K.C.S.E Physics Q & A - MODEL 2014PP2QN017

K.C.S.E Physics Q & A - MODEL 2014PP2QN017

(a) Figure 9 shows two speakers  and S2 which produce sound of the same frequency.
They are placed equidistant from a line AB and a line PQ. (PQ is perpendicular to line AB).

​(i) A student walking from A to B hears alternating loud and soft sounds. Explain why at some point the sound heard is soft. 
(ii) The student now walks along line PQ. State with reason the nature of the sound the student hears. 
(b) Figure 10 shows sound waves in air produced by a vibrating tuning fork. R is an air molecule on the path of the waves.

​(i) Using a line, indicate on the diagram a distance d equal to one wavelength of the wave.
(ii) In the space provided, show with an arrow the direction of motion of the air molecule R as the waves pass. 
(iii) Explain the reason for the answer in (ii).

answers

​(a) (i) (I) sound is soft when the waves arrive out of phase;such waves undergo destructive interference.
(ii) same sound — loud. 
Along PQ the waves undergo constructive interference as they arrive in phase. 

​(iii) As the longitudinal waves pass molecule R moves along to either side.
For a crest. R moves away from source.