[ODE] joint motor(2)

[whitt013]

Well, the slider is going to keep you constrained to always pointing "away" 
from your starting location.  So you'd have to ditch that idea if you wanted 
to turn.  You could add a couple spheres and hinge joints to the box to act as 
wheels and make a simple car (see test_buggy).  If you wanted to keep it to a 
single box, I'm sure there's an equation that would give you the force you 
need to set a force to once you reach the desired speed to maintain constant 
velocity, based on friction, mass, and gravity (mass*gravity*mu????).

Here's an idea that might be a bit more fun.  Fake wind resistance in your 
system by adding a force in the opposite direction of the box's velocity, and 
make the force a multiple of the square of the magnitude of it's velocity.  
Then add forces to push it in the direction you want it to go, and it will 
reach a "terminal velocity" once it gets to a certain speed.  Adjust the 
multiple to adjust the terminal velocity, and you have both constant velocity 
and unrestricted rotation.  Something like this:

//create "wind resistance"
dReal *velocity = dBodyGetLinearVel(box);
dReal k = 1.0;    //this is the multiple.  Increase for lower terminal 
velocity
dBodyAddForce(k * 
(velocity[0]*velocity[0]+velocity[1]*velocity[1]+velocity[2]*velocity[2]), 
-velocity[0], -velocity[1], -velocity[2]);
//that is NOT physically correct!

//now you can add all the forces you wan to the body.
dBodyAddRelForce(100, 1, 0, 0);   //the pushing force
dBodyAddForce(10, 0, 0, 1);   //the steering force (assuming y is up)

Like I said, that is definitely not a physically correct model for wind 
resistance, since it doesn't take into account the surface area or shape of 
the item, but it is a "good enough" approximation if all you need is a 
constant terminal velocity.

Hope that helps,
David

>===== Original Message From "kimyongsum" <vat832@hotmail.com> =====
>Hi:
>
>I got the box moving @ constant speed by using slider joint as David 
suggested. Now I have a new question.
>What if I want to push the box one side harder than the other to cause the 
box to turn (not only changing the direction) but still go at constant speed. 
How can I achieve this?
>
>I think I can use dBodyAddRelForceAtRelPos on both sides the box to make it 
turn but it'll go faster and faster.
>
>Thank you,