Cracking the SAT Physics Subject Test

Questions 10-11

A sound wave with a frequency of 343 Hz travels through the air.

10. What is its wavelength?


11. If its frequency increased to 686 Hz, what is its wave speed and
    the wavelength?

Here’s How to Crack It

10. Using v = 343 m/s for the speed of sound through air, we find that


11. Unless the ambient pressure or the temperature of the air changed, the
    speed of sound would not change. Wave speed depends on the characteristics
    of the medium, not on the frequency, so v would still be 343 m/s. However, a
    change in frequency would cause a change in wavelength. Since f increased by

a factor of 2, to λ would decrease by a factor of 2, to (1 m) = 0.5 m.

12. A sound wave traveling through water has a frequency of 500 Hz
    and a wavelength of 3 m. How fast does sound travel through water?

Here’s How to Crack It

v = λf = (3 m)(500 Hz) = 1,500 m/s.