Apply Force Relative To Rotation?

[bartbes]

Not 2, most (if not all) keyboards I encountered were able to do at least 3 keys at the same time. (how else would you do ctrl+alt+del, all the windows users need this)

[stevec64]

Do not use applyForce() or applyImpulse(). They are broken hard. I've reported the bug and sent a patch, but until it goes through, use setVelocity(). It is not broken, and it's possible to model acceleration and jerk using only setVelocity and the time increment.

I just hit this bug recently also... spent a while trying to figure out where unwanted torque was coming from. For the purposes of those who want to make a local fix to their source (I couldn't find Alex's patch on the Tracker):

The offending code is in box2d\Body.cpp. Make the following changes:

Code: Select all

	void Body::applyImpulse(float jx, float jy)
	{
		body->ApplyImpulse(b2Vec2(jx, jy), body->GetWorldCenter());
	}

	void Body::applyImpulse(float jx, float jy, float rx, float ry)
	{
		body->ApplyImpulse(b2Vec2(jx, jy), b2Vec2(rx, ry));
	}

	void Body::applyTorque(float t)
	{
		body->ApplyTorque(t);
	}

	void Body::applyForce(float fx, float fy, float rx, float ry)
	{
		body->ApplyForce(b2Vec2(fx, fy), b2Vec2(rx, ry));
	}

	void Body::applyForce(float fx, float fy)
	{
		body->ApplyForce(b2Vec2(fx, fy), body->GetWorldCenter());
	}

This seemed to fix the issues I had at least, so happily applying forces and impulses now getting the behavior I expect.

Steve

[LagMasterSam]

Do not use applyForce() or applyImpulse(). They are broken hard. I've reported the bug and sent a patch, but until it goes through, use setVelocity(). It is not broken, and it's possible to model acceleration and jerk using only setVelocity and the time increment.

Aside from that, lejeaunerenard is right. Derive your new x and y using the amount of force you want to apply (R) and the the angle that the object is rotated (theta):

Code: Select all

vx, vy = object:getVelocity()
newX, newY = R*math.cos(theta), R*math.sin(theta)
object:setVelocity(newX + vx, newY + vy)

I've tried using your code. However, my object almost immediately reaches a slow maximum speed, no matter how large I make R.