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B Help a HS teacher understand uniform motion?

  1. Oct 18, 2017 #1
    Greetings all,

    I apologize for joining your forum just to inquire about something that should be relatively simple, but you seem like a group of physics geeks able to articulate coherent arguments, and I need someone/s to help me see a different perspective on how to make an argument. For the record I'll state that I'm not exactly uneducated in this realm myself, but I'm far enough from day-to-day classical mechanics that I can make some pretty stupid mistakes, and am no-longer fluidly conversant in all of the relevant theorems/etc.

    I'm faced with an instructor who believes that an object at rest, is _not_ in uniform motion (bizarrely this seems an awfully common belief online!). "An object in uniform motion has a constant velocity - an object at rest has no velocity at all!"...

    I have tried the route of explaining that constant zero velocity is a constant velocity, I've tried the route of pointing out that any acceleration would cause V to vary from constant zero. These arguments are not leading to understanding, apparently because there's an embedded belief that zero velocity ("no motion") is somehow a privileged third state that is different from both "constant velocity" and "changing velocity" (a third category different from "uniform motion" and "non-uniform motion").

    If only Newton hadn't phrased the first law in terms of "state of rest or uniform motion in a straight line"... At least online, everyone seems fixated on the "uniform motion in a straight line" as somehow being a completely different thing than "state of rest"...

    It seems that there /must/ be a theorem out there, or a proof, that says something like "an object in a state of uniform motion in one inertial reference frame, is in uniform motion in every inertial reference frame", or "an object moving at a constant velocity in one inertial reference frame, is moving at a constant velocity in every inertial reference frame" (or the similar for delta-V).

    Such a theorem would allow proof that (constant) zero velocity is just a specific case of constant velocity, and isn't somehow privileged. Without that, I'm not seeing any obvious way to combat the thinking that a thing with zero velocity somehow has an undefined "no velocity at all" velocity.

    Any suggestions?
  2. jcsd
  3. Oct 18, 2017 #2


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    I agree with you. A constant velocity means that the velocity does not change -- and this includes a velocity that is zero.

    The person you mention would probably not argue that there's a difference between having "no money" and the bank versus having a balance of $0.
  4. Oct 18, 2017 #3


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    Worth noting that velocity isn't a property of an object but a relationship between an object and some other reference. When I am standing on a platform I would regard myself as stationary. But an observer on a train passing by would regard me as moving at a non-zero speed. So even "not moving" is moving by someone else's description.

    It's inconsistent to regard "stationary" as somehow special. Just ask: stationary with respect to what?
  5. Oct 18, 2017 #4


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    You can try getting around this or rendering it moot by asking if they do indeed believe rest is a priveledged third state and if so, what you can do with it. The two possible answers are:
    1. "No, I think they are mathematically equivalent." In which case equal means equal.
    2. "Yes." In which case: what does that do for you? If they cant give you an example (and they shouldnt; there are none). Then #1 must be correct.
  6. Oct 18, 2017 #5


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    You can tell them that in set theory, there is a difference between an empty set { } versus a set having zeroes {0}.

    Then you can tell them that a person with no bank account is different than a person with a bank account but having $0 in it.

    However, in the end, you need to figure out whether this is simply a matter of semantics, or if this is really a fight worth fighting over. Will the difference result in a meaningful deviation in the understanding? You need to pick your battles, and is this a battle worth fighting over?

  7. Oct 18, 2017 #6
    Unfortunately the "this third privileged state doesn't give you anything" argument is, I believe lost on this individual. It doesn't overrule the "magical thinking" that says "I don't care, it's different, therefore it's not the same".

    I'm dissatisfied that the only response I can currently find for that is the equally magical "no, just because you've given it a special name doesn't make it different". There should be a simple and definitive way to prove this, rather than just relying on an assertion.
  8. Oct 18, 2017 #7


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    Just an observation, but the only difference between constant velocity and accelerated motion is whether the acceleration is zero or not...
  9. Oct 18, 2017 #8
    I will admit, it's a fairly small point, but, when faced with an educator who is mis-educating students due to a misunderstanding of basic physics, I think it's worth some amount of effort to assist them in developing a better understanding.

