LSL Wiki : mass

Mass

Mass is the measure of translational inertia.

The mass of a prim or object, as reported by llGetMass or llGetObjectMass, depends only on its size and shape, not on its material type. The units are lindograms.

Functions

llGetCenterOfMass llGetMass llGetObjectMass

SL gravity is <0,0,-9.8> m/s². According to Newton's Second Law, an object may be given a desired acceleration with:

vector desired_acceleration;
float mass = llGetMass();
llSetForce(mass * (desired_acceleration + <0,0,9.8>), FALSE);

Notes

Changing material type has no effect on density and therefore mass.
Hollowed prims tend to have less mass and twisted prims tend to have less mass but there are some oddities (see energy).
In linksets, the parent returns the sum of all the masses in the set, while the children return only their own.
Avatar mass isn't affected by attachments but will change depending on avatar height (but not fatness, thickness, muscle, etc). Changing shoes also changes avatar height and therefore the mass is effected. Attachments seem to adopt/report the avatar's weight.
Sitting down means your weight is added to the sat-on object. It seems implemented as temporarily being one of the children in the linkset.

Physics | Energy