# Thread: Bounce - Dealing With It

1. ## Bounce - Dealing With It

I detect a collision between some object and another, and I get a normal to the surface where the collision occured, and the point of that collision. I try to sort of pretend that the velocity vector of the colliding object and the surface normal make like an 'angle of incidence'. And then I go farther and deal with that whole dealiothingy in put it into the context of a polar system sort of. Looks like this (multiplication operator does dot product):

Code:
```        double incidence = std::acos( -v_ball.getNormalized() * normal );

// Into polar (theta, radius) form
double theta = std::acos( -v_ball.getNormalized() * CVector(1, 0) ) + incidence * 2;

// Into cartesian x, y form
CVector out = CVector( std::cos( theta ) * radius, std::sin( theta ) * radius );
m_ball.setVelocity( out );```
So, basically my logic is like this. It's wrong I guess, because the ball goes all over the place unpredictably after the collision, but, it's my logic.

2. Given you know the normal vector of the surface and the velocity vector of the ball -

Ball.VelocityVector*=Surface.Normal;

Given both are expressed as unit or normalized vectors.

3. I ended up using the formula for vector reflection:

( v_ball.getNormalized() - 2 * ( v_ball.getNormalized() * normal ) * normal ) * v_ball.length();

The multiplication operator denoting a dot product operation when operating on vectors there.

>> Ball.VelocityVector*=Surface.Normal;

What will that give me? It didn't seem to work in my program (even after scaling that vector)

4. I'm not sure if this works, might be worth a try though:

Ball.vel = Ball.vel - 2 * absolute(normal) * ball.vel

Worked for me on normal = (-1, 0) and (0, 1), not sure if it's the general case though.

Good luck

Wazaa