Page 40 - Q & A STPM 2022 Physics

Physics Semester 3 STPM Chapter 21 Sound Waves
Question 2

The figure below is the displacement-distance graph of sound wave travelling
in air in the +x-direction. At which of the points A, B, C and D is the greatest
instantaneous pressure?
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Displacement, y

0
A B C D Distance, x

Answer: B
From the y-x graph, the instantaneous displacements (+ or – ) of air molecules
to the left and right of each of the points are shown in the figure below.

A B C D

Pressure at point B is the greatest because the air molecules on either sides
move inwards.

Question 3
The variation of displacement y from the equilibrium position with time t for
air molecules at a distance x from a point O when sound ware travels in air is
represented by the equation
y = y 0 sin (ωt – kx)
What is the expression for the variation of pressure p with time t for points
distance from O?
[ p 0 is the amplitude of pressure change]
A p = p 0 sin (ωt – kx)
B p = p 0 cos (ωt – kx)
C p = –p 0 sin (ωt – kx)
D p = –p 0 cos (ωt – kx)

Common Error
Semester
1
3 The peak for the curve for p occurs – cycle after and not before the peak for
4
the curve of y.

400

021 STPM Q&A Physics T3.indd 400 17/02/2022 10:24 AM