# Physics

## Gravity

In Schome gravity is simply a force that acts to pull objects down, rather than being the complex inter-object force it is in the real world. First of all the strength of gravity is directly proportional to the mass of the object that is being affected, so an object twice the size has twice the force acting on it. However, as it is twice the volume and mass, objects fall at the same speed, hitting the ground at the same time when dropped at the same time.

Schome gravity acts to accelerate objects by approximately 9.8 metres per second per second. This occurs regardless of the mass of the object (as described above) and also seems to be independant of the height from which the object is dropped, which means that it does not decline with distance as it does in the real world. Further experiments with pendulums are in progress to find the value of g, through the use of the formula g=4π²L/t² (because g=4π²/gradient, while the graph is t² up and L across, so it's g=4π²/(t²/L))

Unfortunately, it seems that LaTeX formulae won't work here :( so I've reformatted it --Decimus 21:27, 11 April 2007 (BST)

Preliminary experiments (by Explo; liable to and human error horribly imprecise) have come up with a value of g that is approximately 8.25. However, scripting may be employed to receive a more accurate figure. Please try running some experiments of your own so as to receive many results and increase reliability.

Further experiments have come up against problems as friction utterly spoils the accuracy of the measurements. However, the period comes out as somewhere between 8.5 and 9 seconds, leading to a value of g of between 6 and 9. Ridiculously inaccurate, but at least they are results.

## Mass

Every object has a mass, which increases linearly with volume. This means that an object with twice the set mass a set distance from a pivot will balance with an object with a set mass twice the set distance. In addition two objects together have the mass of each of the objects combined, as is intuitive. However, the material of which the object is made does not make a difference to the objects mass. The same applies for all other properties such as shape and texture.

## Water

Water makes no difference to the physics engine. Objects fall at the same speed and neither specifically float or sink.

"These are great descriptions of your discoveries. But I am curious about the findings in water. How do objects like boats float in SL? --Davee Commerce 20:15, 30 March 2007"

Boats appear to have many different sets of rules applied to them, through LSL scripts. These involve operating as a boat type vehicle (a set of rules on its own) as well as having specific float commands for sea level (possible llBuoyancy(1);). I'm not sure how these work, but boats are set so they can only move when on water (they can exist anywhere, but not be controlled anywhere but water), as well as having a lower limit to how high they can exist than normal prims. Ask Decimus Schomer for more information as he was the one to find this out.

## The Schome Strange Force

The Schome Strange Force, or SSF, is a repulsive force emitted by every object, except phantom objects. It lasts for a distance of around 0.05 metres, so two objects together will be seperated by 0.1 metres, so long as one of them is physical (and can thus move outside of the forces power). However, as at least one of these objects is physical there are slight variations which means that the distance between the objects is either greater or less by up to 0.01 metres.

A key feature of the SSF is that it acts as though it were part of the object, so something cannot be made physical if it intersects with the forces space. However, a physical object moved into the forcefield and then updated will move out of the space, whether upwards or not, but a physical object intersected with the object itself, falls through the occupied space.

Topper Schomer is at present running experiments into cases of extreme SSF, where objects can be hurled many metres and even off-world. This appears to occur when an object cannot move out of the SSF's range and the subsequent pressures force it to intersect with the objects around it, providing more force to push it away. As you may guess, the best instances of this are in hollows, such as with pendulums.

Very interesting! Can you set up a demonstration experiment to show this in SchomePark? --Mark Cabaret 10:52, 20 March 2007 (GMT)

As yet, SPii doesn;t have a demonstration area, as I for one have been otherwise occupied. This is clearly something that we can do a lot of work on.

## Breaking SSF

As described previously, the Schome Special Force (henceforth termed SSF), can be broken in extreme cases. To break SSF, a physical object must be pushed into another object, either physical or non-physical, with sufficient force. When this occurs the two objects start to intersect, and the physical object involved is pushed away with a great deal of force. A table of the forces required to break SSF, as collected by an automatic experimentation module, can be found here (apologies for the poor formatting). Whether breaking SSF is dependent on velocity, momentum, kinetic energy, or some unknown factor, is at this point unknown.

Warning: If the objects are unable to move apart due to other conditions, the physics engine in Second Life crashes, causing Schome Park to crash. Therefore, experiments on breaking SSF must be performed with a great deal of care.

## Air Resistance

Air resistance does appear to exist in Schome Park, though further experiments are necessary-finding an object has a terminal velocity would be proof of air resistance. In the experiment I ran (at 100,42,120) a force is applied to a wooden block. The distance it travels on the y axis in the time it takes to fall 5 metres is recorded and with appropriate mechanics calculations (thanks go to Irlo for these) the distance without air resistance can be worked out (s=u√(h/4.9)) where s is distance moved, u is the initial velocity, and h is the distance fell). With the values at present (h=5, u=12.5, s=9.99966) s comes out as only 80% of what it should be. This indicates that air resistance is a counter force of a fifth the value of the force (please correct me here).

Decimus, could you format the later formula as well-I've done it according to the wkipedia help section but it obviously isn't working-Explo
Done --Decimus 20:35, 22 April 2007 (BST)

Yet more experiments, measuring the velocity of a block over time, have produced conflicting results for air resistance, with a block launched into the air counting no change in the velocity in the x direction as it moves. As these results are more directly corrected I would be inclined to trust them more-they have greater validity and the results are the same at every test, indicating better reliability. These also match the first results showing that two objects of different surface areas fall at the same speed. However, the question remains as to why the other results showed air resistance. Unfortunately, my attempts to replicate the experiment with a better script has failed.

## Friction

Friction also exists in the form of a counter force acting when two objects move against each other. I have done no experiments on the quantities as of yet but my experiment (100,42,100 again-I sound like an advertiser :P) can show that friction exists. Without friction but with gravity a cylinder that is rolled down a hill would not roll but slide. By releasing a cylinder at different gradients friction can be observed.

Data is now in on quantitive measurements of friction. With a wooden cylinder of mass approximately 3.61 and an initial velocity of 8 the displacement ends up as around 5.757. Running this throught the various equations (provided by Irlo) the overall change in velocities ends up as -4.901504 per second per second. In other words friction and air resistance combined create a negative acceleration of around 4.9. By running a control in which air resistance is the only factor the effect it has (0.355682) can be deducted and the effect of friction can be assumed to be around 4.5 per 8, or abot 56 per cent. Now, I am sure this is wrong, especially as air resistance was previously worked out to be around one fifth (and hence 1.6 in 8) so any experiments would be greatly appreciated. Further tests showed that material makes a difference to friction with metal ending in a displacement of around 6.1 and rubber and flesh ending in a displacement of around 5.4. Prim size increases the effect of friction proportionally, but by using a rolling cylinder as well as a cube it appears that area in contact with the surface doesn't make a difference (though again I doubt this conclusion). I haven't tried situations where more than two prims are in contact with each other.

Further experiments have come up with a more likely figure for the power of friction. With only air resistance a wooden block moved an average of 7.5732719 metres. A block with the same specifications and impulse moved 5.6137094 metres on average when in contact with a wooden platform. Putting this through the equations, as well as comparing it with previous findings, it appears that the block should, without air resistance, move approximately 9.5 metres, so the block encountering both friction and air resistance is around 60% of that figure, so friction has an counter force of again around one fifth of the force.

## Materials

Experiments to test the different values of friction with different materials are not as yet quantitive but a list follows, with the least friction first: glass, metal, plastic, wood, stone, joint flesh and rubber.

Thanks go to Decimus Schomer for his work on acceleration due to gravity and Marco Schomer for his work on mass.