All through kinematics, $F = ma$ sat in the option deck as a trap — nothing had a mass, so it never applied. That diagnosis just flipped. Read aloud: F equals m a — the net force on a thing is its mass times the acceleration it gets. Its quieter cousin is $W = mg$ (W equals m g): the pull gravity puts on a mass, using $g = 9.8\ \mathrm{m/s^2}$, the acceleration Earth hands out for free.

The sorting question is the same as ever: what does the story GIVE you? A mass accelerating wants $F = ma$. A mass hanging, sitting or pressing wants $W = mg$. And steady motion with a distance and a time still belongs to last chapter's $v = \dfrac{d}{t}$ — moving to a new chapter never uninvents the old tools.