    That being accepted as a potential excuse to let something like this slide, there's a possible counter-argument in that this is /highly/ fundamental, and if they're going to teach students that an object at rest does not have a constant velocity, this could propagate into some truly bizarre descriptions of more complex phenomena. For example, it would appear to force one to accept a definition where, for an object with v==0, dv/dt != acceleration. I won't claim that it's impossible to develop a set of consistent definitions that enables "no velocity at all is something uniquely different than constant zero velocity" to be meaningful, but, I believe that any such attempt is likely to get ugly, really quickly, and probably is not a good playground in which to be educating high-school level physics students.
  10. Oct 18, 2017 #9
    Heh - I tried that angle too...
  11. Oct 18, 2017 #10


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    No, the two sides are not equally vacuous: what makes something different is not the name it is the the features. A name is just a name. The fact that there is no relevant reason to consider 0 different means it isn't. Different has to be different otherwise it is the same!

    There may be another way of proving that 0 is the same, though, and that is via the principle of relativity: there are an infinite number of inertial reference frames available, and an object at rest with respect to one could be moving at any speed with respect to others. In other words, you can change 0 to 10 or 42 or any other number just by changing reference frames. In other other words: zero isn't unique because it isn't even always zero!

    If you really wanted to, you could transform this teacher's a problems to another reference frame before solving them. That way, if the teacher specifies an object is moving, you could say "not it isn't" or vice versa and still get the correct answer to the problem. Of course, that just leads to the argument: who gets to decide if an object is stationary or not?
  12. Oct 18, 2017 #11


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    But like I said, if you think this is a fight worth fighting for, then go for it. I don't have the full understanding of your situation, and I wasn't trying to discourage you from countering this.

    You could try another approach. Within both the Galilean and Lorentz transformation, an object at rest (with velocity=0) is equivalent to an object with a constant velocity in another inertial reference frame. This, after all, is the fundamental aspect of both Newtonian and Relativistic mechanics. So already here, you can show that v=0 is simply a special case of a constant velocity.

    If this doesn't get through, then this is no longer a matter of arguing the physics. It is now a matter of stubbornness and other psychological issues that is WAY beyond the scope of physics. Irrationality cannot be countered with rational arguments.

  13. Oct 18, 2017 #12
    I agree with Ibix in post #3. The motion of an observer moving at a constant velocity relative to the "stationary object" can't possibly affect the behavior of the object. Yet, to this observer, the object is moving at a constant velocity. So, an object moving at a constant velocity is entirely equivalent to a "stationary object."
  14. Oct 18, 2017 #13

    Mister T

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    An object that's at rest is indeed an object that's not in motion. Newton's 1st Law tells us that there's no way to distinguish between a state of rest and a state of uniform motion. But there is a way to distinguish between an object that's at rest and an object that's in motion, you just measure its velocity and see if the value you get is zero or not.

    An object at rest has a velocity of zero. To say that this is somehow different than having no velocity seems to be semantics.

    What has this instructor said or done to indicate that a case of zero velocity is somehow privileged? I don't agree that his stance necessarily implies this.

    Edit: You might try asking him about a ball that's been tossed directly upward. What happens to its acceleration as it passes through its highest height. If he thinks it changes sign then you're probably right in thinking that he somehow assigns a privilege to a state of rest.
  15. Oct 18, 2017 #14
    I attended the following lecture by Ben Redford who is an editor of the Skeptical Inquirer.


    The point of the lecture was to lament bad science in journalism, preach against pseudoscience, and promote science literacy. At one point, trying to give an example of people supposedly thinking in an unscientific way, he said

    "People talking about the Sun rising and setting when of course the Sun stands still. It's the Earth that moves".

    He said this in a matter-of-fact way, as of it was self-evident, when, in reality, he was making the same error that the high school teacher the OP was referring to. You can choose your frame of reference to be whatever you want. If you choose the Sun as your frame of reference, the Earth orbits the Sun. If you choose the Earth as your frame of reference, the Sun orbits the Earth. The reason that we usually choose to use the Sun as our frame of reference is because the mass of the Sun is so much larger than the Earth that the center of mass of the Earth-Sun system is very close to the center of the Sun. However, this is just a matter of convenience. You are "allowed" to use the Earth as your frame of reference.

    The problem is that most non-scientists assume that there exists a preferred frame of reference. The reason is because they are conditioned by their daily life. Most people spend their lives walking around on the surface of the Earth, where the ground under your feet is an obvious frame of reference that you can always use. Most people think of the ground as a "place" so they end up thinking a "place" could be frame of reference, and in space, they still assume that a "place", in that case, empty space, could be used as a frame of reference.

    This explains the reason for a high school teacher's error. If the average person saw a ball rolling across the ground, and another one just sitting on the ground, they would think that they are obviously not the same, because one is moving with respect to the ground and the other isn't, which they would describe by saying "one is moving and the other isn't".

