Is Acceleration Truly Separate from Motion in Physics?

[Sterling Monty]

An acquaintance and I are having a dispute and I am looking for an answer that will hopefully end the dispute.

My acquaintance asserts that:

There are uniform motions and non-uniform motions.
Uniform motions are relative.
Accelerations are absolute.
Rotations are absolute.
Some rotations are uniform and some are not.
Non-uniform motions involve accelerations.
Accelerations are not motions.

Of all of those assertions, the last one is new to me. What is the "history" behind that assertion? Is it a "majority viewpoint" or a "minority viewpoint"?

 

[Mk]

Accelerations are not motions.

I'm pretty sure no body thinks that...

Accelerations are motions, just look at the units.
m/s²
meters per second per second

Meters per second, describes speed. If something is moving at 50 meters per second, it has a speed, and its moving. That's a motion.

Acceleration is a change in velocity. Velocity is composed of
A) speed
B) direction

So acceleration is a change in speed and/or direction. Speed is a motion, therefore acceleration can be a change in motion, therefore acceleration is motion.

Explained well?
- Mk

I say "can be," because it can also be a change in direction of motion, that also ends up making acceleration a motion.

Concluding, acceleration is defined as a change in rate/direction of motion.

 

[El Hombre Invisible]

You can't describe the motion of an accelerated object with reference to its acceleration. Likewise you can't do so without knowing its velocity at a given time. So I'd say "acceleration isn't motion" is true to the same extent that "velocity isn't motion" is true.

In a general case, motion is neither velocity nor acceleration, but something that is described by both. In a special case (no acceleration), motion is described entirely by velocity, so playing fast and loose with terminology you could say velocity is motion for these cases. In a different special case (the acceleration of a body from rest), motion is described entirely by acceleration, so again the carelessness of saying motion is acceleration in such a case could be deemed acceptable.

Best stance is don't mix up words that mean different things. Motion is a phenomenon described by ideas of position, displacement, velocity and acceleration. It is not just one of those things, nor is one of those things ever motion, strictly speaking.

 

[Sterling Monty]

THI: So I'd say "acceleration isn't motion" is true to the same extent that "velocity isn't motion" is true.

SM: I would understand that response, if my acquaintance had taken that position. At least, it is consistent and intelligible.

It is possible that that is what my acquaintance is trying to say but, because he hasn't figured out how to say it, I can't help but disagree with what he has said. Unfortunately for us both, he's stated that:

1. Motions can be uniform or non-uniform,
2. Non-uniform motions involve accelerations
3. Accelerations are not motions.

And my problem is that I can't think of a non-uniform motion that does not involve acceleration. Moreover, I can't help but wonder whether he thinks "uniform acceleration" refers to a motion or a non-motion.

THI: Best stance is don't mix up words that mean different things. Motion is a phenomenon described by ideas of position, displacement, velocity and acceleration. It is not just one of those things, nor is one of those things ever motion, strictly speaking.

SM: The classification challenge comes at the second level ...

A. There are motions and there are non-motions.​

A.1. Motions may be relative or absolute.​

A.1.a. Absolute motions may be uniform or non-uniform.​

A.1.b. Relative motions may be uniform or non-uniform.​

A.2. Non-motions may be relative or absolute.​

In that case, I could easily view each motion and each non-motion as having some value of position, displacement, velocity, and acceleration equal to or greater than 0 and go from there to talk about available evidence for each state.

But, because my acquaintance refuses to accept the meaningfulness of the concepts of absolute non-motion, absolute uniform motions (except for "rotations"), and relative non-uniform motions, we end up arguing over whether accelerations are motions or non-motions.

 

[El Hombre Invisible]

Okay, first of all:

1. "There are motions and there are non-motions."

This may well work for everyday experience, but the difference between non-motion (by which I presume you mean in a state of rest) and uniform motion is a consequence only of the observer's frame of reference. In fact, in any meaningful sense, there is no difference, since you can rightly describe a body at rest in one frame as being in motion in another, and vice versa. In short, non-motion is just uniform motion with a speed of 0 in the frame it is observed.

2. "Motions may be relative or absolute."
3. "Non-motions may be relative or absolute."

For the same reason, these are false. All motion in 3 spatial dimensions is relative - that is, will be described differently in two different reference frames. The only non-relative kinda motion I can think of is the intrinsic angular velocity of quantum particles, which isn't really motion. At least, not in any way helpful to this question.

4. "[M]y acquaintance refuses to accept the meaningfulness of the concepts of absolute non-motion, absolute uniform motions..."

Quite rightly so. And should refuse to accept the meaningfulness of absolute uniform rotations also. For some quantity to be absolute, it would have to be the same in any inertial frame. The rotation of the Earth, for instance, will appear as at its known value and roughly uniform to a spaceship pilot headed directly towards the Earth's centre of mass at a constant velocity. However, to another pilot flying tangentially to the Earth at a constant velocity, the rotation would not only be different, but would appear non-uniform.

And like I said, acceleration, velocity, etc AREN'T motion - they are properties of motion.

 

[Crosson]

Prove your friend wrong using the definitions of velocity and acceleration. If acceleration is non-zero, and acceleration is the derivative of velocity, then velocity must be non-zero. Assuming that he considers velocity to be motion.

 

[mezarashi]

After taking a short course in kinematics, I think all of this will easily clarify. Acceleration is defined as the change of motion. You can predict the motion of an object in the future but adding the acceleration to its current motion vector.

There are things like average acceleration, average velocity (in science it's called velocity rather than motion), instantaneous acceleration, and instantaneous velocity.

