Gravity=9,81 right?

Hi! I don't doubt that the gravitational acceleration of 9.81 m/s2 is right but somehow it often looks too low for me. I saw a rally race on TV yesterday and I recognized that if the cars jump big jumps they are on the ground again after about 1 sec :) That means even with high velocities they have contact again after a very short period of time. When I try to reproduce this with a "9.81-sim" like my car simulator or Carmageddon 2 (the only game I've installed) it is as if you're flying like a bird for several seconds. It looks as if the gravity "attacks" to late after jumping. I think in games where jumping is not very common this is not a problem, but if you're making a stunt game (like me) this seems quite unrealistic. Just to have a look I took 20m/s2 of gravity and in many situations it seems much more realistic. I was wondering which other physical fact influences the gravitational motion of the car body, but if it was only the real-world's air friction, the cars in my sim would fall even slower since I don't have air friction implented yet. Did you notice that behaviour? Thanks! Robert

"MothaBrain" <[email hidden]> wrote in message news:[email hidden]...

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> Hi! > > I don't doubt that the gravitational acceleration of 9.81 m/s2 is > right but somehow it often looks too low for me. > > I saw a rally race on TV yesterday and I recognized that if the cars > jump big jumps they are on the ground again after about 1 sec :) > That means even with high velocities they have contact again after a > very short period of time.

Which way is that high velocity? It's along their direction of travel - not vertically. The vertical velocity imparted by a small jump is not a lot. i.e if something is moving up at 9.81 m/s - it'll take 2 seconds to arrive back at the point it started (1 seconds to slow down to 0 - 1 seconds to accelerate back to the same point again) 9.81m/s is infact 35km/h. The verticle element of the velocity of a car is no where NEAR that much - while it's horez. element may be in excess of 200km/h Doug

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> I don't doubt that the gravitational acceleration of 9.81 m/s2 is > right but somehow it often looks too low for me. > > I saw a rally race on TV yesterday and I recognized that if the cars > jump big jumps they are on the ground again after about 1 sec :) > That means even with high velocities they have contact again after a > very short period of time.

The cars are heavy, and travel upto about 120mph. They can't go too far. Remember that they use suspension to absorb the jumps, so it requires a much better ramp for them to take off. > When I try to reproduce this with a "9.81-sim" like my car simulator > or Carmageddon 2 (the only game I've installed) it is as if you're > flying like a bird for several seconds. > It looks as if the gravity "attacks" to late after jumping. I expect it's a case of your simulated cars having less advanced suspension and as such going much higher. Obviously it'll take longer to fall from a greater height ;-)

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> I think in games where jumping is not very common this is not a > problem, but if you're making a stunt game (like me) this seems quite > unrealistic. > Just to have a look I took 20m/s2 of gravity and in many situations it > seems much more realistic. > > I was wondering which other physical fact influences the gravitational > motion of the car body, but if it was only the real-world's air > friction, the cars in my sim would fall even slower since I don't have > air friction implented yet.

The rate at which you jump/land is completely seperate to the speed you are travelling along the road at. Split your thoughts into components. You take off with a velocity of x m/s in the upwards direction caused by the reaction with the ground. This is reduced at a rate of about 9.81m/s2 until you land. I suspect you are using rather large ramps to take off with (you mention a stunt simulator), and have little suspension modelling to absorb the effect of the ramp. If a ramp 1 metre long produces a vertical component of reaction with the tyre of 100N, then the tyre will accelerate upwards with the velocity of x m/s. The suspension takes this velocity and absorbs it into the car's mass which means overall, the car has sustained 100N of upwards force. This model should work until you run out of capacity for stored energy, or it is released before you take off. Having read that back to myself it makes no sense, and I can't think of another way to express it. Can anyone else help? Anyone cope with a translation? Andy -- PC-Based Multimedia System http://www.andylaurence.pwp.blueyonder.co.uk/pcbmms NB: Spamtrap - my name @yahoo.co.uk

"Andy Laurence" <[email hidden]> wrote in message news:9sbhpu$129mk8$[email hidden]...

