What does a physicist mean by "external" force? O An external force is one that does not affect the body. An external force is one that acts at a distance where no contact is required. An external force is one that can affect the velocity of a body but not it's total energy. O An external force is one caused by something outside of the body under consideration. In light of Newton's third law of motion, why can't an internal force alone produce an acceleration of a system? An internal force can't produce an acceleration because it is always balanced by an external force. An internal force can't produce an acceleration because any internal force acting in some direction on one part of a body would produce an equal but opposite force acting another part. An internal force can't produce an acceleration because it has no inherent energy. An internal force can't produce an acceleration because it cannot produce an equal but opposite force in the body