Box2d计算轨迹

我试图让物理物体以随机的速度,以随机的位置撞击目标。我从使用chipmunk在Box2d中运行的web上收集并稍微修改了这段代码

+ (CGPoint) calculateShotForTarget:(CGPoint)target from:(CGPoint) launchPos with:(float) velocity
{
    float xp = target.x - launchPos.x;
    float y = target.y - launchPos.y;
    float g = 20;
    float v = velocity;
    float angle1, angle2;

    float tmp = pow(v, 4) - g * (g * pow(xp, 2) + 2 * y * pow(v, 2));

    if(tmp < 0){
        NSLog(@"No Firing Solution");
    }else{
        angle1 = atan2(pow(v, 2) + sqrt(tmp), g * xp);
        angle2 = atan2(pow(v, 2) - sqrt(tmp), g * xp);
    }

    CGPoint direction = CGPointMake(cosf(angle1),sinf(angle1));
    CGPoint force = CGPointMake(direction.x * v, direction.y * v);

    NSLog(@"force = %@", NSStringFromCGPoint(force));
    NSLog(@"direction = %@", NSStringFromCGPoint(direction));

    return force;
}

问题是我不知道如何把这个应用到我的程序中,我有一个重力为-20的y,但是把20的g和一个更低的速度,比如10的v,只能得到“无射击解”。

我做错什么了?