Submitted by Independent-Choice-4 t3_zy9lu2 in space
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Comments
IronSmithFE t1_j24k2yo wrote
the fastest baseball pitch on record is 0.04697 km/s.
Reggie_Barclay t1_j254x2y wrote
The fastest recorded on Earth.
[deleted] t1_j24m6mj wrote
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IsraelZulu t1_j24kb55 wrote
That's about 5,324 MPH for Americans.
I'm curious about what this "escape velocity" actually means though. Is that just the raw velocity needed to escape lunar orbit as if the moon was the only thing in the universe, or is it just what you need to get out past L1 so that Earth's gravity becomes dominant enough to pull you away?
absoluteally t1_j24mxq3 wrote
Escape velocity is contextual so depends where you are and how fast you are already going. I this case i believe they are starting from static in the surface(usually what is meant).
What it actually means is for the moon the escape velocity is the velocity change you would need from where you are to reach infinity with no excess velocity. Assuming there is nothing else in the universe.
But given they way gravity tails off escape velocity is often not meaningfully different from the velocity to leave the bodies dominant influence.
zoicyte t1_j24x0mn wrote
*contextual relative to the object you're trying to escape.
you need more speed to escape from the gravity of the earth than the moon.
the escape velocity to exit the solar system is really, really high, because you're trying to escape the pull of the sun itself. to date i think we have only launched 4 probes that have achieved the solar system's escape velocity (pioneer 10/11, voyager 1/2). it's possible the new horizons craft has reached the escape velocity for the solar system, it's certainly truckin'.
rondonjon t1_j24m5ul wrote
Even we know that anything moving at multiple kilometers a second is fast. But thanks.
ExNihiloish t1_j24n5i7 wrote
Yep. I measure everything as either fast or not fast. Absolute numbers are irrelevant.
rondonjon t1_j24o6pd wrote
Well, there is also slow, which is not quite the same as not fast.
This_Username_42 t1_j25bsli wrote
There is a finite amount of potential energy that is stored by you being elevated in a gravitational field. That’s why going up a mountain is harder than going down a mountain, you’re putting energy into climbing, and getting it back when you descend
If you imagine being out in space, and you fell to earth (negating air resistance) — you’d be moving very very fast. When you started, there is a lot of energy stored — you’re at the very top of the “mountain”
That means that you have a certain amount of kinetic energy corresponding to your speed and mass.
When you “hit” earth, and imagine a soft landing where you aren’t obliterated — that kinetic energy is exactly equal to you “climbing” the mountain — I.e. being out in space.
When you want to climb the mountain to get out of earth’s gravitational field, then you need to use that much energy to get up to speed to jump out.
Escape velocity is simply how fast you’d have to start out at to get away from earth, negating air resistance.
IsraelZulu t1_j25da60 wrote
Ok but we're talking about escape velocity from the moon, for an object flung towards Earth. At a certain distance from the moon, along such a trajectory, Earth's gravity has more influence on an object than lunar gravity, so it starts being pulled away from the moon without needing to expend more energy than is needed to reach that point.
My question is whether the escape velocity from the moon alone greater than the velocity required to reach the point where Earth's gravity can just take over.
HRDBMW t1_j24nu7q wrote
The escape velocity is dependent on the mass of the two objects close to each other. For one object being much more massive, like a moon and a baseball, the mass of the smaller object can be ignored. There is more roundoff error and truncation error in the math.
IsraelZulu t1_j24o1zu wrote
So, you're saying the baseball is too small for Earth's gravity to be relevant here?
lellololes t1_j24r4q6 wrote
No, they are saying that the baseball is too small for the baseball's gravity to be relevant here.
IsraelZulu t1_j24rn6d wrote
I fail to see how that had anything to do with my question then.
lellololes t1_j24tggk wrote
I was clarifying what they were saying. That's all.
Given the distance earth is from the moon, the gravity it imparts on a small mass like a baseball is also going to be a rounding error on a rounding error. It isn't nothing but it may as well be.
What they are saying is that if you have an object that is big enough to impart some gravitational pull it would then start affecting escape velocity for that object. E.g. The escape velocity of Phobos is 25 miles per hour or something like that ( it's big enough to have some gravitational effect). The escape velocity of Phobos from the surface of the moon would likely be a bit different than that of a baseball.
It all fits together, I don't think they misunderstood you.
HRDBMW t1_j27rlne wrote
Not exactly. What I am saying is that the mass of the ball is mostly irrelevant, and the only relevant mass for calculations of escape velocity are massive bodies very close to the ball. In this case, just the moon. The escape velocity is determined by the moon mass, not the position of other masses.
undercoveryankee t1_j24urhq wrote
The two-body escape velocity (if the moon and the projectile were the only things in the universe, would the projectile’s trajectory be unbounded?) is easy to define and calculate, so when you see a single number quoted as “the” escape velocity from a location it’s usually using the two-body definition.
Jakebsorensen t1_j24nvdg wrote
How much extra velocity would be required to get to earth after escaping the moon’s gravity?
BartyDeCanter t1_j24som3 wrote
None! As long as it's heading in the right direction at the right time. Once something is moving at escape velocity it can go anywhere else, unless it is captured by something bigger.
