Acceleration and uniform motion are not making any sense

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Discussion Overview

The discussion revolves around the concept of uniform motion and its relationship to acceleration, particularly in the context of real-world examples such as a train moving at a constant speed and a dog at rest on a rotating Earth. Participants explore the implications of motion in spacetime and the challenges of defining uniform motion in practical scenarios.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Exploratory

Main Points Raised

  • Some participants argue that uniform motion does not exist in a practical sense due to the constant motion of the Earth and the effects of acceleration.
  • Others propose that uniform motion can be defined in a frame-invariant sense, suggesting that an accelerometer reading of zero indicates uniform motion.
  • A participant mentions that while both the train and the dog are technically experiencing acceleration due to Earth's rotation, this acceleration is often negligible and can be ignored for practical purposes.
  • Some contributions highlight that uniform motion is an abstraction and not perfectly realized in real life, with examples like the Pioneer probe being cited as a close approximation.
  • There is a discussion about the distinction between uniform motion and inertial motion, with some participants seeking clarification on this point.
  • One participant emphasizes the importance of ideal approximations in physics, noting that simplifying assumptions are often necessary to analyze systems effectively.
  • Another participant expresses concern that even small details, such as a spot of dirt on a car, should be accounted for in models, indicating a tension between idealization and real-world complexity.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the nature of uniform motion, with multiple competing views remaining. Some agree on the utility of uniform motion as a concept, while others question its applicability in real-world scenarios.

Contextual Notes

Limitations in the discussion include the dependence on definitions of uniform motion and acceleration, as well as the unresolved nature of ideal approximations and their potential errors in practical applications.

Seminole Boy
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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.

.
 
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Seminole Boy said:
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.

.
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.
 
Seminole Boy said:
It seems like there is no such thing as uniform motion.
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.
 
Seminole Boy said:
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.

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.
 
Seminole Boy said:
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.

.
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.
 
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.
 
1977ub said:
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.

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.
 
Wait, so in the minds of some there is a distinction between uniform and inertial motion or do I misunderstand something here?
 
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.
 
  • #10
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?
 
  • #11
Seminole Boy said:
Are you guys basically saying that I shouldn't read into this too much?
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.
 
  • #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?
 
  • #13
AugustCrawl said:
I have no acceleration (because I am sat still)... but the ball does... relative to me.
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.
 
  • #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.
 
  • #15
DaleSpam:

No, apparently I do not understand the idea of ideal approximations and errors? Please explain this a little more.
 
  • #16
Seminole Boy said:
No, apparently I do not understand the idea of ideal approximations and errors? Please explain this a little more.
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.
 
  • #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.
 
  • #18
Dale:

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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