Physics Paper 2, Nov/Dec. 2011
 Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
Weakness/Remedies
Strength

Question 13

(a)    (i)    Explain interference of waves.
(ii)   With the aid of diagrams distinguish between constructive and destructive
interference of transverse waves.

(b)    A stretched wire under tension T produces a note of frequency 60 Hz when plucked.
Calculate the frequency if the tension in the wire is increased by 125%

(c)    The displacement of a plane progressive  wave is given by the equation
y  = 12.5 sin (240pt – 0.50p)
where the symbols have their usual meanings,
y and x are in metres and t in seconds.
For the wave, determine the:
(i)     frequency;
(ii)    wavelength;
(iii)    speed.

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Part(a)  a good number of the candidates were able to explain interference of waves and applied the knowledge to distinguish between constructive and destructive interference of transverse waves.

Part(b)  majority of the candidates that attempted this question performed poorly.  Majority of the responding candidates only scored for the formula f = k the substitution and simplification was poorly rendered.

Part(c) was well attempted by majority of the candidates.

The expected answers are:

Interference of waves is the super position (coming together) of two or more progressive waves to form a resultant wave.

Constructive interference                                                Destructive interference

(b)    f  =  KT
f1  =  60  =  kT        ………………..  (1)
f2   =  k    T        ………………… (2)

Divide   equation (2) by equation (1)

=     T
T

F2   =     X   60

f2     =    90Hz

(c)            y  =   12.5 Sin  ( 420 t  -  0.50px)
Comparing the given equation with the standard equation
y  =  a Sin   2pt     -     2
T

(i)     W  =  2pft [ 1 ]      =    420pt [ ½ ]
f   =   210Hz                 [ ½ ]

(ii)     2p
l               =    0.50px

l   =

=   4m
(iii)     V  =   f

=  210 x 4

=   840 ms-1