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In this case, escape velocity from the Moon is 2.38km/sec and the orbital velocity of the Moon is 3.68km/sec. So, at most the baseball would be travelling at 6.06km/sec relative to the Earth if it was just at lunar escape velocity. Since that is less than the Earth's escape velocity of 11.19km/sec, it would be captured by the Earth as it moved away from the Moon. At that point, depending on the fine details of the orbits it could either orbit the Earth, crash into the Earth or be re-captured by the Moon.
Client_Hello t1_j24ub52 wrote
It needs to be faster, more like 3.4 km/s
Throwing an object at the escape velocity just means the gravity of the moon will slow the object down until that object is not moving relative to the moon. The moon orbits the earth at 1.02 km/s, so the object would end up orbiting the earth.
Throw something (2.38 + 1.02) km/s in the opposite direction of the moons orbit and it will end up with an orbital speed of 0, causing it to fall down to earth.
ChrisARippel t1_j24jzlr wrote
How fast can you throw?
The Moon's escape velocity is 5,324 miles per hour.
ZealousIdealBat-0 t1_j24miqk wrote
Short answer: No. This is because at the surface of the moon the moon’s gravity is stronger than the earth’s gravity.
Long answer: Newton’s law of gravity says that the force of gravity between two object decreases as the distance between them increases. The moon’s surface gravity causes an acceleration towards the moon at 1.62 m/s^2. The earth’s gravity, on the surface of the moon, causes an acceleration of approximately 0.003 m/s^2 towards the earth. So, not negligible, but also not enough to cause an object to fall towards the earth. In order for the earth’s gravity and the moon’s gravity to be equal to each other, you must be about 10% of the way from the moon to the earth, or about 38,000 km from the surface of the moon. This point is called a “Lagrange point”, where gravitational forces are cancelled out to create a point of equilibrium. This Lagrange point is like balancing a pencil on your finger; one small push towards the earth causes the object to fall towards the earth, but a small push towards the moon causes the object to fall towards the moon. I hope this helps!
Triassic_Bark t1_j24q7gn wrote
That was a great explanation of why Lagrange points exist!
HolyGhostin t1_j24uw1g wrote
And for op's question, if you launch something straight "up" at earth enough to get out of the moon's gravity and into earth's, it then just falls "down" to earth without orbit. If you look up at earth and launch something 90 degrees "ahead" of the moon, it would escape into an orbit around earth. Maybe not a stable orbit, I'm not smart enough to figure that out.
Saoirsenobas t1_j24kecb wrote
The moon is extremely far away from earth, even if you could throw objects from the surface to escape the moon's gravity they would still just fall in a ever so slightly smaller or larger orbit around earth and would never go anywhere near the surfcace.
This_Username_42 t1_j25cdy4 wrote
If you threw a hammer at escape velocity toward earth it would hit earth because it would be pulled into the gravitational field. If you’re adding velocity toward the center of the system and you’re already orbiting the body, you’re going to go eccentric if the speed is low and impact if the speed is high. You’re ripping that hammer and it’s gonna burn up in atmosphere
CrayonDelicacies t1_j24qkpa wrote
OP, I feel that’s a perfectly valid question and I’ve been curious about it myself.
daikatana t1_j24rs1e wrote
There are a few problems with that. The first thing being throwing something toward the Earth won't mean it impacts Earth. This is not how orbits work. In order to get from the moon to the Earth, you have to throw it in the opposite direction from the moon's velocity.
But ultimately that velocity is going to be very high. The moon orbits the Earth at about 3,600km/h and you'd have to throw that hammer at a good percentage of that velocity in order to lower the hammer's lowest point in its orbit so that it would hit the Earth. This is just something orders of magnitude beyond what a human being can do.
Edit: If you could do what you're suggesting then we'd already be mining the moon. The biggest problem with mining something like the moon is getting fuel to return the material to Earth. If all it took were a human-scale toss to return to Earth then we'd be hurling minerals our way like there's no tomorrow.
billcraig7 t1_j250jvw wrote
You don't necessarily need fuel just energy. A rail gun type system could theoretically do it. I am not really sure what the Moon has that the Earth needs. We have plenty of iron, silicon, aluminum, oxygen, ect... We really don't have a need for He3 yet. In some far future SciFi time it might be interesting to get material that was not at the bottom of such a deep gravity well and thick atmosphere into deep space.
[deleted] t1_j2517wk wrote
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ChesterNorris t1_j251ae5 wrote
Others have answered the question. I just want to say that you shouldn't be throwing hammers. Someone might get hurt.
Reggie_Barclay t1_j2541h9 wrote
Is it okay to throw baseballs?
[deleted] t1_j25d2zq wrote
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xNeyNounex t1_j254xkt wrote
It wouldn't keep going. the moon has gravity, just not a lot. That is why people bounce on the moon, and don't float off into space. It would go really far, but eventually it would be pulled down by the gravity of the moon.
RiverDragon64 t1_j24pk84 wrote
You’d have to throw the object where the earth would be when it got there. Moon orbits the earth, which orbits Sol. Nothing is static. In the Robert A Heinlein book “The Earth is a harsh mistress “, the “Loonies” use a computer guided mass accelerator to bomb earth with rocks.
CrayonDelicacies t1_j24qjcs wrote
OP, I feel that’s a perfectly valid question and I’ve been curious about it myself.
EasyReader t1_j24j4dv wrote
Escape velocity of the moon is 2.38 km/s. So if you can throw a hammer than fast, yes. I certainly can't. Probably too small to see as a meteorite though.