collDistance = (self.DIAMETER + particle.DIAMETER) / 2
        dx = particle.position.x - self.position.x
        dy = particle.position.y - self.position.y
        e = self.ELASTICITY * particle.ELASTICITY
        r = self.DIAMETER / 2
        distance = math.hypot(dx, dy)

        if distance < collDistance:
            #if particle in self.collision:
             #   return True
            #self.collision.add(particle)
            vp1 = self.velocity.x * dx / distance + self.velocity.y * dy / distance
            vp2 = particle.velocity.x * dx / distance + particle.velocity.y * dy / distance

            dt = (self.DIAMETER - distance)/(vp1 - vp2)

            self.position.x     -= self.velocity.x * dt
            self.position.y     -= self.velocity.y * dt
            particle.position.x -= particle.velocity.x * dt
            particle.position.y -= particle.velocity.y * dt

            dx = particle.position.x - self.position.x
            dy = particle.position.y - self.position.y

            distance = math.sqrt(dx * dx + dy * dy)
            ax = dx / distance
            ay = dy / distance

            va1 = (self.velocity.x * ax + self.velocity.y * ay)
            vb1 = (-self.velocity.x * ay + self.velocity.y * ax)

            va2 = (particle.velocity.x * ax + particle.velocity.y * ay)
            vb2 = (-particle.velocity.x * ay + particle.velocity.y * ax)

            vaP1 = va1 + (1 + self.ELASTICITY) * (va2 - va1) / (1 + self.mass / particle.mass)
            vaP2 = va2 + (1 + self.ELASTICITY) * (va1 - va2) / (1 + particle.mass / self.mass)

            self.velocity.x = vaP1 * ax - vb1 * ay
            self.velocity.y = vaP1 * ay + vb1 * ax
            particle.velocity.x = vaP2 * ax - vb2 * ay
            particle.velocity.y = vaP2 * ay + vb2 * ax

            self.position.x += self.velocity.x * dt
            self.position.y += self.velocity.y * dt
            particle.position.x += particle.velocity.x * dt
            particle.position.y += particle.velocity.y * dt