[ODE] Mass etc.

[Jon Watte]

1. and 2. -- the units are whatever you want, as long as they are 
consistent. This is answered in the documentation. If your mass 
is kilograms, then typical Earth gravity would be 9.81 (newtons 
per kilo), assuming step size is seconds (0.016 for 60 Hz) and 
distance is meters.

3. Yes, it's world space. If you want object space, get the object 
rotation as a matrix or quaternion, and pre-apply the relative 
orientation you want using regular 3D geometric algebra.

Cheers,

			/ h+


-----Original Message-----
From: ode-bounces at q12.org [mailto:ode-bounces at q12.org]On Behalf Of Yefei He
Sent: Wednesday, March 30, 2005 5:19 PM
To: ode at q12.org
Subject: [ODE] Mass etc.


Hi, 

    Here are a few more newbie question from me. 

    1. Does mass have units? I assumed it is in kilograms and assigned 1000 
to an object and let it rest on top of a plane at the beginning. As soon as 
the simulation starts, the object falls under the plane as much as 4 meters 
below and then bounces back up. When I set the mass to 50, it works more 
realistic. What mass values should I assign to an object to match its real 
world mass, say a one ton car, on concrete road surface? Are there any world

parameters I need to set? 

    2. Does step size correspond to real world time? I'm running a visual 
simulation that renders at 60Hz, so presumably the step size should be
0.0167. 
But it seems that free falling objects fall rather slowly. I had to double
the 
step size to make it look more real. The gravity is set at (0, 0, -0.981). 

    3. Could somebody answer me if dBodySetLinearVel() and 
dBodySetAngularVel() sets velocities in the world frame coordinates or 
the body frame coordinates? They appears to be in world frame... If so, 
how do I apply roll, pitch and yaw rates? 

    Thanks, 

    Yefei
 
     


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