Question

what must be happening, according to newtons second law of motion, if a rigid body of unchanging mass is observed accelerating?
a force was once applied but is now not being applied
the mass must be changing
a force is being applied to the mass
no force was or is being applied to the mass

Explanation:

Newton's second law is \(F = ma\) (where \(F\) is force, \(m\) is mass, and \(a\) is acceleration). If \(m\) is constant (unchanging) and \(a
eq0\) (accelerating), then \(F
eq0\). A non - zero force must be acting on the mass. If no force (\(F = 0\)), then \(a=0\) (from \(F = ma\)). If the mass was changing, the problem states it is of unchanging mass. If a force was once applied but now not, the body would move with constant velocity (by Newton's first law, \(F = 0\) implies no acceleration, constant velocity). So, a force must be currently applied.

Answer:

A force is being applied to the mass