How to apply force at a specific position on a body

Unity Engine
1

Hi,

I have a space ship, and it needs to have a left, middle and right thruster. I was hoping to simply weld 3 thruster rigid bodies to the space ship and simply apply a force directly to the thrusters, but there is no box2D weld joint.

So I think I need to add 3 child game objects which are not physics objects to the space ship, and apply a force to the main spaceship at these positions. Is this the correct approach, and how do I apply a force at those positions…do I apply a force at a world position, or a local position of the spaceship - if world space, how do I get the position vector i need to apply the force at.

Any help appreciated.

Regards

Stephen

2

It is world space. However, you have the child object (engine) transform and rotation, so you can simply apply the force to the parent (ship) at the current transform position.

3

Hi,

Thanks, it was easier than I thought…transform gives world space anyway, even on child objects so no translation required.

Got in a pickle for an hour, because I forgot to Mathf.Abs the horizontal access value and so it only ever turned one way :wink:

public class LanderScript : MonoBehaviour {

public GameObject leftThruster;
public GameObject rightThruster;
public GameObject mainThruster;
public float thrustForce = 2f;
public float sideThrustForce = 2f;
// Use this for initialization
void Start () {

}

// Update is called once per frame
void Update () {
Camera mainCamera = Camera.main;

float leftRightThrust = Input.GetAxis(“Horizontal”);
float mainThrust;

if(Input.GetKey(“joystick button 16”)) {
mainThrust = 1.0f;
} else {
mainThrust = 0f;
}

//Vector3 leftThrustPos = transform.TransformPoint(leftThruster.transform.position);
Vector2 leftThrustPos = new Vector2(leftThruster.transform.position.x, leftThruster.transform.position.y);
Vector2 rightThrustPos = new Vector2(rightThruster.transform.position.x, rightThruster.transform.position.y);
Vector2 mainThrustPos = new Vector2(mainThruster.transform.position.x, mainThruster.transform.position.y);

Debug.Log("leftThrustPos " + leftThrustPos.x + " " + leftThrustPos.y);
Debug.Log("rightThrustPos " + rightThrustPos.x + " " + rightThrustPos.y);
Debug.Log("mainThrustPos " + mainThrustPos.x + " " + mainThrustPos.y);

if(leftRightThrust < -0.2) {
Debug.Log(“applying thrust to turn left”);

rigidbody2D.AddForceAtPosition(transform.up * Mathf.Abs(leftRightThrust) * sideThrustForce, rightThrustPos);
} else if(leftRightThrust > 0.2) {
Debug.Log(“applying thrust to turn right”);

rigidbody2D.AddForceAtPosition(transform.up * Mathf.Abs(leftRightThrust) * sideThrustForce, leftThrustPos);
}

if(mainThrust > 0) {
rigidbody2D.AddForceAtPosition(transform.up * thrustForce, mainThrustPos);
}
//leftThruster.AddForce(Vector2.up * thrust);
}
}