How is AddForceAtPosition() _supposed_ to work?

Unity Engine
1

My aim was to simulate an object being propelled by a thruster, so I wrote:

var m_thrusterPos : Vector3(0, 0, -1);   // local-space thruster position
var m_thrusterForce : Vector3(0, 0, 500);   // thruster force in local-space  

function FixedUpdate()
{
   var wPos : Vector3 = transform.TransformPoint(m_thrusterPos);
   var wForce : Vector3 = transform.TransformDirection(m_thrusterForce);
   rigidbody.AddForceAtPosition(wPos, wForce);
}

On pressing run, I expected my object to start moving under the force of the thruster. Instead, it just sorta kinda sat there. Not moving. At all. :frowning:

After trying a bunch of different things, I got it to move the way I expected by doing this instead:

function FixedUpdate()
{
   rigidbody.AddRelativeForce(m_thrusterForce);
   rigidbody.AddRelativeTorque(Vector3.Cross(m_thrusterPos, m_thrusterForce));
}

I’d expect this to be equivalent to the version I wrote above, except it’s not. So… what am I missing here? How is AddForceAtPosition supposed to work?

2

Well, you switched the order of the parameters…

Before I’m gonna try and break my brain at 3am on the rest of the code - try and put them in the correct order and post the results :slight_smile:

3

iSuck indeed! :wink:

Thanks for spotting the blatantly obvious error my brain failed to register! Changing the order of the parameters, things work as advertised now. Yay!

I think I’ll keep the implementation that uses RelativeForce and RelativeTorque, though.