# Elastic Collision

A set of randomly initialized partiles move inside a two-dimensional box. Each particle \(p_i\) is initialized with some mass \(m_i\), position \(\mathbf{x}_i\), and velocity \(\mathbf{v}_i\). The color of the particle indicates its speed \(|\mathbf{v}|\) and its size indicates its mass \(m\).

Collision detection is done by testing each of the \(n\) particles, which requires \(\mathcal{O}(n^2)\) time. If we increase \(n\), a more sophisticated collision detection algorithm would be advantageous. I test for interception and movement direction because otherwise, one collision might lead to multiple collisions due to the numerical inaccuracy.

```
intersect(particle) {
return this.position.sub(particle.position).length() < (particle.radius() + this.radius())
}
collide(particle) {
let epsilon = 0.001;
let p = this.position;
let q = this.velocity.mul(epsilon).add(p);
let p1 = particle.position;
let p2 = particle.velocity.mul(epsilon).add(p1);
// intersection of bodies and particles do not move away from each other
return this.intersect(particle) && p.sub(p1).length() > q.sub(p2).length();
}
```

For computing the new velocity after the collision, I use the projection to reduce the problem of the two-dimensional elastic collision to the one-dimensional case.

```
// this is the first and other is the second particle
let nVec = other.position.sub(this.position);
let p = nVec.project(this.oldVelocity);
let o = this.oldVelocity.sub(p);
let op = nVec.project(other.oldVelocity);
let mul = 1;
let velX = (this.mass * p.x + other.mass * (2 * op.x * mul - p.x)) / (this.mass + other.mass);
let velY = (this.mass * p.y + other.mass * (2 * op.y * mul - p.y)) / (this.mass + other.mass);
this.velocity = new Vector(velX, velY).add(o);
```

I move the particles forward by using Euler’s method.

```
move(delta) {
this.time += delta;
this.position = this.position.add(this.velocity.mul(delta));
this.positions.unshift(this.position);
if (this.positions.length > this.historySize) {
this.positions.pop();
}
}
```

There are numerical inaccuracies. Therefore, the energy of the system is unstable.