Physics Paper 2, May/June. 2014  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 Main

General Comments

Question 10

(a)    State the three characteristics of sound and the factor on which each of them depends.
(b)    Explain resonance as applied to sound.
(c)     What role does echo play in the construction of a concert hall?
(d)     The surface of an ear drum (assumed circular) has a radius 2.1 mm.  It resonates with an amplitude of 0.8 x 10-7 m as a result of impulses received from an external body vibrating at 2400 Hz.  If the resulting pressure change on the ear drum is 3.6 x 10-5 Nm-2 calculate the:
         (i)     period of oscillation.
        (ii)     velocity:
       (iii)     acceleration:

  1.  force.

[   =  3.14]


Part (a):  A popular question among the candidates.  Most candidates were able to list the characteristics but failed to correctly match or even mention the corresponding factors.
Part(b):  Most candidates gave general irrelevant definitions of resonance rather than explaining the term resonance.  Performance was poor.

Part(c):  Those candidates that attempted it did not explain properly because they were ignorant of the demand of the question.  Majority of the responding candidates merely defined echo and stated its consequences.  Performance was poor.

Part(d):   Majority of the candidates that attempted this part got the period and missed others.  Conversion of unit from ‘mm to m’ was an uphill task to some candidates.  Applying concepts in physics to real life situation is an unusual situation to candidates.  Teachers should endeavour to apply physics principles to situation outside the textbooks.

The expected answers are:

10.   Characteristics of Sound and their corresponding factors






-   Pitch
-   Loudness/intensity
-   Timbre/quality


-   Frequency                              
-   Amplitude                              
-   Harmonics/overtone           

  (b)   Resonance
         When an external vibrating source of sound such as a sounding tuning fork set another system such as
         molecules of air column into vibration a situation is reached when the later system vibrates with
         maximum amplitude producing a loud sound. This phenomenon is termed resonance.                                                                                                             
(c)    It ( echo) sensitises the construction engineers on the need to line the interior wall with         appropriate sound absorbing materials.                                                                                                                            
(d)     (i)       T   =                                                                                             
                          =   4.2 x 10-4 s                                                                          

         (ii)        v = r = 2                                                                                
                                  =   2 x 3.14 x 2400 x 0.8 x 10-7                                           
                                  =  1.2 x 10-3  ms-1                                                              

         (iii)      a =  2r    =   (22r                                                                      
                                      =  (2x3.14 x 2400)2 x 0.8 x 10-7                                     
                                                         = 18.2 ms-2                                                                     

         (iv)     P =                                                                                                  
                    F  =   PA  =  3.6 x 10-5 x 2
                                  =  3.6  x 10-5x 3.14 x (2.1 x 10-3)2                                                 

                                     =   5.0 x 10-11N                                                                            





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