We will never know why it does, even though it is a well-motivated prediction of classical and modern theories. It is possible for us to test whether or F=ma,

We will never know why it does, even though it is a well-motivated prediction of classical and modern theories. It is possible for us to test whether or F=ma, though, and so far, it (with relativistic corrections) checks out. An example of this sort of test from our research group: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.150801 https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98... (I'd go dig up more citations on the subject, but we have a grant proposal due in two days... :) )