I just had a quick look and I couldn't find any exact info, so I just need to know does Rigidbody mass variable simulate its weight or mass (where of course weight is mass * gravity). This link http://docs.unity3d.com/ScriptReference/Rigidbody-mass.html states that you should never set the mass higher than 10 , but the car in the standard assets demo is set to a mass of 1000 (which makes it sound like a weight figure). My thoughts are that you should just set object masses relevant to each other, but what happens when you want something really light (ie. tennis ball) and something heavy (ie. truck) to collide (where if the truck was set to 10, then we would need the ball at .00002 (based on a collision between 5 tonnes and 100 grams)?