question_answer

Two loudspeakers M and A are located 20 m apart and emit sound at frequencies 118 Hz and 121 Hz, respectively. A car is initially at a point P, 1800 m away from the midpoint Q of the line MN and moves towards Q constantly at 60 km/hr along the perpendicular bisector of MN. It crosses Q and eventually reaches a point R, 1800 m away from Q. Let n(t) represent the beat frequency measured by a person sitting in the car at time t. Let n, n and n be the beat frequencies measured at locatons P, Q and R, respectively. The speed of sound in air is 330ms"1. Which of the following statement (s) is (are) true regarding the sound heard by the person

A) \[{{n}_{p}}={{n}_{R}}=nQ\]

B) The rate of change in beat frequency is minimum when the car passes through Q

C) The plot below represents schematically the  variation of beat frequency with time  

D) The plot below represents schematically the variation of beat frequency with time