A fireman in a truck turns on a flashing light and sees it generate a flash every 1.5 s ...

Question

An observer outside of the truck measures the time between flashes as 2.5 s. How fast is the truck moving relative to the Earth?

Answer 1

0.8c

Any more stupid questions?

Answer 2

V = c*sqrt[1 - (1.5/2.5)^2] = .64*c = 1.92E8 m/s