Relative Motion of two cars are moving with constant velocities

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Discussion Overview

The discussion revolves around the concept of relative motion as it pertains to two cars moving with constant velocities and the observation of a ball thrown straight up and down within one of the cars. Participants explore how different frames of reference affect the observations of the same event, particularly in the context of relativity theories.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether a person in one car would observe the same behavior of the ball as someone in another car, emphasizing the need for an understanding of frames of reference.
  • Another participant explains that an outside observer will see the ball go straight up and down relative to the car, but will also observe the car moving forward during the ball's flight, leading to different perceived distances traveled by the light beam when compared to an observer inside the car.
  • It is noted that both observers, despite being in different frames of reference, will measure the speed of light as the same, suggesting complexities in the relationship between time and space.
  • A participant highlights a common misconception regarding relativity, clarifying that the effects are not merely due to light "lag" but involve the relative experience of space and time, including disagreements on simultaneity of events.
  • Another participant adds a humorous example of relativistic effects, suggesting that under certain conditions, one could theoretically store a longer pole in a shorter shed due to relativistic effects.
  • A participant seeks clarification on what is meant by "light lag," indicating a need for further explanation of the concept.
  • A later reply refines the discussion by stating that differences in observation can arise from the varying distances of observers from an event, while also noting that some relativistic effects require explanations involving changes to spacetime itself.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of relativistic effects and the implications of different frames of reference. The discussion remains unresolved with respect to the nuances of these concepts.

Contextual Notes

Some participants mention limitations in common understandings of relativity, particularly regarding the concept of light lag and its implications for observations in different frames of reference. There is an acknowledgment that even after accounting for light lag, observers may still disagree on the simultaneity of events.

uzair_ha91
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Before I start studying the relativity theories, I have to grasp this concept:
Suppose two cars are moving with constant velocities in any direction and a ball is thrown straight up and comes back straight down. This will happen in both cars.
But what if a person in one car observes the experiment done in the other car, will he observe the same?
ALSO please explain this in terms of frame of references.
Thanks
 
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Someone observing the ball from outside the car - i.e. in a different frame of refernce - will indeed see the ball go straight up and down **with respect to the car** (this is the key bit - they won't see the ball land in the back seat or something daft... they will see it land in the same place as an observer inside the car will see it)

Where it gets interesting is when you try it with a beam of light - so instead of a ball, you send a beam up from the bottom of the car to a mirror at the top of the car and back down again. So far so good...

Outside the car you will still see the beam go straight up and down **with respect to the car**, but with respect to you, the car has moved forward in the interviening time and so you see the beam of light travel further than an observer in the car would see it (inside the car: straight up and down... from outside the car: up, down and along a bit).

So... two observers in different frames of reference, see the same beam of light travel different distances. Not a problem until the observers measure the speed of light. Both of them will come up with the same speed.

Same speed, same light beam, different distance travelled. Something has gone weird with time. And space :-)
Welcome to the highly entertaining world of relativity!

Enjoy!
 
Last edited:
Also, if you are about to start studying relativity, please keep in mind something that I've found isn't made very clear in relativity textbooks and is a very common misconception among new students:

The effects of relativity are not simply a result of light "lag". Although the finite speed of light does result in interesting observed effects, the real study of relativity has to do space and time itself being experienced in a relative way. In fact, even after having corrected measurements of observations for light lag, two observers can still disagree on the simultaneity of two events.
 
Good point! The effects of lag are interesting in their own right, but the more you get into the subject, the more fascinating - not to mention mind bending - it becomes.

Have you ever wanted to store a 10 metre pole in a 5 meter shed? Well travel fast enough and you can do. For a fraction of a second anyway:-)
 
What is light "lag"?
 
Me being lazy... I meant the difference in observation caused by two observers being different distance from an event :-)

Many reletavistic effects can be explained by this, but many others have to be explained through changes to spacetime.
 

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