, but always miss it, going
instead into a circular path around it, guided only by gravity. It will orbit the Earth in
a circle, taking about 90 minutes to go around the planet once.
Circles are simple orbits. The speed at which the orbiting satellite travels depends on
the mass of the object it’s orbiting, and its distance from it. The farther it is, the
weaker gravity is, so it doesn’t have to travel as quickly to maintain the orbit.
Roughly 400 years ago, the astronomer Johannes Kepler realized that there can be other shapes
of orbits as well. He discovered the planets orbit the Sun on ellipses, when previously
it was thought they orbited in perfect circles. An elliptical orbit happens when you throw
the rock sideways even harder than it takes for a circular orbit; it goes up higher on
one end of the orbit than on the other.
In fact, the harder you throw the rock, the more elongated the orbit gets. An orbit like
this is still closed; that is, the orbit repeats itself and the rock is still bound to the
Earth by gravity. At some point, though if you throw the rock hard enough, an amazing
thing happens: It can escape.
Remember, gravity gets weaker with distance. If you throw a rock hard enough, while gravity
can slow it down, the gravity gets weaker the farther away the rock is. If the rock
has enough velocity, gravity weakens too quickly to stop it. The rock can escape, moving away
forever, so we call this the escape velocity.
The escape velocity of an object like a planet or star depends on how much mass it has and
how big it is. For the Earth, that turns out to be about 11 kilometers per second — for
Jupiter, it’s about 58 kilometers per second, and for the Sun it’s a whopping 600 kilometers
per second. Whatever the particular escape velocity for your cosmic location is, if you
fling a rock away from it faster than that, I hope you kissed it goodbye first, ‘cause
it ain’t coming back. One way to think of it is that the rock is always slowing, getting
ever closer to stopping, but it never actually stops. If it could travel infinitely far away,
it would stop, but that’s kind of a long trip.
This works in reverse, too. If I go way far away from the Earth and drop a rock, it’ll
accelerate. When it hits the planet it’ll be moving at escape velocity, that same 11
kilometers per second. And if I give it a little sideways kick, it’ll miss the Earth
but still pass us at escape velocity. An escape orbit is open — it doesn’t come back — and
is shaped like a parabola.
What if you throw the rock even harder than that? The rock doesn’t come back, and moves
away even faster. The orbit is now a hyperbola, which is similar to a parabola, but is even
more open. The rock never stops, even at infinity. It just keeps movin’ on.
Like all forces, gravity gets weaker with distance. But its force never quite drops
to zero; it just gets smaller and smaller as you get farther and farther away.
So why then are astronauts on the space station “weightless”?
Gravity is still pulling on the astronauts! In fact, at the height of the station, Earth’s
gravity has only decreased by a little bit; it’s still about 90% as strong as it is
on the Earth’s surface. If they were in a tower 320 kilometers high they’d weigh
90% of what they do on the Earth’s surface. But the big difference is that the astronauts
are in orbit, falling around the Earth. Weight is actually not just the force of gravity
on a mass, but how hard a surface pushes back on that mass. For example, when you stand
on the ground, the ground pushes back. Otherwise you’d fall through! The force of the ground
back on you is what causes you to have weight.
In free fall, there’s nothing pushing back. You’re falling freely, and so you have no
weight. NASA likes to call this condition “microgravity,” since there are subtle
forces acting on you.
This actually highlights the difference between mass and weight. In space you have the same
mass as you do on Earth, but no weight. If another astronaut pushed on you they’d have
to exert a force, but if you stood on a scale in space it wouldn’t register anything.
Space is weird. Well, compared to Earth.
One more thing, and this is truly weird: Photons, particles of light, have no mass, yet they
can be affected by gravity, too, bending their direction of flight as they pass a massive
object! It turns out gravity can actually warp space! Light travels along the fabric
of space like a truck on the road, and if the road curves, so does the truck. I know
this is an odd concept, and we’ll be dealing with it later in more detail when we push
escape velocity to its limits… with black holes.
Today you learned that gravity is a force, and everything with mass has gravity. Gravity
accelerates object, changing their speed and/or direction. An object moving along a path controlled
by gravity is said to be in orbit, and there are many different kinds: straight lines,
circles, ellipses, parabolae, and hyperbolae. You can’t ever escape gravity, but if you
travel faster than escape velocity for an object you’ll get away from it without falling
back. And if you’re in orbit, in freefall, you have no weight, but you still have mass.
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Start Here. Go Anywhere.
Crash Course Astronomy is produced in association with PBS Digital Studios, and you can head
over to their channel and find more awesome videos. This episode was written by me, Phil
Plait. The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.
It was co-directed by Nicholas Jenkins, and Michael Aranda, edited by Nicole Sweeney,
and the graphics team is Thought Café.
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