[God's Dice] Science & Technology
Posted: Tue Apr 22, 2003 5:29 am
Just a place to discuss science and tech and physics stuff about God's Dice.
For reference, the current God's Dice document can be found here.
My first thought is something that's been bouncing around in my head for some time. It occurs to me that knowledge of basic physics is well advanced in the 30th century. In elementary school, most kids should have a grasp of Newtonian physics that would put most 21st century adults to shame.
It's really a matter of necessity... when you live on a huge space habitat, and you can look out the window and see that your world is visibly spinning full circle every five minutes... when you can watch giant moons orbiting around Jupiter (which fills half your sky, BTW... your childhood questions suddenly move well beyond "Why is the sky blue, Daddy?"
But it was just such questions that inspired the following piece:
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[Elementary School Physics]
A basic grasp of Newtonian physics comes early to 30th century denizens of space. Evidence of physics in action are everywhere and impossible to hide from:
“Mom, why do Io and Jupiter spin around in the sky like that? It makes me dizzy if I stare at them too long.”
They’re not spinning, honey; we are. Lincoln is shaped like a giant wheel, and we stand on the inside of it, like this.
“Okay, if Lincoln is spinning, why don’t we fall when we get to the top?”
Because the spinning is what keeps us in place. It’s like this basket. If I turn it upside down, what will happen?
“All my cubes will fall out?”
Right—but watch what happens when I swing it in a big circle over my head. See? Even when the basket was upside down, the cubes stayed in. How come?
“The spinning?”
Yup—just like that. If you start something moving, it wants to keep moving in a straight line until something stops it. So when I swing the basket, is it moving pretty fast?
“Yes.”
And is it moving in a straight line?
“No.”
That’s right. I got it moving, and it wants to move in a straight line, but it can’t, because my hand keeps pulling the basket back in a circle. The cubes keep trying to move in that straight line away from me, but my hand keeps pulling the basket back against them. So as long as I keep spinning, the cubes are stuck to the bottom of the basket. Same thing with Lincoln—it’s spinning pretty fast (over 500 klicks where we are), so our bodies want to keep moving in a straight line away from the center, but the curved shape of Lincoln keeps it moving in a circle instead, holding us in.
That’s centripetal force.
“So if I was on the outside and let go, I’d just go flying off into space?”
If you let go—yes.
“Then why don’t the Spacers go flying off? I’ve seen them walking on the outside all the time.”
Well, for one thing, Spacers never let go. That’s what their tails are for. Plus, they usually have at least two safety lines that tie them to the hab as well.
“Well Mom, is Jupiter spinning then? Because it looks like it is.”
Yes—Jupiter spins. In fact, on the surface, Jupiter spins a heck of a lot faster than Lincoln does.
“Okay, then why doesn’t everything on Jupiter go flying off into space?”
Because Jupiter is big.
“I don’t get it.”
It’s called gravity. All physical things in the universe have mass—substance. Mass generates gravity, which is a force that pulls all other mass towards itself. So everything in the universe is pulling on every other thing out there.
“So I have gravity that tries to pull stuff towards me?”
Yes—all things have gravity. But gravity is so weak, that you can’t even notice it unless it comes from something very very big.
“Like Lincoln?”
No—not even like Lincoln. To have gravity you would notice, a thing needs to be really huge—like a planet or a star or a moon. The bigger the object, the stronger the pull. But then the pull of gravity also gets weaker as you move farther away.
“Well, we learned that Jupiter is the biggest planet in Sol System, so it should be pulling us in, right?”
Yes, Jupiter’s gravity pulls very hard on Lincoln, but we don’t ever fall and hit the planet, because we are in orbit.
“Huh?”
Let me explain: Remember what happens to something when you start it moving? What does it want to do?
“Move in a straight line?”
Right. So imagine that this big ball is Jupiter, and imagine that this is us flying sideways above it here. We’re moving pretty fast, so what does our ship want to do?
“Keep moving in a straight line.”
That’s right. But Jupiter has a lot of mass and a lot of gravity, so what does it do to our ship?
“Pulls it down?”
You got it. So let’s watch what happens: if our ship is going too fast, then Jupiter’s gravity can’t pull us down fast enough, and even though it bends our path a little at first, soon we are far enough away that we don’t feel its gravity anymore.
On the other hand, if we are moving too slow, then gravity pulls us down to Jupiter really fast and then…
“CRASH!”
That’s right—we crash into the planet. But look what happens when our speed is just right: gravity keeps pulling us down, so we never get away, but we’re going fast enough that we never quite fall into the planet either. As our path bends, we just make a giant circle going all the way around the planet over and over. That’s our orbit. And as long as nothing stops us, we can keep going like this for a long long time.
“So are Io and Ganymede and Europa all in orbit around Jupiter too?”
You bet—all the moons are.
“Then why don’t we all move at the same speed? You said we all have to move at exactly the right speed to stay in orbit. But Io doesn’t keep up with us. Sometimes it’s really close to Lincoln, and sometimes it moves way off or even goes behind Jupiter or something. Is Io going too slow? Is it going to fall into Jupiter some day?”
No honey—actually, Io is going a lot faster than we are, because it’s closer to Jupiter. I said you have to be at just the right speed to stay in orbit, but the right speed changes by how far from the planet you are. As you get closer to the planet, what happens to the pull of gravity from the planet?
“It gets stronger.”
Exactly. And if gravity is pulling down harder on us, then we need to be moving even faster to stay in orbit. It takes Lincoln almost nine hours to orbit around Jupiter, but Io is faster and takes less than 6½ hours to make the same trip. And the really close moons go even faster. Amalthea is very close to Jupiter, and it has to go so fast to stay ahead of Jupiter’s gravity that it goes all the way around Jupiter in just over an hour.
