Coming to terms with the velocity addition formula

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SUMMARY

The discussion centers on the application of the velocity addition formula in special relativity, specifically regarding two rocket experiments conducted from a laboratory frame of reference. In the first experiment, both rockets approach each other at 0.75c, while in the second, one rocket is stationary at A and the other moves towards it at 0.96c. Despite the differing configurations, the calculated relative speed of the rockets remains consistent at 0.96c, leading to the conclusion that both experiments yield the same damage results when accounting for relativistic effects such as time and length contraction. The key takeaway is that understanding these relativistic effects resolves the perceived discrepancies in the laboratory frame.

PREREQUISITES
  • Understanding of special relativity principles
  • Familiarity with the velocity addition formula
  • Knowledge of time dilation and length contraction concepts
  • Basic grasp of reference frames in physics
NEXT STEPS
  • Study the implications of the Lorentz transformations in special relativity
  • Explore examples of time dilation and length contraction in practical scenarios
  • Learn about simultaneity in different reference frames
  • Investigate the effects of relativistic speeds on physical interactions
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Physics students, educators, and anyone interested in understanding the complexities of special relativity and its implications on high-speed interactions.

adimare
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I have two rockets, one in point A and another in point B, I'm going to crash one against the other, twice, and I will be observing safely from my laboratory at C, which is right in the middle between A and B.

The first time the experiment takes place, I see both rockets rushing forward to one another at a constant speed of 0.75c, they meet and crash right in the middle point between A and B.

The second time, the rocket at A is at rest relative to me, and I see the other rocket moving towards it from B at a speed of 0.96c, they meet and crash at A.

Using the velocity addition formula to calculate what the speed of B relative to A was in both experiments, we'd get precisely 0.96c, which I believe means that for the rocket at A, both experiments should produce the exact same results (let's define results as the amount of damage in the ships), which means that both experiments should produce the exact same results in every reference frame.

I'm just having trouble digesting this from the reference frame of the laboratory, where the configuration of each of the two experiments seems somewhat different as to produce the exact same results. In the first experiment, considering the laboratory frame exclusively, after one meter of time has passed, the rockets are 1.5 meters closer than they were before, whereas in the second experiment, after one meter of time has gone by, they're only 0.96 meters closer to each other.

I think I'm just trying to find a way to visualize these results without having to analyze the experiments from the reference frame of either of the ships.
 
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adimare said:
I'm just having trouble digesting this from the reference frame of the laboratory, where the configuration of each of the two experiments seems somewhat different as to produce the exact same results. In the first experiment, considering the laboratory frame exclusively, after one meter of time has passed, the rockets are 1.5 meters closer than they were before, whereas in the second experiment, after one meter of time has gone by, they're only 0.96 meters closer to each other.
Have you accounted for
1] time contraction when measuring simultaniety of events?
2] length contraction of the rockets?
When you do, you should find the discrepancy evaporates.
 

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