Physics 41 Chapter 17 Sample Problems
1. The variation in the pressure of helium gas, measured from its equilibrium value, is given by
P = 2.9  10–5 cos (6.2x – 3000t) where x and t have units m and s, and P is measured in N/m2.
Determine the frequency (in Hz), the wavelength, (in m) and the speed (m/s) of the wave.
2. The decibel level of a jackhammer is 130 dB relative to the threshold of hearing. Determine
the sound intensity produced by the jackhammer.
3. A point source emits sound with a power output of 100 watts. What is the intensity (in
W/m2) at a distance of 10.0 m from the source? What is it in dB?
4. Calculate the intensity level in dB of a sound wave that has an intensity of 15  10–4 W/m2.
a.
b.
c.
d.
e.
20
200
92
9
10
5. By what factor will an intensity change when the corresponding sound level increases by
3 dB?
a.
b.
c.
d.
e.
3
0.5
2
4
0.3
6. How far apart are two layers of tissue that produce echoes having round-trip times that differ
by 0.750s? What minimum frequency must the ultrasound have to see detail this small? The
speed of sound in human tissue is 1540m/s.
7. A car approaches a stationary police car at 36 m/s. The frequency of the siren (relative to the
police car) is 500 Hz. What is the frequency (in Hz) heard by an observer in the moving
car as he approaches the police car? (Assume the velocity of sound in air is 343 m/s.)
a.
b.
c.
d.
e.
220
448
5264
552
383
8. A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite direction. If
the frequency of the siren relative to the police car is 500 Hz, what is the frequency heard
by an observer in the truck as the police car approaches the truck? (The speed of sound
in air is 343 m/s.)
a.
b.
c.
d.
e.
9.
396
636
361
393
617
A truck moving at 36 m/s passes a police car moving at 45 m/s in the opposite
direction. If the frequency of the siren is 500 Hz relative to the police car, what is the
frequency heard by an observer in the truck after the police car passes the truck? (The
speed of sound in air is 343 m/s.)
a.
b.
c.
d.
e.
361
636
393
396
383