Author's Notes:Un-beta'd, so any factual errors are entirely mine. And Wikipedia's. I always did like physics, but it's been a long time.
The Laws of Physics
An object at rest remains at rest until acted upon by an external force.
Trust Sirius to be dramatic about it, storm out, slamming the door behind him like a common Muggle instead of something sensible, like the Floo or Illegal Apparition.
Force equals mass times acceleration.
The door is closing fast, ancient hinges squealing protest.
The speed of sound is 343.4 m/s at 20 degrees Celsius in the standard atmosphere.
It's probably slower in an atmosphere that consists of three parts dust and sixteen parts tradition for every one part air.
Also, it's closer to 25 degrees Celsius, now that he thinks about it.
How odd that it should be so warm and bright outside when his world has suddenly become so dark and cold.
His brother's hair flashes in the sunlight as he reaches the street, scattering light in his wake like a prism.
It does not reach him on the stairs, is swallowed by dust and shadows and ancient malice as soon as it darts through the narrowing doorway.
The speed of light in a vacuum is 299,792,458 m/s.
It's not a vacuum either and all that really matters is that light is faster than sound and even light can't reach his brother now.
But he tries.
"Sirius, don't-"
They say that time stands still when you are dying.
They are right.
The door clicks shut.
"-leave me."