elastic collision simulation loses 400 kgm/s momentum in a few hours

I have made a 2D simulation of 20 balls enclosed within 4 walls, each with a velocity magnitude of 20 units/s, each with a mass of 1. all rigidbodies and colliders are elastic - 0 for friction, 1for bounce.

Initial conditions

• The velocity sum along the x axis is 240.

• The velocity sum along the y axis is 160.

• The sum of |V|*m was 400 (20*20) at start, and I continuously printed that sum on screen, while the balls went on colliding within these 4 walls.

During runtime

• after 1 second : sum = 360

• after 10 minutes : sum = 250

• after 4 hours : sum = 0

Motivation

Why did I do this simulation? because the ball in my arkanoid-style game loses speed over time, and I wanted to find out why. Is this due to the physics engine? shouldn't I rely on it to be accurate enough for at least a few minutes, without a need for runtime-corrections-by-code?

I know this sum does not represent how conservation of momentum works, but I start with non-zero momentum on both axis, so the simulation should not never get to 0 speed for all 20 balls.

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