SR - car with headlights - new twist

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In summary, the conversation explores the concept of velocity composition and relativistic velocity composition when two objects are traveling at speeds of 1/2c and 3/4c in opposite directions. The discussion touches on the possibility of the separation rate between the two objects being greater than the speed of light, but it is clarified that this is not the actual speed of any object. The conversation also mentions the importance of considering different frames of reference when determining speeds and distances.
  • #1
elegysix
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What happens if two objects travel at speed 1/2c in opposite directions?

Like suppose two ships left the ISS in opposite directions, each going speed 1/2c.

Wouldn't either of the ships determine that the other is moving away at the speed of light?
Could they see each other?

What if they both went 3/4c in opposite directions? would they conclude the speed between them was greater than c?
 
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  • #3
I understand that for two frames, but there's three frames here - one for each ship and one 'stationary' that they move away from.

We can determine that an object is moving at 1/2c or 3/4c, relative to us, right?

which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
 
  • #4
elegysix said:
which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
Yes, the separation rate as measured by that third frame can be greater than c. But that's not the speed of any object.
 
  • #5
elegysix said:
which means if a different object is moving at 1/2c or 3/4c in the opposite direction, we would conclude that the distance between them is increasing by c or 3/2c, respectively, right?
Just because we would conclude that does not mean the people on the spaceships would. They would see the other spaceship as moving at 4/5 the speed of light in your first example (we see two spaceships each moving at 1/2 c in the opposite direction), and at 24/25 c in your second example (we see two spaceships each moving at 3/4 c in the opposite direction).

Read the link I provided.
 
  • #6
By the way, this is not a "new twist". It's over a century old.

It would be good to read the suggested links.
 

1. What is the SR in "SR - car with headlights - new twist"?

The SR stands for "safety rating", which is a measure of how safe a car is to drive.

2. What does "car with headlights" mean?

This phrase refers to a car that has functioning headlights, which are essential for safe driving at night or in low visibility conditions.

3. What is the "new twist" in this concept?

The "new twist" refers to an innovative feature or design element that sets this car apart from others in terms of safety and functionality.

4. How is the SR of a car with headlights determined?

The SR of a car with headlights is determined through rigorous testing and evaluation by organizations such as the National Highway Traffic Safety Administration (NHTSA) and the Insurance Institute for Highway Safety (IIHS). They assess factors such as crashworthiness, crash avoidance, and overall vehicle safety features.

5. Are there any limitations to the SR rating for a car with headlights?

While the SR rating is a useful tool for comparing the safety of different vehicles, it is important to note that it is not the only factor to consider when choosing a car. Other factors such as driver behavior, road conditions, and maintenance also play a significant role in overall vehicle safety.

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