When an accelerating force acts on the case, the mass appears to swing in which direction?

When an accelerating force acts on the case, the mass appears to swing in the opposite direction to the accelerating force due to the principles of inertia and the effects of acceleration. This phenomenon can be understood through Newton's first law of motion, which states that an object at rest remains at rest, and an object in motion continues in its motion unless acted upon by a net external force.

If the case is accelerating in one direction, the mass inside will tend to remain in its original position relative to the non-accelerating frame of reference. As the case accelerates forward, for instance, the mass will appear to swing backward because it is resisting the change in motion due to inertia. This means that the direction of the swing is opposite to that of the accelerating force acting on the case.

The other options do not accurately describe the behavior of the mass relative to the case under acceleration. Thus, the correct understanding is that the mass swings opposite to the direction of the accelerating force.