The Gravity of the Situation: Crash Course Astronomy #7

Summary

This 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

00:00

🌌 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.

05:02

🚀 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.

Keywords

💡Planet

A planet is a celestial body that orbits a star, is massive enough to be rounded by its own gravity, and has cleared its neighboring region of planetesimals. In the video, the concept of living on a planet is used to contrast with the vast emptiness of space, emphasizing the unique conditions that allow for life as we know it.

💡Gravity

Gravity is a natural force that attracts two objects with mass towards each other. The video script explains how gravity is a fundamental force on Earth, causing objects to fall back to the surface when thrown. It also discusses how gravity varies with distance and is responsible for the motion of celestial bodies in space.

💡Mass

Mass is a measure of the amount of matter in an object and is a key factor in determining the strength of gravitational forces. The script defines mass in a layman's term as 'how much stuff makes up an object,' and it is crucial in understanding how objects interact through gravity, regardless of their size or density.

💡Density

Density is defined as mass per unit volume. The script uses the concept of density to illustrate that two objects can have the same mass but different sizes, hence different densities. This is important in understanding how mass distribution affects the gravitational interaction between objects.

💡Orbit

An orbit is the curved path of an object in space as it moves around another object due to gravity. The video describes different types of orbits, such as circular and elliptical, and explains that orbiting is essentially falling around an object without ever hitting it, which is a direct result of gravity.

💡Escape Velocity

Escape velocity is the minimum speed needed for an object to break free from the gravitational attraction of a celestial body without further propulsion. The script mentions that this concept is key to understanding how objects can leave the gravitational influence of a planet or star.

💡Inertia

Inertia is the resistance of any physical object to any change in its velocity. The script implies this concept when discussing how objects with more mass are harder to set in motion, which is a direct result of their inertia.

💡Free Fall

Free fall is the motion of an object under the sole influence of gravity. The video script explains that astronauts in orbit experience free fall, which results in the sensation of weightlessness despite gravity still acting upon them.

💡Weightlessness

Weightlessness is the perception of an apparent lack of gravity. The script clarifies that astronauts in space stations are not weightless in the sense that gravity is absent; rather, they are in a state of free fall, which gives the sensation of having no weight.

💡Spacetime

Spacetime is a four-dimensional continuum that combines space and time. The script touches on the concept that gravity can warp spacetime, affecting the path of light, which is a fundamental aspect of understanding gravity's influence on the universe at large scales.

💡Photons

Photons are elementary particles, the quantum of light and the force carrier for the electromagnetic force. The video script points out that even though photons have no mass, they are affected by gravity, which illustrates the universal nature of gravitational influence.

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.