# What to call an object which is in motion but not accelerating?

I began learning about acceleration and according to the source i learned from an object only accelerates when there is change in vel. &/ direction... what i am wondering is what do you call it when the object does not accelerate? i.e. when the object travels at a constant speed and direction.

• "the object travels at a constant speed and direction" - Relative to what? How about relative to the walls inside an elevator that happens to be in a free fall? Or how about relative to the building, in which this elevator is? The answers obviously would not be the same. So your question is too ambiguous for a correct answer while the answer you've accepted is incorrect. – safesphere Jul 26 '18 at 22:52
• @safesphere, if it has constant velocity in any inertial frame, it has constant velocity in all inertial frames. It's fairly common to assume we're measuring these things in an inertial frame if it hasn't been explicitly stated otherwise. – The Photon Jul 27 '18 at 0:46
• @ThePhoton One can assume nothing on this site. Besides, when was the last time you were in an inertial frame ;) – safesphere Jul 27 '18 at 2:08
• @safesphere Are you saying my answer is incorrect just because the OP didn't list certain assumptions, or is there actually something wrong? On this site I try to be helpful based on what I think the intent and level of the OP is rather than nit pick at every single detail, but if there really is something actually incorrect please let me know so I can make the proper edits. – Aaron Stevens Jul 27 '18 at 3:37
• @AaronStevens It is OK for the OP to make mistakes and omissions. It is not OK for the person answering a question for the whole world to see to take it lightly and make the answer ambiguous and blurry. You should have explained that motion is relative, the abstraction of inertial frames, how we deal with gravity in real life frames (e.g. see my elevator example above), and so on. Instead your post sounds like you don't really want to go through the trouble. It's not an incorrect answer, it's not really an answer at all and should've been just a comment instead. – safesphere Jul 27 '18 at 21:19

I don't think you call the object anything. You might hear people use the phrase "uniform motion". Although this term can also be applied to circular motion as "uniform circular motion" which is motion in a circle with a constant speed, but like you said there is still acceleration due to a direction change.

So I would say "uniform motion", "uniform linear motion", "motion under $0$ acceleration", or maybe just "constant velocity". Maybe other people know some things I haven't heard of or thought of though.

It should be mentioned that this all depends on the reference frame you are observing the object in. All motion is relative, so you need to first specify your frame of reference. For example, a ball in free fall will appear at rest to someone falling along with the ball, but to an observer on the Earth the motion of the ball will not be uniform linear motion. Since you made minimal assumptions, I just stuck with a simple answer. If the object is observed from an inertial (non-accelerating) frame, then we will see uniform linear motion in all inertail frames.

• uniform linear motion? – Oktay Doğangün Jul 26 '18 at 22:17
• Isn't this an inertial frame of reference. From Wiki; An inertial frame of reference in classical physics and special relativity is a frame of reference in which a body with zero net force acting upon it is not accelerating; that is, such a body is at rest or it is moving at a constant speed in a straight line – foolishmuse Jul 26 '18 at 22:18
• @foolishmuse I am not sure about this. You can be in an inertial frame of reference without referring to the motion of an object. And you can have an object undergoing acceleration observed from an inertial frame of reference. I see where you are coming from though. – Aaron Stevens Jul 26 '18 at 22:20
• A rest frame need not be an inertial one. For example, an accelerating object has a rest frame... but that frame isn't inertial since, when viewed from that accelerating object's frame, inertial objects won't travel in straight lines with constant velocity. Example: a car that is suddenly braking. – robphy Jul 26 '18 at 22:31
• @OktayDoğangün I'm always big on creating new words. So I'm starting SAIM. Society for the Adoption of Inertial Motion. And "saim" pronounced "same" even sounds right for the topic at hand! – foolishmuse Jul 27 '18 at 15:47

"inertial motion".

that's what i would call it.