Question

What does each of the letters stand for F=d^2s/dt^2?
I know F stand for force, the rest don't know.

Answer 1

Well force F = rate of change of momentum P
momentum P = mass * velocity = m V
so
F = d/dt P = d/dt ( m V)
if mass is constant (usual assumption in rigid body physics at speed much less than light speed
F = m dV/dt
BUT
V = dx/dt where x is position
so
F = m d^2 x/dt^2
I suspect that this is what you intended to write
however if s = m x
then what you typed works

Answer 2

x