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Acceleration and uniform motion are not making any sense

  1. Mar 30, 2013 #1
    It seems like there is no such thing as uniform motion.

    Example: I just took my pup to the beach. She crashed into many waves. We're back and she's asleep (at rest). However, her position in spacetime is changing because she's attached to the earth, which is moving, and the earth is constantly changind its direction as it rotates.

    A train moving at a constant 80mph on a "straight track" is changing its direction (through spacetime) even if it doesn't realize it.

    Thanks for any help on this.

  2. jcsd
  3. Mar 30, 2013 #2


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    True, but if you insist on perfect rigorousness when discussing any aspect of physics, you'll never make any progress in learning nor will you make a good teacher.
  4. Mar 30, 2013 #3


    Staff: Mentor

    This is not nearly as complicated as you seem to think. Attach an accelerometer, if it reads 0 then the object is in uniform motion in the only frame-invariant sense of the term.
  5. Mar 30, 2013 #4


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    In this context, uniform motion means unaccelerated motion, and you are right that neither the train nor the pup is undergoing uniform motion; both are experiencing acceleration because of the rotation of the earth about its axis. However this acceleration is small enough (it takes seriously sensitive instruments to even detect it) that we can generally ignore it and use uniform motion as a very very good approximation in both cases.

    It is important to remember that acceleration (not counting gravitational acceleration here - that's a different kettle of fish, requires a more sophisticated definition of "uniform motion") is something that we can directly observe without reference to any external body so is not relative the way speed is. Thus, we can measure the acceleration, decide if it large enough to matter or if we can safely approximate it as zero and use the uniform motion math. So there's never any ambiguity about the physics.
  6. Mar 30, 2013 #5


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    I think you are beyond help. Uniform motion has been explained to you by many poster and you don't seem to have grasped any of it.
  7. Mar 30, 2013 #6
    One cannot draw a "perfect circle." Magnification will show the line as jagged and discontinuous. Uniform motion is, likewise, an abstraction which is not encountered perfectly in real life.
  8. Mar 30, 2013 #7


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    The best approximation to uniform motion I can think of would be something moving in empty space far from any source of gravity. The pioneer probe would be a good example.
  9. Mar 30, 2013 #8
    Wait, so in the minds of some there is a distinction between uniform and inertial motion or do I misunderstand something here?
  10. Mar 30, 2013 #9
    Are you guys basically saying that I shouldn't read into this too much? I think the one guy said it perfectly--there is no perfect circle, and magnification would prove this. I'm not trying to be difficult. I'm just trying to understand this better.
  11. Mar 30, 2013 #10


    Staff: Mentor

    What exactly do you think you misunderstand still? Do you understand the relationship between accelerometers and uniform motion? Do you understand the idea of ideal approximations and errors? What is left?
  12. Mar 30, 2013 #11


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    Yes, I think so. Uniform motion is a very useful concept when learning the laws of motion (example). I'd probably make a similar example of uniform motion as pervect did above.
  13. Mar 30, 2013 #12
    Here is a very simple way of thinking about this.
    I am sitting and I roll a ball away from me....

    I have no acceleration (because I am sat still)... but the ball does... relative to me.

    Now consider things from the point of view of the ball.... which might decide that it is not it which is moving but everything around it...

    Uniform motion is possible if the only two things that exist are you and the ball.... and you remove awkwardness like friction, gravity, coefficients of restitution and compression of the ball, the fact the ball spins... if you make your model simple enough then the equations will give you the answers in the mechanics textbooks.

    If the question is "does uniform motion apply absolutely to the world I live in" then I would say it doesnt.

    Have I understood the sense of your question correctly?
  14. Mar 30, 2013 #13
    One of the most important findings of Einstein's theory is that you do accelerate by sitting still. Before Einstein nobody seem to have realized that.
  15. Mar 30, 2013 #14
    August Crawl:

    Yes, that is exactly what I'm saying. Thank you for explaining it in more efficient language. You understand what I'm trying to say (which may be pointless and useless). I've just been struggling with this working definition of uniform motion, and how it's different from acceleration.
  16. Mar 30, 2013 #15

    No, apparently I do not understand the idea of ideal approximations and errors? Please explain this a little more.
  17. Mar 30, 2013 #16


    Staff: Mentor

    Whenever you are doing any physics problem of any kind you always make some simplifying assumptions that you can neglect a whole mountain of factors. Instead of completely representing all possible minute complexities of the system you ignore unimportant details and analyze a simplified and idealized model of the system.

    For instance, if you are analyzing the acceleration of a car you might use the manufacturer's data for the mass of the car and ignore the fact that there is a spot of dirt on the rear bumper that adds some mass not accounted for by the manufacturer. So, you are not analyzing THAT car, but an idealized model of that car.

    Those approximations and idealizations can produce errors, but there are techniques for estimating them. Then, you can determine how exact you need your analysis to be, and from that you can decide if your approximations are OK, or if you need to make your model more complicated.
  18. Mar 30, 2013 #17
    Dale Spam: that is a very good way of explaining what I wasn't understanding. Thank you for making it so simple. However, I still think the system is a bit contrived. That spec of dirt on the car is there for a reason and it must be accounted for. I know that sounds trivial, but I believe it. Either way, thank you for taking the time to make sense out of what I was missing.
  19. Mar 30, 2013 #18

    Ignore my last post. I just re-read this: "Whenever you are doing any physics problem of any kind you always make some simplifying assumptions that you can neglect a whole mountain of factors. Instead of completely representing all possible minute complexities of the system you ignore unimportant details and analyze a simplified and idealized model of the system." and it made perfect sense.
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