Question

The pitch or frequency of the siren of the coming train appears to be increasing because of:
a) Big-bang theory
b) Doppler’s effect
c) Charles law
d)Archimedes law

Answer 1

Hint: The above phenomenon observed is basically named after a person who observed it and formulated the theory. The effect is observed when there is a relative motion between the source of sound and the observer. Knowing who observed the above phenomenon will enable us to select the correct alternative
Formula used:
${{\gamma }^{'}}=\dfrac{v}{v-{{v}_{s}}}\gamma $

Complete answer:
Let us say a source of sound is with speed ${{v}_{s}}$ is approaching a stationary observer. The frequency of the source is $\gamma $. If the sound wave travel with the speed ‘v’, then the apparent frequency (${{\gamma }^{'}}$ )heard by the observer is given by,
${{\gamma }^{'}}=\dfrac{v}{v-{{v}_{s}}}\gamma $
From the above equation we can clearly see as the value of the speed of the source increases, the term in the denominator decreases. Hence the frequency of the sound heard by the observer increases. This phenomena of apparent change in the frequency of sound from any source of disturbance when the source, the observer and the medium are in relative motion is termed as Doppler’s effect.

Hence the correct answer of the above question is option b.

Note:
Big –bang theory gives the idea of the formation of the universe from nothing to something. It basically speaks about how the universe began. Charles law speaks about the behavior of gasses during compression and expansion. The Charles law states that the volume occupied by the gas is directly proportional to the temperature provided the pressure remains constant. The Archimedes principle which explains the floating of the bodies in liquid. The Archimedes principle states that when a body is partially or fully immersed in fluid, it experiences an upthrust equal to the weight of the fluid displaced by it and the upthrust acts through the center of gravity of the displaced fluid.