For reference, the current God's Dice document can be found here.
My first thought is something that's been bouncing around in my head for some time. It occurs to me that knowledge of basic physics is well advanced in the 30th century. In elementary school, most kids should have a grasp of Newtonian physics that would put most 21st century adults to shame.
It's really a matter of necessity... when you live on a huge space habitat, and you can look out the window and see that your world is visibly spinning full circle every five minutes... when you can watch giant moons orbiting around Jupiter (which fills half your sky, BTW... your childhood questions suddenly move well beyond "Why is the sky blue, Daddy?"
But it was just such questions that inspired the following piece:
----------------------
[Elementary School Physics]
A basic grasp of Newtonian physics comes early to 30th century denizens of space. Evidence of physics in action are everywhere and impossible to hide from:
“Mom, why do Io and Jupiter spin around in the sky like that? It makes me dizzy if I stare at them too long.”
They’re not spinning, honey; we are. Lincoln is shaped like a giant wheel, and we stand on the inside of it, like this.
“Okay, if Lincoln is spinning, why don’t we fall when we get to the top?”
Because the spinning is what keeps us in place. It’s like this basket. If I turn it upside down, what will happen?
“All my cubes will fall out?”
Right—but watch what happens when I swing it in a big circle over my head. See? Even when the basket was upside down, the cubes stayed in. How come?
“The spinning?”
Yup—just like that. If you start something moving, it wants to keep moving in a straight line until something stops it. So when I swing the basket, is it moving pretty fast?
“Yes.”
And is it moving in a straight line?
“No.”
That’s right. I got it moving, and it wants to move in a straight line, but it can’t, because my hand keeps pulling the basket back in a circle. The cubes keep trying to move in that straight line away from me, but my hand keeps pulling the basket back against them. So as long as I keep spinning, the cubes are stuck to the bottom of the basket. Same thing with Lincoln—it’s spinning pretty fast (over 500 klicks where we are), so our bodies want to keep moving in a straight line away from the center, but the curved shape of Lincoln keeps it moving in a circle instead, holding us in.
That’s centripetal force.
“So if I was on the outside and let go, I’d just go flying off into space?”
If you let go—yes.
“Then why don’t the Spacers go flying off? I’ve seen them walking on the outside all the time.”
Well, for one thing, Spacers never let go. That’s what their tails are for. Plus, they usually have at least two safety lines that tie them to the hab as well.
“Well Mom, is Jupiter spinning then? Because it looks like it is.”
Yes—Jupiter spins. In fact, on the surface, Jupiter spins a heck of a lot faster than Lincoln does.
“Okay, then why doesn’t everything on Jupiter go flying off into space?”
Because Jupiter is big.
“I don’t get it.”
It’s called gravity. All physical things in the universe have mass—substance. Mass generates gravity, which is a force that pulls all other mass towards itself. So everything in the universe is pulling on every other thing out there.
“So I have gravity that tries to pull stuff towards me?”
Yes—all things have gravity. But gravity is so weak, that you can’t even notice it unless it comes from something very very big.
“Like Lincoln?”
No—not even like Lincoln. To have gravity you would notice, a thing needs to be really huge—like a planet or a star or a moon. The bigger the object, the stronger the pull. But then the pull of gravity also gets weaker as you move farther away.
“Well, we learned that Jupiter is the biggest planet in Sol System, so it should be pulling us in, right?”
Yes, Jupiter’s gravity pulls very hard on Lincoln, but we don’t ever fall and hit the planet, because we are in orbit.
“Huh?”
Let me explain: Remember what happens to something when you start it moving? What does it want to do?
“Move in a straight line?”
Right. So imagine that this big ball is Jupiter, and imagine that this is us flying sideways above it here. We’re moving pretty fast, so what does our ship want to do?
“Keep moving in a straight line.”
That’s right. But Jupiter has a lot of mass and a lot of gravity, so what does it do to our ship?
“Pulls it down?”
You got it. So let’s watch what happens: if our ship is going too fast, then Jupiter’s gravity can’t pull us down fast enough, and even though it bends our path a little at first, soon we are far enough away that we don’t feel its gravity anymore.
On the other hand, if we are moving too slow, then gravity pulls us down to Jupiter really fast and then…
“CRASH!”
That’s right—we crash into the planet. But look what happens when our speed is just right: gravity keeps pulling us down, so we never get away, but we’re going fast enough that we never quite fall into the planet either. As our path bends, we just make a giant circle going all the way around the planet over and over. That’s our orbit. And as long as nothing stops us, we can keep going like this for a long long time.
“So are Io and Ganymede and Europa all in orbit around Jupiter too?”
You bet—all the moons are.
“Then why don’t we all move at the same speed? You said we all have to move at exactly the right speed to stay in orbit. But Io doesn’t keep up with us. Sometimes it’s really close to Lincoln, and sometimes it moves way off or even goes behind Jupiter or something. Is Io going too slow? Is it going to fall into Jupiter some day?”
No honey—actually, Io is going a lot faster than we are, because it’s closer to Jupiter. I said you have to be at just the right speed to stay in orbit, but the right speed changes by how far from the planet you are. As you get closer to the planet, what happens to the pull of gravity from the planet?
“It gets stronger.”
Exactly. And if gravity is pulling down harder on us, then we need to be moving even faster to stay in orbit. It takes Lincoln almost nine hours to orbit around Jupiter, but Io is faster and takes less than 6½ hours to make the same trip. And the really close moons go even faster. Amalthea is very close to Jupiter, and it has to go so fast to stay ahead of Jupiter’s gravity that it goes all the way around Jupiter in just over an hour.