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Speed of two spaceships relative to each other observed by an earth observer

  1. Dec 15, 2011 #1
    Edit. I think the topic is misinformative and I dont know how to change it.


    I have to translate this into english so bear with me.

    Two spaceships are approaching earth from two directions along the x-axis. Earth observer(EO) calculates their speeds to be 0,8c each. Calculate at what speed EO observes them to be approaching each other.

    Is c the maximum speed in situations like this or can you just add the speed together -> 1,6c?
  2. jcsd
  3. Dec 15, 2011 #2
    Is this a homework question?
  4. Dec 15, 2011 #3
    Nope, Im just going through the questions from past exams.
  5. Dec 15, 2011 #4


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    Gold Member

    The term that is usually applied in situations like this is "closing speed" and it is equal to the value you got but it is just a calculation and not related to the actual speed that anything is traveling at. Look up "velocity addition in wikipedia if you want to see what each spaceship is actually traveling relative to the other one.
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