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> > I don't doubt that the gravitational acceleration of 9.81 m/s2 is > > right but somehow it often looks too low for me. > > > > I saw a rally race on TV yesterday and I recognized that if the cars > > jump big jumps they are on the ground again after about 1 sec :) > > That means even with high velocities they have contact again after a > > very short period of time. > > The cars are heavy, and travel upto about 120mph.

The weight of the car has nothing to do with the height it will jump and fall. i.e a 100 kg weight projected up at 9.81 ms will take 2 seconds to fall back to earth. The same is true of a 1g lead shot. (ignoring air resistance) Doug

Douglas Ellison wrote:

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> "Andy Laurence" <[email hidden]> wrote in message > news:9sbhpu$129mk8$[email hidden]... > >>>I don't doubt that the gravitational acceleration of 9.81 m/s2 is >>>right but somehow it often looks too low for me. >>> >>>I saw a rally race on TV yesterday and I recognized that if the cars >>>jump big jumps they are on the ground again after about 1 sec :) >>>That means even with high velocities they have contact again after a >>>very short period of time. >>> >>The cars are heavy, and travel upto about 120mph. >> > > The weight of the car has nothing to do with the height it will jump and > fall. > > i.e a 100 kg weight projected up at 9.81 ms will take 2 seconds to fall back > to earth. The same is true of a 1g lead shot. (ignoring air resistance) >

Yes, but a heavy car will deform the suspension further, reducing the effect of a small jump ramp. Dave

WRC cars have aerodynamics load which are note insignificant. Plus they rarely jump but only fall from bumps, so inital vertical velocity is 0. Usually they never jump more than 2 meters high. x1 = x0+v0*t + 1/2at^2 v0 = 0 x0 = 2 x1 = 0 t = Sqrt+( 4 / a ) a = -9.81 t = 0.6 sec without any aerodynamics load. in Formula One Family on AbMotors they said that with actual wings on F1 at 200kmh the car could drive on a ceiling. Obviously WRC wings are much smaller than that ;o) So maybe t final could something like 0.45sec wich is small !!! usualy WRC do 1 meter high jumps. => t ~= 0.3 sec Try to fall an object from 1 or 2 meter in your sim a time it with your watch. regards, -- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

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> > > I don't doubt that the gravitational acceleration of 9.81 m/s2 is > > > right but somehow it often looks too low for me. > > > > > > I saw a rally race on TV yesterday and I recognized that if the cars > > > jump big jumps they are on the ground again after about 1 sec :) > > > That means even with high velocities they have contact again after a > > > very short period of time. > > > > The cars are heavy, and travel upto about 120mph. > > The weight of the car has nothing to do with the height it will jump and > fall. > > i.e a 100 kg weight projected up at 9.81 ms will take 2 seconds to fall

back > to earth. The same is true of a 1g lead shot. (ignoring air resistance) That is indeed true, but a 100N force will send a 1Kg radio controlled model a lot further into the air than a 1000Kg rally car, and as such it will take longer to come back down. Andy (not making himself very clear) -- PC-Based Multimedia System http://www.andylaurence.pwp.blueyonder.co.uk/pcbmms NB: Spamtrap - my name @yahoo.co.uk

Sure ;o) but if you launch 1000kg of lead and 1 kg of feather in a empty space from the same height they'll touch the ground at the same time :o) were not talking of the same thing. your talking about the initial impulse that cause the jump and we're arguing about the time to touch back the ground ;o) Andy Laurence wrote:

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> > That is indeed true, but a 100N force will send a 1Kg radio controlled model > a lot further into the air than a 1000Kg rally car, and as such it will take > longer to come back down. > > Andy (not making himself very clear) > -- > PC-Based Multimedia System > http://www.andylaurence.pwp.blueyonder.co.uk/pcbmms > NB: Spamtrap - my name @yahoo.co.uk

-- Sebastien TIXIER - Game Developer Dynamics and Car Physics http://www.eden-studios.fr GPLRank Normal:-44.24 Monster:-124.44

