LibGdx Bow and arrow game physics - libgdx

Question

I'm working on a new game written with LibGdx Engine and Java.
I've got a problem with some of the physics in this game.
I want to shoot the arrow in a ballistic trajectory (angry bird style)
and can't find the equation to do so .
I am using these velocity equations:
float velx = (float) (Math.cos(rotation) * spd);
float vely = (float) (Math.sin(rotation) * spd);
I add this to the current position and the arrow shoots in one direction - straight.
I thought maybe changing the rotation would help me achieve what I want (a ballistic path).
It does help, but I want to have the trajectory as well.
I saw this
ProjectileEquation class that someone already posted but didn't know how to work with it:
public class ProjectileEquation
{
public float gravity;
public Vector2 startVelocity = new Vector2();
public Vector2 startPoint = new Vector2();
public Vector2 gravityVec = new Vector2(0,-10f);
public float getX(float n) {
return startVelocity.x * (n ) + startPoint.x;
}
public float getY(float n) {
float t = n;
return 0.5f * gravity * t * t + startVelocity.y * t + startPoint.y;
}
}
I'm looking for some help to help me use this class for ballistic trajectories.
This is how I tried using it:
for(int i =0;i<30;i++)
{
Texture f = ResData.Square_1;
ProjectileEquation e= new ProjectileEquation();
e.gravity = 1;
e.startPoint = new Vector2(bow.getX(),bow.getY());//new Vector2(-bow.getX(),-bow.getY()); //My bow is opposite so it suppose to work fine
e.startVelocity = getVelocityOf(bow.getRotation());
Vector3 touchpos = new Vector3();
s.draw(f,e.getX(i) ,e.getX(i),5,5);
}

Answer 1

The ProjectileEquation class you post looks like it'll calculate the X and Y position given a time delta, so the float you pass in should be the time delta since you started the arrow moving (in seconds).
That code will not give you the angle of the arrow though. To find that, I would suggest you keep hold of the previous X and Y, then you can use Math.atan2() to calculate the angle based on the previous XY and the current XY. Google atan2 for a load of info on how to use it.
The very best way to do this however would be to use Box2d and model the scene correctly. Then you wouldn't have to get involved in the maths at all. I read somewhere that that's what Angry Birds uses, and is an excellent choice for modelling these sorts of physics games.
I hope your game goes well.