A spaceship flies from Earth to a distant star at a constant speed. Upon arrival, a clock on board the spaceship shows a total elapsed time of 8 years for the trip. An identical clock on Earth shows that the total elapsed time for the trip was 10 years. What was the speed of the spaceship relative to the Earth?

35 288 mile/sec

Explanation:

This is a problem of special relativity. The clocks start when the spaceship passes Earth with a velocity v, relative to the earth. So, out and back from the earth it will take:

If we use the Lorentz factor, then, as observed by the crew of the ship, the arrival time will be:

Then the amount of time wil expressed as a reciprocal of the Lorentz factor. Thus:

solving for v, gives = 35 288 miles/s