# The unit of Universe

This is derivative from Reddit post I started.

Godot’s Physics2D engine operates on internal, pixel-based units. ΔV needs the data in real-world units, so I need to translate one to another.

### Why?

I want the physics of the game to be relatable. Todays spacecraft use $kN$ for thrust and $m \over s^2$ for acceleration, and I want ΔV to use the same, so the player who ever saw an actual space launch can relate the values.

### How?

This is the fun part. First, I need to now in what units internally Physics2D engine operates. I found no information on the subject in docs, so i resolved myself to educated guesses:

• Distance is in pixels
• Mass is in whatever you enter (assuming $gmu$ as Godot Mass Unit)
• Weight seems currently broken
• Velocity is in pixels per second

### The translation.

First, my assumptions. My mass unit is $1000kg$ and my pixel is $10cm$ wide. That should allow all the necessary conversions from Godot units to real ones:

• The distance is conveniently $1[m] = 10[px]$ and $1[px] = 0.1[m]$.
• Mass is assumed $1000[kg] = 1[gmu]$.
• Linear velocity: $1 [{ px \over s}] = 0.1 [{ m \over s }]$.
• Force is $1 [{ gmu * px \over s^2 }] = [{ 1000kg * 0.1m \over s^2 }] = 100 [{ kg * m \over s^2 }] = 100[N]$.
• Impulse is the momentum: $1 [{ gmu * px \over s }] = [{ 1000kg * 0.1m \over s }] = 100 [kg * { m \over s }]$.
• Kinetic energy can be derived from mass and velocity as $E_k ={{ m * v^2 } \over 2 } [ gmu * ({ px \over s })^2] = [{ 1000kg * 0.01m^2 \over s^2 }] = [10 { kg * m^2 \over s^2 }] = 10[J]$

That’s about all the units I need now – I can translate real thrust to impulse applied and compute impact energy in J (more likely – GJ). Armed with this data we can resume modeling out our game in real units.