    During this talk by Ben Redford, he was also talking about that unfortunate flap in the media where it was erroneously reported that neutrinos went faster than light, not realizing that this would allow you to send a message backwards in time. It was obvious that Ben Redford initially thought that the neutrinos were going faster than light, until it was explained as an experimental error, but worse than that is that Ben Redford was under the mistaken impression that physicists also initially thought that the neutrinos were traveling faster than light until it was determined to be an experimental error. The truth is that there was not a single physicist who thought that the neutrinos were going faster than light because that would allow faster light communication, which we know is impossible, because it would allow you to send a message backwards in time, as explained here.


    I know there are things which are, in some sense, "faster than light", like "spooky action at a distance" in quantum mechanics, or the expansion of the Universe, but those things do not allow faster than light communication. However, if neutrinos went faster than light, then they could be used for faster than light communication, so we know that is impossible.

    The people doing the experiment got a spurious meaningless unphysical result so they knew there a systematic experimental error. They initially couldn't find the source of the error, so asked others to help them find the source of the error, which they promptly did. Yet, all the journalists just assumed that the neutrinos were literally going faster than light, not realizing that this would allow you to send a message backwards in time, and Ben Redford initially believed it, and he assumed that physicists also believed it, when in reality, there was not a single physicist who believed that. In Ben Redford's lecture, he brought this up to try to make the point that physicists are willing to throw out their cherished ideas, and what they used to consider "facts", contrary to crackpot's claims that the only reason their crank theories are not accepted is because physicists are unwilling to throw out their cherished ideas or reconsider their assumptions. He brought this up as a way of defending physicists against crackpots or other people supporting pseudoscience who accuse physicists of being unwilling to reconsider their assumptions. It is unfortunate that Ben Redford chose that example because the fact that faster than light communication is impossible is an example of a known fact, that is proven to be true, and will never be discovered to be untrue. I also think is is counterproductive to tell followers of pseudoscience that scientists are not sure of anything, or could change their mind about anything.
    Last edited: Oct 18, 2017
  16. Oct 18, 2017 #15
    High school student here. It sounds like your instructor doesn't understand the concept of a frame of reference. I'd approach the situation Socratically: e.g. ask them "What's your velocity right now?" If they say zero, then point out that they're on a planet which is orbiting the sun and are, in fact, moving very quickly relative to the sun. OK, then maybe the sun has zero velocity. Repeat argument ad infinitum.

    That was how the concept of a reference frame was explained to me, and it has stuck with me.
  17. Oct 18, 2017 #16


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    Well, maybe they need to understand the modern motivation for treating them equivalently. We are now several hundred years after Newton, so we have refined things a bit since his days.

    One of the most important concepts in modern physics is symmetry. This was not fully appreciated in Newton's day, but currently all of the deepest laws of physics are formulated in terms of symmetry.

    The most basic symmetries are space and time translation, spatial rotations, and boosts. The laws of physics work the same in Chicago as they do in Paris, and they work the same tomorrow as yesterday. The laws of physics work the same in one direction as in any other direction. And they work the same at any speed.

    Newton believed that there was an undetectable universal state of rest. So he phrased his law that way. But in modern physics the fact that there is no universal state of rest is important because it is a symmetry and symmetries have deep consequences. This is something that Newton couldn't have known in his time.
  18. Oct 18, 2017 #17
    To be entirely fair, it's only semantics because we understand that a velocity can be pinned on anything for which a position can be defined and is always differentiable. This instructor is, I believe, thinking of velocity somewhat like what ZapperZ mentioned - the difference between having a bank account with zero balance, and not having a bank account. I believe she thinks that the property of velocity is _undefined_ for a stationary object.

    Again, "An object in uniform motion has a constant velocity - an object at rest has no velocity at all!" This is her reasoning for why a zero velocity is not a constant velocity. There's no way to read this other than a belief that at rest, the velocity is somehow _not_ a constant zero.
  19. Oct 18, 2017 #18


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    Ask her to draw the v versus t graph of an object tossed vertically upwards. Does she think the the graph is undefined when it crosses the horizontal axis?

    Like I said, at some point, there’s no reasoning of an irrational person.

  20. Oct 19, 2017 #19

    Mister T

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    I think then that graphs might convince her.

    Draw several horizontal lines on a xy-plane, with the lines being parallel to the x-axis. Ask her what is the slope of each line. If one of those horizontal lines happens to lie on the x-axis it still has a slope of zero.

