What's possible with physics?

So far my attempts at physics have been good for simulating tumbleweed. I've messed with many of the physics functions but they rarely do what I want.

Some things I'd like to do with physics:

- tethered balloon that reacts to wind or things bouncing off it
- swingset
- pendulum
- windmill that turns to face the wind and reacts to things hitting it
- teeter-totter
- catapult
- wheel on axle on parallel bars
- weight on a spring

Are these sort of things possible with the Second Life physics engine? Where do I start for handling constrained movement?

Hi,

so far as I know, the physics engine has the following limitations...

no air resistance (so no wind effects whatsoever)
no hinged joints (so no swings / pendulums)
no object deformations (so no springs)

which between them rule out most of the things you list.

However, I am far from an expert on physical prims, so I may well be wrong, and there will certainly be ways to emulate the functionality of so of the items.

The tethered balloon is doable with llMoveToTarget, the swingset and pendulum will be very difficult to do until they update hinges mechanics.

the windmill can turn to face the wind using a combination of llRotLookAt and llWind.

Catapults are a perennially popular fixture in the sandboxen. they get pretty big.

weight on a spring? Fascinating idea, though our current physics engine doesn't handle dynamically deformable objects.

see also http://www.secondlife.com/badgeo/

We have a non-physics swingset set up in Awa (coords 185-185-25) you can try.... not sure where the group got it really, but it's basic to operate.. you sit in a swing, and use up and down arrows to 'push' in both directions and swing higher.

It doesn't use physics but it does simulate very well. it makes me think of the real thing.

no object deformations (so no springs)...

Actually you can deform physical objects. Come see my inchworm vehicle at the Foo Zoo on Foo Island. Anyone can ride it around. The physics simulation is surprisingly good! You and a few friends sit on the middle segment, and while the inchworm moves forward the entire body changes shape, which causes the riders to move up and down. Even though the riders bounce up and down, I'm not changing the height of the vehicle manually, tweaking hover parameters or anything like that. The movement that you see when you ride this is being done exclusively by deforming the object while it moves. I realize this isn't spring mechanics, but it is definately changing the shape of the object. Here's a couple of pictures.

http://www.sluniverse.com/pics/pic.aspx?id=83474&sort=PictureID+desc&Name=Unoti+Quonset

http://www.sluniverse.com/pics/pic.aspx?id=83457&sort=PictureID+desc&Name=Unoti+Quonset


I have some other toys that move exclusively by being physical and changing their shape dynamically. You can make some pretty fantastic things this way. Try it-- make something physical, then in child prims set their positions and rotations, and behold the chaos!

For another example of something cool that's possible with physics, come check out my giant pink elephant that juggles people.

http://www.sluniverse.com/pics/pic.aspx?id=80897&sort=PictureID+desc&Name=Unoti+Quonset

Anyway, come check out my stuff on Foo Island and play with it!

Anything that is free-hanging and involves multiple points of support is going to be erratic and unstable, unless done on a very large scale (2+ meters) and with very few objects (5 or less).

A swingset using actual chain links is impossible. Due to engine limitations the chain will unlink and/or explode, and will cause massive lag when physics is first turned on... until the unlinking or explosion occurs.

A balloon on a string faces similar problems because a piece of string functions exactly like a linked-chain, except the pieces are joined together and don't separate.

There's nothing currently built into SL that can simulate a string for your balloon other than flexible prims. Those are limited to "phantom only" to totally prevent them from being physics/collision enabled and causing massive engine lag. And currently, nothing can be attached to the free end of a flex prim.

I have a physics swing that I made that works pretty well. Rez a torus, rotate it 90 degrees, put a cylinder through the center hole. Attach a thin cylinder for a rope to the torus then attach a seat to the other end. Make the torus physical. Put end caps on the cylinder that goes through the torus hole to keep the torus from sliding off the ends. Link the torus, rope and seat together.

Sit on the seat and give yourself a good push.

The tourus and cylinder need to be pretty big to keep the torus from breaking through the cylinder. and sometimes it still does, but not very often.

- Sam

I experimented with making a little humanoid robot type physical object, with arms, legs, feet, etc, and used a simple balance-point algorithm to move his limbs, and apply translational and rotational impulses, so as to get him to stand up straight, and not fall over as he walked about. I eventually gave up, mainly because I found that although it would sort of work for a while, inevitably after a few minutes something would happen which turned all the scripts in the object off, so I would find poor Mr. Prim lying motionless on the floor. Plus every now and again he would rocket off into the sky for no apparent reason.

I decided to wait for the next version of the physics engine, and try again then.

If you want to make a windmill (there's one in Io, near 128, 128 that's non-physical) it's not that bad. Having been a sailor in SL I decided AGAINST making the windmill react to the wind, because the wind in SL is a fickle *****. In about 2 minutes it will swing throughout a full circle for direction. The speed is a little more stable if you're static but still varies from howling gale to totally still in less than an hour.

Small changes in horizontal position can result in huge differences in wind direction, I have seen (been in one of) two sailing boats both running pretty much straight down wind straight into each other and have to dodge to avoid contact. That was a bow-to-bow contact, the wind was running in opposite directions at two points within a few metres of each other. Less than the span of a windmill blade.

If you're still determined to do it you have to fake it. But there is llWind() which will let you calculate a speed and direction. You can use that to, as I did briefly, alter the spin speed and sail facing. You could equally use it to apply suitable torques.

Can be done with particles...

If you want to make a spring, it is possible. But you would need to do some equation figuring with how your spring would be positioned and how much force it would give, as well as figure in the HAVOK engine's limitations on how physics are run.

It would have to have a veeeerrrrrry small spring constant. Heh.

when we try to insert cyl with physics through torus, we get told one can not interposition object with physics into torus.

I know I have created toruses that interpenetrated eachother (bounding box wise) and had them behave properly...


- d.

i recant. we have it. make the cyl(s) the physicals

Teeter-totters are possible if you use a hollowed out half circle in the middle, or one that isn't completely closed. I remember experimenting with that stuff on Help Island. I was trying to find out all the fun things I could do without using script - Roller coasters and marble sets among other things . I had a gravity-powered car that was heck to reset because I'd have to realign everything and a few times I accidentally unlinked it .

- Jolan

I think there has to be a 10 cm gap between them, so you might have to open up the hole of the torus some more, or shrink the cylinder some.