Question 889173
Relativistic particles speed through the atmosphere of the Earth at 0.6c. If the upper atmosphere is 500 km from the ground, from the perspective of an observer on Earth, how long does it take the particles to travel that distance?
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You didn't specify c.  300000 km/sec is commonly used.
0.6c = 1.8e5 km/sec
d = r*t
t = d/r = 500/1.8e5 =~ 278e-5 sec
= 2.78e-3 seconds
= 0.00278 seconds
= 2.78 milliseconds