Question

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

Explanation:

Newton's second law of motion is \(F = ma\) (where \(F\) is force, \(m\) is mass, and \(a\) is acceleration). If \(m\) is unchanging (\(m
eq0\)) and \(a
eq0\) (accelerating), then \(F
eq0\). So a force must be applied. The option "The mass must be changing" is wrong as problem states mass is unchanging. If no force (\(F = 0\)), then \(a=0\) (from \(F = ma\)), so options "A force was once applied but is now not being applied" and "No force was or is being applied to the mass" are wrong.

Answer:

A. A force is being applied to the mass.