> Sure ;o) but if you launch 1000kg of lead and 1 kg of feather in a emptyspace > from the same height they'll touch the ground at the same time :o) Yeah (I assume you mean negating air resistance). > were not talking of the same thing. your talking about the initial impulsethat > cause > the jump and we're arguing about the time to touch back the ground ;o) Yeah, but the time taken to get back to the ground is proportional to the height of the jump. This in turn is proportional to the impulse. I was just trying to say that the vehicle could be going higher in his simulation, yet seem a similar height because of camera angles. I'm not disagreeing with anyone on this thread (so far), just adding a different perspective. Sorry for the confusion. Andy -- PC-Based Multimedia System http://www.andylaurence.pwp.blueyonder.co.uk/pcbmms NB: Spamtrap - my name @yahoo.co.uk

But the weight does change how much of the jump will be absorbed by a given suspension, thus changing the height of the jump. Jesse

So THAT's how those mogul skiers don't launch off of every mogul! ;-) ;-) ;-) Great analogy for those strict "in a vacuum neglecting all friction" people. Jesse

Yeah, 9.81 fps/s if you're a NASA rocket scientist ;-)

Jesse Black <[email hidden]> wrote in message news:9sbstd$5g6$[email hidden]... > But the weight does change how much of the jump will be absorbed by a given > suspension, thus changing the height of the jump. Actually, that's not the weight per se but a combination of the weight and the suspension characteristics. Ignoring dampers, it probably involves the static deflection but you can't really ignore dampers. Jonny

<snip> >Did you notice that behaviour? >Thanks! > >Robert > I noticed it in my sim too :0) It didn't seem like gravity was working correctly, but sure enough, dropping the car from a set height caused it to take xx seconds to hit the ground, just like it should... It looks different than on tv though, I agree :0) For a stunt type game, you might want to play with gravity to get it however "feels" best, and is most fun... You might check to see how high your cars are being launched off the ramps, they might be beyond current world records :0) Todd Wasson --- Performance Simulations Drag Racing and Top Speed Prediction Software http://PerformanceSimulations.Com

On 07 Nov 2001 22:26:11 GMT, [email hidden] (J. Todd Wasson) wrote:

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><snip> > >>Did you notice that behaviour? >>Thanks! >> >>Robert >> > > I noticed it in my sim too :0)

People 'complained' about that in my sim as well. Weird. Tested it, and it's ok. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

I think if you always played your sim with that 9.81-thing and saw it all the time it doesn't seem strange to you because it's usual. In my case I already realised it (however used to "overlooked" it) but when I saw the rally racers on TV I was very surprised how unrealistic my gravity looks. The cars on TV look heavy but mine are like paper boxes. I'm testing gravities of 1,5*g to 2*g now, and my colleague also says it looks much better now (he's a graphics artist ;)) A problem that arises with the higher gravity is that things like loopings are harder to manage, so I think I should find some compromise. Robert

MothaBrain wrote: > Hi! > > I don't doubt that the gravitational acceleration of 9.81 m/s2 is > right but somehow it often looks too low for me. The acceleration of gravity near the Earth's surface of course varies from place to place, but that is a good estimate. ;) I have discussed the same concern with game producers. The problem as I see it is that few people have watched cars hit abrupt steep ramps at 250 kph and leave the ground. The effect of a 5 degree flight path angle at 250 kph are, well, roughly 2.5 times as great ;) as at a more normal speed of 100 kph. Most games also have visual fields of view that are compressed. If you aren't used to compensating for the effects you can't really evaluate the motion properly. Even if you drop the car from rest they will note the apparent slow motion at the start of the acceleration and ignore how fast the car has gotten just moments later as it hits (as it does in real life.) Another problem is that most people are bad at telling time in small increments. I was once told that a 4 sec drop test took 7 sec, with back of the envelope calculations to show that our gravity must be very wrong. The actual problem in this case was in the calibration of the one-Mississippi two-Mississippi three-Mississippi experimental equipment ;). > > I saw a rally race on TV yesterday and I recognized that if the cars > jump big jumps they are on the ground again after about 1 sec :) > That means even with high velocities they have contact again after a > very short period of time. Depends on the upward component of the velocity as they leave the ground. (How steep they are flying and how fast they are going.) In games, an artist can easily put in several degrees of ramp without noticing it. The "Looks flat to me" problem ;) Also, see comments above about off the cuff time measurements :) -- Matthew V. Jessick Motorsims [email hidden] http://www.moto1.net Vehicle Dynamics Engineer (972)910-8866 Ext.125, Fax: (972)910-8216

