[periodontal]

Of course, even your example units don't always behave like you want. For velocities, 150 megameters/second + 150 megameters/second ≈ 240 megameters/second.

[all-fakes]

Sorry, can you explain why the velocities should add that way? I've always just seen velocities add like regular old vectors, so this looks very strange.

[identity0]

Relativity. If they could add that way, you would get 300 megameters/s, which is breaking the speed of light.

[Gladdyu]

That'd be special relativity for you - nothing ever exceeds the speed of light, hence a corrective factor needs to be applied.

https://en.wikipedia.org/wiki/Special_relativity

[JadeNB]

Relativistic effects.