Science question: Archery on a moon.

Would archery (or anything to do with gravity) be effected in any noticeable way by having a (solid, non-gas giant) planet overhead in the sky? Assume the moon's standard gravity is earth like.

edited 1st Dec '10 7:48:06 AM by deuxhero

Probably not, no. Space Is Big. However, lots of things will be affected if the moon's gravity is far less than Earth's.

The planet does have higher gravity than the moon (I'm wonderering how much the difference would have to be).

edited 1st Dec '10 7:49:38 AM by deuxhero

Gravity is about 10 meters per second on earth. On OUR moon it is about 8 meters per second.

Horizontal velocity is how far it travels divided by the time.

Vertical velocity will decrease as it goes up, and as it goes down, it will increase to it's initial velocity (velocity decreases/increases at acceleration times time, with the sign making the difference)

Momentum, which is what make things hit hard, is mass times velocity. Velocity is distance divided by time.

Mass is a set number. To fine mass, either take the kilograms (which is mass), or take the pounds, times by 4 (to convert to newtons) and then divide by 10 (Earth's approx. gravity) to get mass.

But I am digressing. To get the distance, you need the horizontal velocity (the cosine —use a calculator in degree mode— of the angle times the initial velo) and the time.

Time is need to be found by the vertical velocity (the sine of the angle times the initial velo). The vertical velocity will then be divided by the gravity value, to get the amount of time it takes to rise. The time it takes to fall is the same number, so the total time would just be twice your answer.

Now, how to GET initial velocity (make up the angle if you want, the best angle for distance is 45), is where it gets fuzzy. You need the force (in newtons, which is pounds times 4), as mentioned previously.

Force is basically the amount of weight you are putting on it.It's messy, and best to just start off with how fast you want the arrow to start off.

SO. Distance would equal velocity horizontal times time or, in other words, (V(i) times cosine of angle) times (V(i) times sine of angle divided by gravity times two) equals how far it will travel.

It's momentum will always be just as strong, since it is mass (unchangeable) times velocity. It will move farther, but not necessarily hurt anyless.

Keep in mind, I only have a year of regents physics, and a dropped AP Phys course on my plate, so I might be a bit off.

Also, air resistance is something I did not take into account. So this is in a perfect setting.

Also, any problems you solve should be in the metric system, since that's how the formulas work.

For reference:

  ^key

v = d/t

a = change in v / t

f = ma

f = m(v/t)

f = change in p / t

p = mv

v(h) = v(i)cos

v(v) = v(i)sin

v^2 = 2ad

t = v(v)/g

mg = weight

edited 1st Dec '10 8:13:10 AM by MrAHR

Does your moon have atmosphere?

Yes. It is the setting.

@Mr AHR

Uh... ok?

edited 1st Dec '10 8:38:48 AM by deuxhero

It would be affected but it's something you'd get used to over time or barely notice at all. Unless your planet is significantly closer, it won't make it do corkscrews or anything. Also, the moon's gravity isn't eighty percent of earths, it's eighty percent less than earths, putting it at twenty percent.

To give you an idea of the scale, the Earth's gravity accelerates something on the surface of the moon at 0.0027 m/s^2, compared to the 1.6m/s^2 caused by the moon itself.

Also, what effect are you going for? There are equations that relate gravity to maximum height of a projectile, total distance travelled, and so on, but those won't be useful if you don't know what numbers you want out the other end.

edited 1st Dec '10 9:01:57 AM by Yej

Oh I was just wondering if it would cause less drop than usual.

A little, yes. Significant amounts? Probably not.

Another thing to consider is the effect of distance on gravity. On Earth, for instance, the Moon will deflect the course of an arrow, just as it deflects the oceans to create tides. The question is, how much of an effect will it have? The strength of gravity decreases by the square of the distance, i.e. a lot when you're talking astronomy. Realistically, you don't have to correct for the pull of the Moon, and you'd need an exceptionally sensitive instrument to detect it.

Okay, numbers. On the Earth's surface, you're about 5,000 miles from the center, with gravity of 1G. On the Moon, you're about 250,000 miles away, or 50 times further, with gravity of 0.0004G. Work it out for Moon vs Earth and you get an even smaller number.

Jesus Christ, it's fiction. It's literally based solely on the concept of escaping from reality. And realism is a bit of a moot point when you're talking about moon archery anyway.

If the moon were close enough to the planet for the planet to have a noticeable effect on its surface gravity, the moon would probably be torn apart by tidal forces.

Moon gravity +/- planet gravity/(orbital radius/planetary radius)^2

Roughly speaking, that is the inverse square law.

Gravity is in accelleration (metres or feet per second squared while the radii can be in miles of kilometres.

• Earth gravity is 9.81 m/s^2
• Earth radius is approximately 6371 km
• Moon's orbital radius is approximately 384,400 km

So when you are on the moon, the earth's gravity is only about 0.0027 m/s^2 compared with the moon's gravity of 1.6 m/s^2, which is only a difference of +/- 0.17%

Not significant.

• Jupiter gravity = 23.1
• Jupiter radius = 71,500 km
• Europa gravity = 1.314
• Europa Orbital Radius = 671,0000 km

So Jupiter's gravity on Europa would be about 0.26, or almost +/-20%

That's significant. Taking marksmanship instead of archery, your bullet drop on the far side would be half again what your bullet drop would be on the near side of the moon (at a range equal to the velocity of the round).

That almost seems wrong because that implies that you would weigh 50% more if you walked on the far side of Europa. Can someone check the math?

The smaller, closer moons would be affected even more.

edited 7th Dec '10 3:02:49 PM by 66Scorpio

The fact that it's fiction doesn't give you carte blanche to avoid the laws of physics.

Basic point here: the distance from the planet to the surface of its moon is such that there should be no generally noticeable effect of gravity from the planet on the trajectory of projectiles on the moon. There's just too much distance involved. However, should the moon have liquid water on it, there would be tidal forces from the planet — quite high tides, in fact.

Tomoe: ...Yes, yes it fucking does. If you want reality, go live it. If you want physics, take a physics class. Fiction is supposed to be different, it's literally the entire point.

Uh... No.

Fiction gives you the ability to ignore the laws of reality if you want.

The same as Real Life gives us the ability to throw the book out in disgust at the stupidity of it.

So moon archery not being depicted realistically (despite that being something that has literally never happened in reality) is disgusting? Now you're just being ridiculous.

It really depends on what kind of story you're doing, doesn't it? If you're doing something fantastic on the level of The Adventures of Baron von Munschausen, by all means, do whatever fits the story's flow regardless of possibility. If you're instead going for hard science-fiction, as the OP seems to be asking, you of course have to keep a close tie to real-world physics.

A close tie to real-world physics when talking about things that have never and likely will never happen in the real world.

While I understand the sentiment to a certain point, I'm not seeing the appeal of that kind of thing. Take, say, Ringworld. That book isn't even 200 pages long but I just couldn't finish it - because it goes out of its way to explain the tiger people and the storm that looks like a giant's eye and the invisible spaceship. Why bother including fantastic elements if you want it to be real?

Kid, if you don't like hard science fiction, you don't have to read it. There are those of us who do enjoy such things — we won't force it down anyone else's throats.

No, it's not disgusting.

We CAN be disgusted.

Where did I ever say I was disgusted? Please, show me where.

By the by- the whole point of fiction is not to remove yourself from this reality, it is to immerse yoursef in a different reality. Big difference.