borisb2 0 Posted July 20, 2015 (edited) we know that 1 unit in houdini is 1m .. is it really? Thought I just test it: Placed a cube 10 units in Y above the grid, RBD-object, gravity 9.8 .. uups, takes 35 frames to hit the ground. according to physics: g = 9.81 m/s^2 v= s/t a= v/t -> a = s/t^2 t = sqroot(s/a) = sqroot(10m/9.81m/s^2) t = ~1s = 24 frames .. not 35 !! whats wrong ? placing the cube 5 units in Y it needs exactly 24 frames to hit the ground by the way Edited July 20, 2015 by borisb2 Share this post Link to post Share on other sites
Skybar 360 Posted July 20, 2015 I'm guessing your object is perfectly static at the start, in other words you need to calculate for the acceleration as well. Instead, disabling gravity and setting the cubes initial velocity to -9.80665 in Y makes it hit the ground at approximately the 25 frame-mark (if your scene is set to 25 FPS). Share this post Link to post Share on other sites
anim 1,264 Posted July 20, 2015 well, since equation for free fall with initial 0 velocity is d = 1/2gt2 then t = sqrt(2d/g) so t = sqrt(20/9.80665) = 1.42809s = 34.2741 frames (for 24fps) which seems to check out, considering that you probably start at f = 1 2 Share this post Link to post Share on other sites
lukeiamyourfather 191 Posted July 20, 2015 Tomas' math is correct. Houdini is accurately simulating what would actually happen and one Houdini unit is equal to one meter as far as dynamics go. Share this post Link to post Share on other sites
borisb2 0 Posted July 20, 2015 I was sure that I had to be wrong ..didn't know about that one: d = 1/2gt2 thanks Share this post Link to post Share on other sites
