Is Kinetic engergy relative to an observer?

Click For Summary

Discussion Overview

The discussion centers on the relativity of kinetic energy and motion, exploring whether kinetic energy is dependent on the observer's frame of reference and the implications of this for concepts like speed and energy conservation. Participants also delve into hypothetical scenarios involving observers moving at relativistic speeds and the nature of motion in isolated conditions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that kinetic energy is only relevant when measured from an inertial frame of reference, questioning if it varies for different observers and whether this affects energy conservation or relativity principles.
  • Others argue that speed is inherently relative, suggesting that in a hypothetical scenario of traveling at the speed of light in an empty universe, speed could be perceived as an illusion.
  • A participant questions whether heat, being related to particle motion, could also be considered relative, proposing a scenario where a sudden change in Earth's speed might affect global temperatures.
  • Another participant raises a question about two objects moving away from each other at half the speed of light, inquiring if this would imply one is moving at the speed of light relative to the other and whether they could see each other.
  • One response clarifies that the two observers would not see each other moving at the speed of light, but rather at a calculated relative speed of 0.8c, referencing the relativistic velocity addition formula.
  • A participant notes that while kinetic energy is frame-dependent, the change in kinetic energy during interactions, such as collisions, remains consistent across different inertial frames, although they express some uncertainty about this claim.
  • Another participant agrees that motion in a hypothetical isolated universe does not violate relativity but emphasizes that such scenarios are highly theoretical.

Areas of Agreement / Disagreement

Participants express a range of views on the relativity of kinetic energy and motion, with no consensus reached on the implications of these concepts. Some agree on the relativity of motion, while others challenge or refine these ideas without settling the debate.

Contextual Notes

Limitations include the highly hypothetical nature of some scenarios discussed, such as traveling alone in a universe, and the dependence on specific definitions of kinetic energy and motion. There are unresolved mathematical steps regarding the calculations of kinetic energy changes across frames.

mceddy2001
Messages
21
Reaction score
0
Since all motion is relative to an inertial frame of reference, is a bodies kenetic energy only relevant when the observer is the intertial frame of reference? (i.e. would the kinetic energy be different for another observer and if so does this violate any of the principles of energy conservation or relativity?

This is not for a homework question this is just out of interest.

Me and my physics teacher think this is an interesting topic. Because motion is relative, is speed simply just an illusion created by an observer? If you a traveling at the speed of light in an empty universe (you would be stationary using urself as an inertial frame of reference) so would it be possible to speed up even faster. Since there would be nothing to say your speed is relative to, then speed really is an illusion.

Please answer any of the questions asked or contiue the converstation by adding ideas. Any arguments or criticisms to my statements above are welcome.
 
Physics news on Phys.org


Originally posted by Ambitwistor

It would violate the laws of relativity for an observer to travel at the speed of light. (Massless objects such as photons aren't really "observers"; their worldlines are not timelike.) The laws of relativity also say that any object that travels at the speed of light is massless, and massless objects can only travel at the speed of light.

But it would also violate relativity for an object that is alone in a universe of its own to be traveling at all. If, as you have mentioned, speed is relative to an observer, and you are the only observer, then no matter what, you're stationary (in any inertial frame).
 
since heat is to do with the motion of a particle (vibrations) could heat be relative, would the speed of the Earth suddenly changing cause a fluctuation of tempereatures all over the world, or have i just made a question up on complete lack of back ground knowledge?
 
What happens if you have two objects (or observers, or people) moving away from each other at half the speed of light? Does this mean that one object is moving at the speed of light relative to the other? Would either observer be able to see the other?
 
Originally posted by Rocksicle
What happens if you have two objects (or observers, or people) moving away from each other at half the speed of light? Does this mean that one object is moving at the speed of light relative to the other? Would either observer be able to see the other?

No they would not see each other moving at the speed of light, they'd actually see each other moving at 0.8c (v = (u + v')/(1 + uv'/c2).
 
Originally posted by mceddy2001
Since all motion is relative to an inertial frame of reference, is a bodies kenetic energy only relevant when the observer is the intertial frame of reference?
While kinetic energy is frame dependent (practically by its very definition), a change in kinetic energy is consistent from one inertial frame to another. Do a calculation for a colission between two massive bodies that is not perfectly elastic. You will find that the loss of kinetic energy is the same in any frame that you perform the calculation (I'm pretty sure of this, but not absolutely positive. I have done the calculation myself for a perfectly inelastic collision, because I had the same concern about kinetic energy).
 


Originally posted by LURCH
But it would also violate relativity for an object that is alone in a universe of its own to be traveling at all. If, as you have mentioned, speed is relative to an observer, and you are the only observer, then no matter what, you're stationary (in any inertial frame).
I don't think it violates relativity, but I do agree that motion doesn't make sense in this HIGHLY HYPOTHETICAL situation.
 

Similar threads

Undergrad Is calling fictitious forces "not real" just about terminology?
  • · Replies 114 ·
4
Replies
114
Views
8K
High School Conservation of KE in a moving frame
  • · Replies 3 ·
Replies
3
Views
1K
Undergrad Kinetic energies in two reference frames
  • · Replies 13 ·
Replies
13
Views
2K
Undergrad Same old question, I still don't get it: E=1/2mv^2 - more E with V?
  • · Replies 41 ·
2
Replies
41
Views
5K
Undergrad I am confused about the meaning, and value, of kinetic energy
  • · Replies 7 ·
Replies
7
Views
2K
High School How does the Gallilean Invariance Puzzle challenge our understanding of motion?
  • · Replies 34 ·
2
Replies
34
Views
3K
Graduate Is Mach's Principle truly a paradox in rotational frames?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Energy and reference frames
  • · Replies 87 ·
3
Replies
87
Views
6K
Undergrad Understanding Einstein's second postulate of special relativity
  • · Replies 57 ·
2
Replies
57
Views
8K
Undergrad Which inertial frame is used when resolving toward the centre
  • · Replies 15 ·
Replies
15
Views
2K