Already some good explanations and I'd like to add some of my own comments to the assertions bit. As far as most things are concerned, ALL velocity (motion) and acceleration are RELATIVE. There is another point as to whether you can tell if you are in a moving frame or an accelerated frame, but I'll leave that out for now.

Uniform motions are relative.
Accelerations are absolute.
Rotations are absolute.

If you happen to be accelerating at the same rate as whatever you're observing, it will appear to have no acceleration. Same goes for circular motion. We are going in circles around the Sun right now on Earth. We don't see it as being absolute. In fact we say we are stationary... so who's right? The guys on the sun seeing us going in circles or us on Earth thinking we're at peace. Oh, and yes, all rotations (circular motion) involve acceleration.

 

[ZapperZ]

There is something not quite right here... Let's ignore Einstein's equivalence principle and look at this via simple classical mechanics.

First of all, let's look at Newton's 1st Law. It says that an object in equilibrium will either be not moving, or will move with a constant velocity. It means that those two are the same thing, and implies that the object has no way of knowing BY ITSELF it if it moving or not, AND if it is moving, at what velocity it is moving. The object (let's say YOU), cannot know what velocity you have unless you use an external reference object, or reference frame. This is what is meant by "relative". Your velocity, or the velocity of anything, is measure WITH RESPECT to something else. Without this "something else", you have no way of knowing your velocity, or even if you are moving.

Now, what happens when you accelerate? If you are moving with constant velocity with respect to me, and then suddenly you start accelerating, would you be able to tell that you are accelerating EVEN IF YOU HAVE YOUR EYES CLOSED? What if you're in a spaceship, and the same thing happens, but you were holding onto a mass-spring system. Can you tell without having to use another reference frame that you are actually accelerating? Or what about if you were holding onto a simple pendulum that was swinging? Can you tell that you went from constant velocity to suddenly accelerating? And what if you are in a spaceship that is accelerating, while I am in a spaceship that is moving with a constant velocity? Would our identical spring-mass and simple pendulum system behave in the SAME way? Now compare that to when both of us are moving with a constant velocity (need not be with the same velocity with respect to a 3rd reference frame). Would the spring-mass and simple pendulum system behave in the SAME way?

You will find that you CAN tell if you are accelerating without having to resort to looking at another reference frame. While there is nothing to differentiate between "not moving" and "moving with constant velocity" (look at Newton's First Law again), there IS something different between "moving with constant velocity" and "accelerating" that you can tell without having to use another frame as a reference.

Zz.

 

[cdm1a23]

There is nothing to differentiate between accelerating and feeling the effects of gravity though.

 

[ZapperZ]

There is nothing to differentiate between accelerating and feeling the effects of gravity though.

Which is why I stated in the BEGINNING that we will not deal with the equivalence pinciple. This issue is clearly dealing with "motion" devoid of any gravitational field. If you wish to open that can of worm, then let's start ANOTHER thread.

Zz.

 

[Pengwuino]

I also don't think accelerations are absolute but I am trying to determine exactly why I'm thinking that.

 

[jcsd]

Accelartion in kinematics is absolute (if we allow oursleves to ignore gravity by either assuming no gravity or neglible separation, this applies to Newtonian kinematics, special relativity and general relativty) as someone in acclerated frame of reference will feel a pseudo-force which is directly proportional to their acceleration.

Gravity and the equiavlence principle complicates the issue as we can sit in a frmae with no psedu-forces and watch and someone else who is also in a frame with no pseudo-forces yet observe a cooridnate acceleration in that person.

 

[lightgrav]

There are certainly BETTER phrases, to describe these motional quantities, than the op's acquaintance has used.

If you want to call motion equivalent to velocity (I prefer momentum), then uniform motion is equivalent to constant velocity. Constant velocity can be measured in the object's center-of-momentum frame, and all frames with constant momentum are *equivalent*, so uniform motion is *irrelevant*.

Certainly "absolute non-motion" has *NO physical significance*.
Same for "absolute uniform motion".

Non-uniform motion implies acceleration, and coordinate systems with different acceleration are *NOT equivalent*. But an object's acceleration is the same, as measured from any frame with uniform motion. In that sense, non-uniform motion is *physically significant*, so we base dynamics on it.
:
:
If you equate "motion" to "velocity", acceleration is not motion. Even a *change* in some quantity is not the same thing as the original quantity (deficit is not debt)! So if this guy wants to make the definition of "motion" unambiguous by equating it to velocity (which I'm guessing he has done), acceleration cannot be the same thing as velocity ... here, the UNITS are even different!

Is acceleration IMPORTANT in motion? Well, duh, yes! but that doesn't make it equivalent to motion. The trouble with this approach ("motion" = "v") is that it makes the word "motion" irrelevant, reducing the richness of the language. This is perhaps what MK was getting at, wanting "motion" to encompass all time-derivitives of location (but not location itself).

 

[lightgrav]

The op's acquaintance seems to be equating "motion" to "velocity". Some others want "motion" to include all higher time-derivitives of location as well, but not location itself. I think the first approach is more typical of physicists. When textbook-problem authors ask where a pendulum-bob will be, they want x(t) - when they want a(t), they ask for its acceleration - but when they ask you to describe its motion, you say "it goes ..." , meaning moves, ie, v(t).

of course this first definition, if used as an equivalence, reduces the richness of the language.

Better phrases:

Uniform motion is *irrelevant*, since it has *no physical significance*.

Non-uniform motion has *physical significance*, and acceleration of an object is *measured the same* from any frame having uniform motion.

Structures built exclusively from containment relations ("are","is") are not very interesting ... Physics, which builds Dynamics on properties ("has","does"), is much more fruitful than Kinematics.

Sorry for the "double-post", my browser claimed the first one was lost in transit ... .