ApplyLinearImpulse doesnt work when called within UIPanGestureRecognizer function

I am using Box2d and Cocos2d to develop an iPhone game. I have a ball in the center of the screen with a simple boundary created using fixtures etc. What I want to do is add the functionality so that a user can "swipe" the ball to apply force and basically flick it across the screen.

I am using the following code to achieve this:

b2Vec2 force = b2Vec2(30, 30);
_body->ApplyLinearImpulse(force, _ballBodyDef->position);

If I apply a linear impulse during the init function the ball fires off in the correct direction and behaves as it should do. It doesnt do anything, however, if I put it into the handlePan function that is called when a gesture is done by the user. Here is the full code for the function: (Note that the NSLog writes out the correct information.

- (void)handlePanFrom:(UIPanGestureRecognizer *)reco开发者_运维问答gnizer {
    if (recognizer.state == UIGestureRecognizerStateEnded) {
            CGPoint velocity = [recognizer velocityInView:recognizer.view];

            NSLog(@"Vel: %f, newPos: %f",velocity.x,velocity.y);

            b2Vec2 force = b2Vec2(30, 30);
            _body->ApplyLinearImpulse(force, _ballBodyDef->position);
    }      
}

The force you are applying in handlePanFrom is always b2Vec2(30, 30). It will never change with your current code, and is therefore not going to move in your direction.