The simple way to distinguish "motion" and "action" is:
"motion" is a process
"action" is the actual doing
In a word, moving doesn't necessarily mean making a move ! (take a look at Newton's 1st law of motion)
Since you mention about "機械結構的"動作"那麼用motion對嗎", let take the Newton's 3 laws of motion as an example:
1st Law of motion (law of inertia): if the total external force is zero, then whatever is moving will move at a constant velocity, otherwise, it will stay motionless. In this law, it talks about motion.
2nd law says, if the total external force is NOT zero, then the object will move with an acceleration proportional to the force. It talks about action.
Now, let's go back to your question: 機械結構的"動作"那麼用motion對嗎?
The answer is, "yes" or "no". If you ONLY want to describe the movement of a mechanism then YES.
If you want to describe the cause of the movement, then NO.
In a subject called "Dynamics" (usually is studied by Mechanical Engineering or Physics major). It separates into Kinetics and Kinematics. Where Kinematics studies only the motion, and Kinetics studies the relationship between motion and action.
In a word, action is the cause of a motion, but motion doesn't always need action.
2014-02-20 22:39:20 補充：
My explanation is based upon the Physics. Master AP's saying is based up "purpose" or "will".
2014-02-20 22:43:10 補充：
let take the Newton's … => let's take the Newton's …
2014-02-20 22:44:18 補充：
My description of Newton' law of motion is in my own word. For the exact statement, you shall refer to your Physics textbook.