Sunday, February 11, 2007

Getting Physical

Written 8 February, 2007

Getting Physical

I’ve not hereto paid much attention to the Physical setting in the Edit menu. In fact, I’ve paid it no attention it all. In fact, I think I actively blocked it out.

But I noticed it the other day when things were slow. I turned a prim—a piece of lumber 3 x 4 x 1 meters in size— physical by checking the box.

Nothing happened until I clicked off of the edit box. Then, to my astonishment, the prim fell slowly to the ground and toppled onto its side.

Well, that’s cool, I thought, and filed the event away in my For Later Exploration folder.

Even so, the idea of physical prims began to tumble about in my mind.

The next time I had a chance to play, I made a square prim, sized it to 3 x 3 x 10 meters, cut it so it had sloping sides, and hollowed it in a circular shape. This made a nice chute down which I could roll objects—if they would indeed roll.

I linked three of the chute prims, gave them a nice oak texture—Sweetie doesn’t approve of plywood—and tilted it at an angle of twenty degrees. Then I made a polished steel ball two meters across and positioned it in the air a good half meter above the top of the chute.

Then I turned the ball physical.

The first thing it did was fall. Even though the distance was short, it struck sparks when it hit the chute.

Then it began to roll—slowly at first, then it gained momentum.

When it reached the end of the ramp, it fell to the ground.

When it hit the ground it didn’t exactly bounce, just gave a short of shudder.

And then it began to roll. I barely got out of the way in time.

It went past me, slowing, and then, with its last bit of momentum, rolled into the canal, where it promptly sank to the bottom.

Way cool!

This little experiment taught me several things about Second Life physics.

Specifically, it taught me that SL physics—those I could judge from my experiment, at least—follow the physics of the universe pretty closely. To wit:

Gravity acts upon physical objects.

Physical objects rest upon other objects in a logical manner. That is, if you set a physical glass on a table, it will stay there. If you remove the table, the glass will fall. And, I suppose (and here’s another experiment!), if you tilt the table enough, it will topple (or slide off).

Objects gain speed as they fall.

The rate of acceleration doesn’t seem to be 32 ft. per second per second, as is the case in real life; it’s considerably slower—but they definitely accelerate. (I confirmed this later, when I raced a 40 meter sphere from 500 meters to the ground. I drew ahead at first, but it soon passed me).

The physical behavior of objects depends upon their shape.

The plywood board tumbled onto its side and lay flat on the ground. The metal ball rolled.

Physical objects are slowed by friction.

When the steel ball hit the ground, it rolled rapidly at first, and then slowed, much like a real ball does.

Physical objects interact with one another.

I learned this when I dropped a physical ball on a physical board I had balanced upon a triangular-shaped nonphysical prim, making an elementary teeter-totter. The impact of the ball caused the teeter to totter.

Physical objects interact with avatars.

I learned this when I walked along the teeter-totter and caused it to rotate on its fulcrum (the balance point). I further learned it when I dropped a steel ball on the t-t and was launched high into the air.

The interactions of physical objects occurs along x, y, and z axes.

I learned this when the ball I dropped on the teeter-totter not only caused it to totter, but knocked the board off its balance prim.

I learned a lot.

But there are things I do not yet understand.

Tell me about it.

Why did the teeter-totter board fall right through the non-physical triangle on which it was balanced?

(I have an idea about this, and that is that it did so because the triangular prim upon which the board was balanced had a very sharp point and the board suddenly had a lot of downward force because of the impact of the dropped ball. In other words, perhaps the triangle pierced the board).


Some unanswered questions.

Is there a top limit to the speed at which objects fall? Do they have a terminal velocity? And if so, does it depend upon their shape? And is there wind resistance?

Will, for instance a flat board tumble if it is dropped from a great height? And will it drop more slowly than the more aerodynamic sphere?

Is there a universal coefficient of friction, or does friction vary according to the nature of a substance? Do metal and rubber objects cause the same amount of friction?

Are other forces present? (Centripetal and centrifugal forces, for instance, or magnetism). Are physical objects impacted, as are particles, by the SL sun?


My experiments raised more questions than they answered.

In other words, they were a success.

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