Physics are part of the physics engine on ROBLOX. The original physics engine was considered a very realistic engine by the staff members, but has been questioned due to the amount of fails and inaccuracies with the engines. In 2013, there was an Articulated Physics engine being worked on but was stopped because of it being slow and not solving collisions, and in mid 2015, a new collisions system was released first known as experimental physics but is now known as PGS Physics. Before PGS Physics, ROBLOX used a faster but more inaccurate solver known as Spring Physics.
Released Solvers
As of September 2015, two different physics engines are available to use, which is Spring Physics and PGS Physics.
Spring Physics
Spring Physics was the first physics engine to be in use by ROBLOX, and is still being used in most games. It is a faster engine for physics, but has a lot more give for things like collisions, which allows parts to go a little bit inside of other parts, allowing more bend. However, the engine is a lot more unreliable when used with a lot of physics. Hinges will sometimes glitch out and cause parts to go to random directions, leading to the model to spaz out.
Projected Gauss Seidel
Projected Gauss Seidel Physics or PGS Physics is a more reliable but more costly physics solver released in summer of 2015. The solver has a lot less "give" than spring physics, in terms of parts won't want to go into each other at all, making joints less flexible. This allows for parts to have less of a tendency to go through each other. As of this time, the joints called Glue Joints are not supported in the system and do not work as intended, and it takes more processing power to work over spring physics, which will cause games to act slower.
Comparing Spring Physics and PGS Physics
Before the solver was released, a comparison video was released that show the differences between the types of solvers.
Pro's of Using Spring Physics:
1. The solver is faster and takes less processing power.
2. Has more give (useful for some cases).
3. Better handles parts inside other parts.
4. Glue joints are supported (will be supported using PGS physics soon).
Pro's for Using PGS Physics:
1. The solver is a lot more reliable.
2. Mechanisms fail less due to less give/bending.
3. Will support more joints in the future like ones seen in Articulated Physics.
game.Workspace.PGSSolverEnabled
For enabling either physics solver, there is a property in Workspace called PGSSolverEnabled. When it is set to false in studio, the Spring Physics solver will be in use, and when set to true will enable the PGS solver. However, this property can't be set, or even read in-game.
Unreleased Solvers=
In late 2013, a very complex and very realistic system was in the testing phase called Articulated Physics, but was not released due to a lot of issues that came up.
Articulated Physics
Articulated Physics was a solver that was being worked on in late 2013. It allowed for extremely complex joints, allowing for extremely complex machines without the person needing to do a lot of CFrame math with welds known as Inverse Kinematics.
Why it was Unreleased
Many rumors existed why the solver was stopped, a big one being the person behind the solver left. However, according to Zeuxcg, the solver was canceled because it was an extremely slow solver and did not solve for collisions between moving parts.
Blog Articles on Articulated Physics
The admin behind the solver, Kevin He, released two different blog articles about the idea of Articulated Physics. The first one released in August can be found here, and the second blog article posted in September can be found here.
Videos on Articulated Physics
Three videos were released by ROBLOX that show different machines all working realistically with the solver.
Articulated Physics Demo Front-end Loader
Articulated Physics Demo Mech
Articulated Physics Demo Tank