Physically, inertia is a feeling that you just can't move. Mentally, it is a sluggish mind. Even if you try to be sensitive, if your mind is sluggish, you just don't feel anything intensely. You may even see a tragedy enacted in front of your eyes and not be able to respond meaningfully. You may see one person exploiting another, one group persecuting another and not be able to get angry. Your energy is frozen.
You are not deliberately refusing to act, you just don't have the capacity.
(A) Inertia makes your body and mind sluggish. They become insensitive to tragedies, exploitation and persecution because it freezes you energy and decapitates it.
(B) When you have inertia, you don't act although you see one person exploiting another or one group persecuting another. You don't get angry because you are incapable.
(C) Inertia is of two types - Physical and Mental. Physical inertia restricts bodily movements. Mental inertia prevents response to events enacted in front of your eyes.
(D) Physical inertia stops your body from moving, mental inertia freezes your energy and stops your mind from responding meaningfully to events, even tragedies, in front of you.