On 8 Nov 2001 07:22:21 -0800, [email hidden] (MothaBrain) wrote:

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>I think if you always played your sim with that 9.81-thing and saw it >all the time it doesn't seem strange to you because it's usual. >In my case I already realised it (however used to "overlooked" it) but >when I saw the rally racers on TV I was very surprised how unrealistic >my gravity looks. The cars on TV look heavy but mine are like paper >boxes. > >I'm testing gravities of 1,5*g to 2*g now, and my colleague also says >it looks much better now (he's a graphics artist ;)) > >A problem that arises with the higher gravity is that things like >loopings are harder to manage, so I think I should find some >compromise.

Use a 3D vector gravity; 19.62 vertical force, and 9.81 in the other directions. As seen from the body. Or the world, ehm. *Exploding head* Might be something though so that when the car gets vertical it's relative gravity vector (as if such things exist!) pulls downward with only 9.81. Problem is then back again when reaching the top of the looping, but you may have to use 2 directional vectors for the Y, so Y down gives the usual 9.81. You may also ingore this babble, as it will not make things more realistic. ;) Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/

You know what? Most of the stuff that's been covered in this topic, I actually understand. I guess that even though my grades aren't really reflecting it, I AM learning something in (Advanced) Physics class this year. :) I'm going on about a low to mid B grade because my homework is always bad--the book we use is VERY VERY difficult. None of the questions are straightforward. :( BTW, we're doing impulse/impact right now, and I'm kinda confused. :( ----------------------------------------- Dan Belcher Webmaster, http://www.gplworld.racesim.net/simcrash

Just remember: impact and impulse forces in physics are highly theoretical and make little intuitive sense (at least in my classes). I spent a week trying to think through the limits and stuff and only really got it to work when I discarded my interest in reality. Dave Dan Belcher wrote:

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> You know what? Most of the stuff that's been covered in this topic, I actually > understand. I guess that even though my grades aren't really reflecting it, I > AM learning something in (Advanced) Physics class this year. :) I'm going on > about a low to mid B grade because my homework is always bad--the book we use > is VERY VERY difficult. None of the questions are straightforward. :( > > BTW, we're doing impulse/impact right now, and I'm kinda confused. :( > > ----------------------------------------- > Dan Belcher > Webmaster, > http://www.gplworld.racesim.net/simcrash >

"MothaBrain" wrote ... > > I don't doubt that the gravitational acceleration of 9.81 m/s2 is > right but somehow it often looks too low for me. > Back to your original question, yes <g>. At least on this side of the pond, gravitational acceleration is approximately 32 ft/s2, which my simple math tells me is the same thing. And I think the biggest problem with how "it often looks" is that the "hang time" of anything is one of the most deceptive things there is, especially when there's a high lateral movement involved. Our eyes just don't do a very accurate job of measuring secondary movement, which is what the vertical component of a rally jump is, horizontal movement being the primary one.

On Mon, 12 Nov 2001 16:03:12 -0500, "Don Jennings" <[email hidden]> wrote:

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>"MothaBrain" wrote ... >> >> I don't doubt that the gravitational acceleration of 9.81 m/s2 is >> right but somehow it often looks too low for me. >> > >Back to your original question, yes <g>. At least on this side of the pond, gravitational acceleration is approximately 32 ft/s2,

On another note; my gravity didn't suck <g> once because when jumping, the suspension would get so loaded that its velocity got a bit big. I then, in my check for an impossible suspension position, forgot to reset the wheel velocity once the susp bottomed out. This meant if wheelVel was 60 m/s, it would still be that way, and the spring would take a long time to get the velocity back to 0 again. In the mean time, some mean forces kept on pushing the car in the air. So my gravity was there, but didn't do its job like it was supposed to. Ruud van Gaal Free car sim : http://www.marketgraph.nl/gallery/racer/ Pencil art : http://www.marketgraph.nl/gallery/