# HW 14 Question 8

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An alien spaceship traveling at 0.630c toward the Earth launches a landing craft. The landing craft travels in the same direction with a speed of 0.870c relative to the mother ship (as measured by aliens on the mother ship). As measured on the Earth, the spaceship is 0.100 ly from the Earth when the landing craft is launched.

Q: When you did this question in class, you wrote v=0.630c is relative to earth. How do you assume that ? The question simply says its travelling at 0.630c toward the earth...this could be its proper speed? How do you know this is speed relative to Earth?

asked Nov 24, 2013

## 1 Answer

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The judgment is based on this given sentence:"An alien spaceship traveling at 0.630c toward the Earth launches a landing craft." The wording can be improved to explicitly state that the velocity is relative to the Earth. Meanwhile, the velocity cannot be relative to the space ship as it is always stationary if looking at itself. If the velocity is relative to some other object, then that object has to be mentioned. So typically if people say moving with a velocity towards something without referencing a third object, the velocity is usually relative to the thing that the motion is aimed at.
answered Nov 24, 2013 by (21,750 points)
Thank you that was very helpful