    Draw a set of velocity-time graphs that are all horizontal lines, with the lines being parallel to the t-axis.. She will hopefully agree that they all represent the motion of an object with zero acceleration. If one of those horizontal lines happens to lie on the t-axis it still represents the motion of an object with zero acceleration because it still has a slope of zero. Surely she will have to agree that an object remaining at rest has an acceleration of zero!

    It may be, though, that she is dug in and will not be rational, as @ZapperZ has pointed out, in which case you will have to remind yourself that there are some people with whom you simply cannot argue.
  21. Oct 19, 2017 #20
    Here's a thought experiment for your teacher. (S)he's on a train riding on a long straight track. The ride is perfectly smooth and quiet. The windows of the car are covered so no one can see out. (S)he has fallen asleep, and when (s)he wakes up, she would like to know if the train is still moving or has come to a stop and is "at rest." Can (s)he think of an experiment (s)he can do inside the compartment to determine this (without uncovering the windows)? If not, then she has proven that, physically, there is no difference between moving at a constant speed and being "at rest."
  22. Oct 21, 2017 #21


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    “… have the ship proceed with any speed you like, so long as the motion is uniform and not fluctuating this way and that. You will discover not the least change in all the effects named, nor could you tell from any of them whether the ship was moving or standing still.”

    – Galileo, Dialogue Concerning the Two Chief World Systems
  23. Oct 23, 2017 #22
    If only Galileo had gone far enough to say that not only can you not tell, but there is actually no difference, other than point of view, between these states.

    As much as I think it's important to attempt to help this instructor properly understand the subject she's teaching, it doesn't seem possible to get her there by providing examples from which the equivalence can be reasoned.

    She's not a physicist, and doesn't have the experience to understand proofs in this world. As a result, there's an equivalence in her mind between "rational" decisions she has made (e.g., "uniform motion has the word 'motion' in it, motion means moving, therefore unless something is moving, it's not in uniform motion"), and rational decisions that others have made (e.g., "if there is no experiment that you can perform to determine whether something is in uniform motion, or at rest, then these states are identical), and since there's an equivalence between the rationality of these, she feels free to choose her rationality over that of others.

    The only thing that I think is going to sway her at this point is someone with recognized authority "telling her" which rationality is more correct. I'm desperately avoiding trying to cast myself as an authority in the discussion with her (and geez, is this difficult - graduate students you can just lead around and let them trip over the walls that their misconceptions have set up, until they get their heads straight. I'm finding it's a fairly large luxury to usually be educating people who don't have the option of checking out of the conversation when they decide that they disagree...), but this means that I need to find someone credible who has made this argument in a fashion that's clear for the non-scientist.
  24. Oct 23, 2017 #23
    If she is using semantics (like this) to provide her rationale (in place of solid scientific reasoning), then she is in the same category as my mother-in-law who espoused the concept that a "blanket of snow" in the winter keeps the ground warm so that the crocuses can come up early. After all, blankets keep things warm.
  25. Oct 23, 2017 #24

    Mister T

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    It's only with the benefit of hindsight that we can say this. Physicists in the latter half of the 1800's were convinced that there was a way to tell, and it was only through the use of some very sophisticated measuring techniques that they came to believe otherwise. And it took a revolution in scientific thought to understand it.

    I don't take your point. I think she's right about this. I think your objection is pedantic.

    But there is an experiment you can do. Point a radar gun at it! Of course, there's no way to tell if your radar gun is moving, but there is a way to tell if something is moving relative to the gun. Again, semantics.

    As I said in my earlier posts, it depends on what consequences she draws from these claims. That is the ultimate test of whether or not her stance is valid. Did you try having the discussion of the slope of the velocity-time graphs I described in Post #19?
  26. Oct 23, 2017 #25


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    Here is another suggestion. Present your teacher with the following hypothetical situation.

    Suppose the two of us are driving on a straight highway at 60 mph. If I asked you "Am I in uniform motion, what would you answer?"

    I see three answers, to which there are three responses from you depending on what the teacher says.
    1. Teacher says "yes".
    Then you say, "Nonsense. Do you see me move? Wouldn't you say that my velocity is zero? By your reasoning I should not be uniform motion."
    2. Teacher says "no".
    Then you say, "Nonsense. Look at the speedometer. It reads 60 mph. If that's not uniform motion, what is?"
    3. Teacher is savvy enough to say, "With respect to what?"
    Then you say, "Why does that matter? Can I simultaneously be and not be in uniform motion?".

    If, after this, your teacher remains unconvinced, give up.
    Last edited: Oct 23, 2017
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