page 08 7. A spacecraft passes the Earth at a speed of 0.4c. A light on the spacecraft pulses on and off. A passenger on the spacecraft measures the time between the pulses as 2.5 s. An observer on Earth measures the time between the pulses as: A 2.3 s B 2.5 s C 2.7 s D 3.0 s E 3.2 s. 8. A student makes the following statements about the expanding Universe: I The evidence supporting the existence of dark matter comes from estimations of the mass of galaxies. II The evidence supporting the existence of dark energy comes from the accelerating rate of expansion of the Universe. III The peak wavelength of radiation emitted by hotter stars is longer than that for cooler stars. Which of these statements is/are correct? A I only B II only C III only D I and II only E I, II and III 9. A police car is travelling at a constant speed of 31.0 m s−1 towards a stationary observer. The siren on the car emits a sound with a frequency of 820 Hz. The speed of sound in air is 340 m s−1. The frequency of the sound heard by the observer is: A 745 Hz B 751 Hz C 820 Hz D 895 Hz E 902 Hz.