mass/gravity question

Blitz3D Forums/Blitz3D Programming/mass/gravity question

kochOn(Posted 2007) [#1]
Hi, I m working on a vector engine (Vectrex like), and it needs some little physics.

what I have for now is:

; Computing Gravity Vector
gx = Cos(-90 + gravity_dir) * gravity_force * object_mass
gy = Sin(-90 + gravity_dir) * gravity_force * object_mass

; Adding Gravity to the Object's Direction 
dx = dx + gx
dy = dy + gy


I can't think an objet of 50kg falls 50x faster than a 1kg Object.

Any help on the formula?


Buggy(Posted 2007) [#2]
I thought two falling bodies of exactly the same size fall at exactly the same rate...

...but I don't know physics either.


_33(Posted 2007) [#3]
check the code archive and find the example called MIRRORIZE.bb that I sent last week.

http://www.blitzbasic.com/codearcs/codearcs.php?code=2029


Who was John Galt?(Posted 2007) [#4]
Everything accelerates at the same rate, regardless of size or
mass. Drop a lead weight and a feather in a vacuum and they both hit the ground at the same time.


H&K(Posted 2007) [#5]
Everything accelerates at the same rate, regardless of size or
mass. Drop a lead weight and a feather in a vacuum and they both hit the ground at the same time.
Only in Highschool Physics. In proper grownup physics although the lead wieght and the feather are pulled towards the earth at the same rate, the lead weight (assumeing that the example means the lead weight has more mass), will pull the earth towards it, more than the feather pulls the earth towards it.

As a theoretical exercise imagine a Mass the size/shape of the earth being created within the gravity well of the earth. Now this new mass will fall towards the earth at a certain rate (When its close 9.8m per s^2). However the earth would also accelerate towards the mass (at 9.8m per s^2 when close enough), so the gap between the two objects is decreceing at 19.6m per s^2. For each object it appears that the other is falling towards it at twice the normal rate/acceration.

So although the acceleration from rest for a mass and a greater mass is the same, the greater mass will hit first, because whatever it is falling towards will accelerate towards it to a greater extent


_33(Posted 2007) [#6]
More the object is dense, the faster it accelerates with gravity.


H&K(Posted 2007) [#7]
More the object is dense, the faster it accelerates with gravity.
Nope, the less it seems to be affected by air resistace. The density of the object is unmportant, only its mass is important


Vertigo(Posted 2007) [#8]
Granted an object with large mass would hit first however, when we are talking about objects in the gravitational influence of a MUCH greater mass, those two figures would be painfully close... Gravity is a weak force, in comparison. When the effect of an iron ball the size of a house hitting the earth is compared to that of a birds feather... in a vacuum mind you, the ball would hit and the feather would be a few pico-meters behind. In other words, yes they fall at the same rate on any logical scale. And I failed physics... geesh haha


H&K(Posted 2007) [#9]
they fall at the same rate on any logical scale
As I said
Only in Highschool Physics



Michael Reitzenstein(Posted 2007) [#10]
The OP is talking about a 'little physics' for a game. There's no need to jump on and start correcting people based on 'proper grownup' physics - 'high school' physics is what he is looking for, so Noel was in fact correct.


H&K(Posted 2007) [#11]
'little physics' for a game
Ive seen a few games where its Planets or Astorids


Who was John Galt?(Posted 2007) [#12]
@H&K - 'Proper grown up' physics is my day job, but you seem best placed to help this guy out. Don't forget to include quantum and relativistic effects in those equations. ; )


H&K(Posted 2007) [#13]
lol,

Ha, I dont mind being wrong, its just when ppl say things that they were indoctrinated with pre-specialization, and think thay are hard and fast, such as this one.
(And that the world can be shown not to be flat [Agian, the sphere is the simplest solution, not the only one], and The Big Bang is contra creationalism)

Can I really mesh Gravatational effects on a planetry scale with quantum? Nice Unifiying thorey coming up.

And relativly the two bodies are infact accelerating towards each other at the same rate, so any effect would be common to both.
(One could argue that the Kg mass droped from the plane, is accelerating towards the earth more than the earth is accelerating towards the Kg mass, but if you replace yourself with the mass, it definatly seems that its the earth speeding to meet you)

Given that the original equations where for a 2d model, I feel confident in saying that the observer is some distance away at a tangent to both these dimentions, and as no indication was given that these bodies would have a life span, I feel quite convident that any relativistic effects can be ignored.

I However still feel that insuficant information was given to assume that one mass was substantaly greater than the other, infact the inclution of Dy and DX for the vector of the acceleration, would continue to give me the belief that we are talking about comparible masses orbiting each other.


Vertigo(Posted 2007) [#14]
Since we are talking simple, why not just remove the whole object_mass variable out of the equation? Personally the more wacky a games play is, the more I like it. Ala, if you were playing mario of any flavor, and your character was limited to jumping only that of what a normal human could... could you honestly say that game would ever be fun, or even playable? haha. Just make something simple and looks nice. As long as you are able to produce the same results from your formula's then I say go for it. You are trying to get the results Valve did with HL2 are you?


kochOn(Posted 2007) [#15]
Thanks to all,

I never thought my question will spawn so much disagreements. I only want to know if and how the mass of an object should interact with gravity forces.

I m now going to check the Memorize example as _33 has suggested.


sswift(Posted 2007) [#16]
kochon:

For the purposes of a game, you should ignore mass when dealing with gravity.

Unless you're trying to model celestial bodies orbiting one another. :-)

In everyday life, objects with different masses will appear to fall at the same rate, unless said objects are light, and have a lot of air resistance, like a feather. In those special cases, you could either model the air resistance, or simply cheat and turn gravity down on that object. You might as well cheat, because the effect will look roughly the same. And you can then model things like balloons by using negative gravity, rather than trying to model their bouyancy by calculating how dense they are relative to the atmosphere. :-)


kochOn(Posted 2007) [#17]
Thank sswift,

It seems that I won't use mass property anymore in my engine, as the user can define different gravity force and dir for each object.


sswift(Posted 2007) [#18]
Don't give up on mass entirely. If you're doing collisions, mass can be useful for that. :-)