The motion detector used in a physics lab sends and receives 39 khz ultrasonic pulses. a pulse goes out, reflects off the object being measured, and returns to the detector. the lab temperature is 20 c. a) what is the wavelength of the waves emitted by the motion detector? ) how long does it take for a pulse that reflects off an object 2.2 {\rm m} away to make a round trip? ms

Explanation:

frequency of sound = 39 kHz

= 39 x 10³ Hz

A ) Wave length of sound = velocity / frequency

velocity of sound at 20 degree = 343 m / s

Wave length = 343 / 39 x 10³

= 8.79 x 10⁻³ m

= 8.79 mm

B ) Distance travelled  in round trip

= 2 x 2.2 m

4.4 m

Time taken in round trip

Distance / velocity

4.4 / 343

= 12.8 ms.

=

the answer is d. 4

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