[填空题]
1.(1)According to the big bang theory, the universe is expanding. One of the supporting pieces of evidence for this claim is the receding of galaxies from the earth and themselves. Which wave phenomenon is used in the discovery?
(2)One of the galaxies is receding from the earth at a constant speed. It emits radio waves in different frequencies. The difference between observed frequency by the receiver on the earth and emitted frequency $f$ from the galaxy is $Δf$. Sketch a graph that shows how $Δf$ varies with $f$. (no need to add values on the axis)
(3)State the assumption you made for the answer in (a) (ii)
2.(1)A stationary person by the side of a straight road records the frequency of the siren of an ambulance driving on the road. When the ambulance is not moving, the frequency is $f$.
The graph shows how the recorded frequency $′f′$ varies with time.
Describe the motion of the ambulance that would result in the graph above.
(2)The person decides to record the frequency of the horn of a car while moving in another car. A car of speed $24\,ms^{−1}$approaches another car of speed $s$ from behind on a level road.
The car behind uses its horn to clear its way. The frequency of the emitted sound wave is $10.0\,k\,Hz$ and the speed of the sound wave in air is $340\,ms^{−1}$. Calculate $s$, in $ms^{−1}$ if the detected frequency in the other car is $10.2\,k\,Hz$. s = $ms^{-1}$
