# Why doesn't the conservation of angular momentum modify mass instead of velocity?

Background

Kepler's second law says that planets sweep equal areas in equal times in their orbits; the closer a planet is to its parent body, the faster it moves; the farther it is, the slower it moves. This is explained by the conservation of angular momentum, which is $m * v * r$.

$m$ is the planet's mass

$v$ is the planet's orbital velocity

$r$ is its distance from the parent body

As $r$ decreases, $v$ must increase to maintain the momentum.

Question

Why is $v$ "chosen by the Universe" to be the value that changes in this phenomena and never $m$? I've only heard of mass being modified theoretically, when something is attempting to increase $v$ beyond the speed of light. If that's true, why does mass remain immutable until that point?

• People, don't flag this question for deletion: it doesn't show lack of effort. It shows a confusion about basic concepts that would be pretty hard do solve your own. Aug 31, 2017 at 20:19