# Half-life

A half-life is the amount of time required for a quantity to fall to half its value as measured at the beginning of the time period. Suppose that a radioactive element has \(10\) seconds of half-life. If this element moves with a constant velocity of \(260,000\) km/s for \(20\) seconds, as measured by an observer at a fixed spot, how much of it is left compared to the initial amount? (Assume that the velocity of light is \(300,000\) km/s and \(\frac{26^2}{30^2}=0.75.\))