✔ 最佳答案
1. Option B
wavelength = 8/2 m = 4 m
Distance between two crests along the wall = 4/sin(pi/6) m = 8 m
Hence, point 1 m apart along the wall = (1/8) wavelength = (2.pi)/8 radians = pi/4 radians
2. Option B
If you sketch out the resultant wave, the peak of the resultant first occurs at position of 1/8 period on the time axis.
Since the first peak of wave P occurs at 1/4 period along the time axis, the difference in time of the two waves = (1/4 - 1/8) period = 1/8 period, which corresponds to a phase difference of 360/8 degrees = 45 degrees
3. Option A
It is clear from the graphs of P and Q that the peak of P appears earlier in time than the peak of Q by 1/4 period, which corresponds to a phase difference of 2.pi/4 radians = pi/2 radians
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2011-09-04 20:36:16 補充：
1. The question is to find the phase difference between 2 points 1 metre apart on the wall.
2011-09-04 20:39:14 補充：
Since 1 wavelength (which is 2.pi radians in phase angle) corresponds to a distance of 8 m apart along the wall, by simple proportion, distance of 1 m on the wall corresponds to (2.pi)/8 radians.
2011-09-04 22:49:10 補充：
The diagram is already given in the question.
SImply, the distance between 2 crests measured ALONG THE WALL is 8m. But this corresponds to 1 wavelength, and 1 wavelength corresponds to phase angle of 2.pi radian (pi = 3.14159...)
2011-09-04 22:51:57 補充：
(cont'd)... hence, the phase angle between 2 points 1 m apart ALONG THE WALL, by proportion, = (1m/8m) x (2.pi) radians = (2.pi/8) radians = pi/4 radians.
