Physics teachers tell us force equals mass times acceleration, but, as
any rocket scientist would tell you, this is false.
Force is actually the change in momentum over time. Since momentum (p) is
mass times velocity, force is actually:
F = dp/dt = d(m*v)/dt = m*dv/dt + v*dm/dt = ma + v*dm/dt
In other words, force is mass times acceleration PLUS velocity times
change in mass per unit time.
For items with fixed mass, dm/dt=0, and this reduces to Newton’s equation.
For items with changing mass (like rockets, which expel mass to gain
velocity), Newton was wrong.
[Of course, Einstein also showed F=ma isn’t 100% accurate because of relativistic effects]