A beam of particles travels at a speed of 2. 4×10⁶ m/s. Scientists in the laboratory measure the average lifetime of the particle in the beam as 3. 7×10−6s. Calculate the average lifetime of the particles when they are at rest. [ans: 22×10−6s] A spacecraft passes you at a speed of 0. 80c. The proper length of the spacecraft is 120 m. Determine the length that you measure as it passes you. in [ans: 72 m ]