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. Letter by letter: $F$ is the net force in newtons, $m$ the mass in kilograms, and $a$ the acceleration in $\mathrm{m/s^2}$; whichever one the question leaves blank is the one you solve for. Its quieter cousin is $W = mg$ (W equals m g): the pull gravity puts on a mass, where $W$ is the weight in newtons, $m$ is that same mass in kilograms, and $g = 9.8\ \mathrm{m/s^2}$ is 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 $v = \dfrac{d}{t}$ — speed $v$ in m/s, distance $d$ in metres, time $t$ in seconds — because moving to a new chapter never uninvents the old tools.