Question

According to Newton's second law of motion, if a rigid body of unchanging mass is observed accelerating, what must be happening?

Answer 1

First off, Newton's second law of motion is the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
So therefore, it would be a force is being applied to the mass

Answer 2

Force acting on the body

Step-by-step explanation:

From Newton's second law of motion:

F = m a

where, F is the force, m is the mass and a is the acceleration.

The acceleration produced in a body having mass m is directly proportional to a net non-zero force acting on the body.

If the mass also changes, the acceleration depends on both the force and the mass.