[Black Echo]

I was working on a game concept that would allow ships to move in the tactical phases of the game by accelerating or decelerating in 'gravities' like many Sci-Fi authors explain in their books.

My question is this: What *IS* a gravity? If you were moving at 1G acceleration, how fast would you be travelling? How much distance would you cover in one second of time?

I know that one gravity is equal to 9.81 meters per second per second (M/S squared), so is an acceleration of one gee actually just a ship moving at 9.81 meters per second per second or is it something else?

If you were moving at 100 G's of acceleration, assuming compensation of occupants for the force, would you be moving at 981 meters per second per second?

This area of sci-fi math has confused me for some time now, and I can't find any resources on the web, in game books, etc. that adequately defines what one G of acceleration is in hard terms.

Thanks for any help you can give.

[Solero_58]

Black Echo, I'm not quite sure I understand your question fully but there are a couple of things I can tell you... From what I know (which isnt a great deal), gravity isn't referred to in units, like one gravity or 100Gs. The 9.81 m/s squared you referred to is the particular gravitational acceleration of the Earth. So if you drop something from 300m in the sky, it will accelerate towards the ground at that rate, regardless of mass.
Gravity varies according to the total mass of the particular planet, so the Earth's gravity is different to probably every other planet in existance. So anyway, what I don't understand is why a ship moving in space is accelerating at the rate of Earth's gravity, if it's not entering the Earth's atmosphere.

If all the above is completly wrong, don't blame me I've only just started Year 11 Physics, and i'm kindof new to it all/.

[Black Echo]

Exactly! I'm working with apples and oranges and trying to figure out what the 'universal' constant of a "G" is. Like in Dan Simmon's books (Hyperion series), it talks about a ship decelerating at 200Gs or accelerating away at 80Gs (with the crew safely tucked inside containment fields to prevent them from becoming raspberry jam).

In another book that was horrible (White Wing), one fighter gets its gravitic compensators knocked out and is undergoing several hundred Gs, the crew are, of course, long dead and the ship is accelerating out of control.

In the game "Traveller", ship manuever drives were rated from 1G to 6G in acceleration, I was often wondering what "G" they were using and what was the universal quotient. Trying to cannibalize and dissect their travel times at a specific G didn't do much to find the magic number I'm looking for.

I was looking for something a little more 'realistic', I guess, than just saying that little ships went faster than big ships, etc. I had the idea of doing the map in "G-seconds" or the time it took to cross a certain amount of distance at 1G acceleration in one second.

Amazing that I can't find any astronomical or other type of calculations / units on the web to try to solve this... : (

THANKS! for the help though...

[Solero_58]

Hmmm interesting, maybe the G used in those books & games isn't really an actual measurement of acceleration. It might just be purely a unit which only has a use in science fiction and not in reality. That would probably explain the lack of astronomical information about it... But then again I could be wrong. Anyway, good luck with the game, Black Echo

[Eventine]

Guys,
Basically, a G is a measure of accelleration. It is 9.81 m/s squared. So even though you migh be on another planet, we base it on Earth's accelleration due to gravity. (Another example of this is years - even though other planets have different year lengths, we still tend to measure time in Earth years)
So when a ship is accellerating at 10G's, it means it is accelerating at 981 m/s squared.
So to answer blak echo's question (sort of) a hip moving at 10g is accelerating at this rate. Of course it can only accellerate until it reaches a certain velocity however, otherwise it would keep on going until it passed the speed of light.

You also have to keep in mind that acceleration is a vectored quantity - ie it must have direction. So the moon for example, has constant acceleration because it does not travel in a straight line. It's velocity is also constantly changing (velocity is vectored as well), but it does have constant speed (a non vectored measurement).

Keep going with the physics Solero_58 - it will all become clear by the time you sit the HSC (hopefully!!)

Send me an email if you want some better descriptions.

[karljah]

Hey! I'm new to this site, and this is probably a very old topic, but if Black Echo still needs some info, I think I can help you out. Acceleration expressed in terms of Gs is both a vector quantity, and a symbol of G-Force (not the cartoon!). So if you accelerate at 1G, you ARE travelling at 9.81 m/s squared and the "gravity" on the ship is equal to normal gravity on Earth (at sea level). Obviously, this is only helpful if you want to establish how many Gs each different alien race can withstand.
Maybe this was all "understood" before I attempted to explain it, but this was the only thing on the discussion board that I had any idea how to answer. If it was too obvious, my apologies.

[Penumbra]

Earth Gs become moot on Mars or anywhere else, except for calculating in Earth terms which is impractical. For example, the exact opposite of gravity is centrifugal force. On Earth, this zone occurs approximately 22,300 miles above the surface and is the place where satellites can exist without the need for expensive retro rockets (to prevent them from falling into the atmosphere) Every planet has its own unique zone because its gravitational parameters are different. I hope that clears it up for you.

[Penumbra]

Sorry. After re-reading, I was unclear. The zone where gravity is exactly counterbalanced by centrifugal force, and thereby neutralized, is the satellite zone. This is not to say that satellites can not be made to ride at a lower elevation if reinforced with propulsion. In fact, LEOS, low earth orbiting satellites, provide most of the telecommunication wizardry we now experience, but are very expensive because of the need for huge power supplies, solar wings and rockets.

[Corwwyn]

You do realize that "Centrifugal Force" is just a term of convenience and not a real force?

"Centrifugal Force" comes from the apparent force one experiences from a ride on/in a centrifuge. http://www.sffworld.com/ubb/smile.gif

The "Satellite Zone" or "Orbit" (stable orbit?) is due to the force of gravity fighting the tangential momentum of the body it is acting upon.

When the force of gravity is equal to the tangential momentum of the subject, that subject will orbit as a satellite.

There's more to it though. The amount of momentum directly towards or away from the gravity source will determine whether or not the subject can be "captured" (achieve stable orbit).

Each sub-concept is straightforward in itself, but the overall concept can get complicated, hence Centrifugal Force becomes a very convenient way to describe it.

Apologies for any boredom caused by this post.

:P

[DarthV]

Here are a few quick blurbs that might help clear up this mess:

I'll go out on a limb and believe that when an author says an X bumber of g's of acceleration that they are using 1g as the acceleration of gravity on Earth's surface. This is 9.807 m/s^2.

So if an object is in free fall, neglecting air resistance, it will accelerate at 9.807 m/s^2. After 1 second, it will accelerate from rest to a velocity of 9.807 m/s. After 2 seconds, it will have a velocity 19.61 m/s and so on.


Corwwyn, maybe this will clear up some of your statements about centripetal force just being a term of convenience. Try not mixing force and veloctiy in equal terms with momentum http://www.sffworld.com/ubb/smile.gif You have the concept right, but just not explained in the right way.

Consider an object moving in a circle of radius r with constant angular velocity. The tangential speed is constant, but the direction of the tangential velocity vector changes as the object rotates.

Centripetal acceleration is the rate of change of tangential velocity.

The direction of the centripetal acceleration is always inwards along the radius vector of the circular motion.


Centripetal force is the net force causing the centripetal acceleration of an object in circular motion.

Its direction is always inward along the radius vector.

So in the case of the Earth and a satellite, you get a stable orbit when the centripetal force on the satellite is equal to the gravitational force.

[This message has been edited by DarthV (edited January 28, 2002).]