Bukhovtsev-et-al-Problems-in-Elementary-Physics

OSCILLATIONS AND WAVES 133
648. A plane capacitor incorporated into an oscIllatory circuit
is so designed that its plates can move relative to each other.
How can the circuit be made to oscillate parametrically by
moving the plates?
4-3. Waves
649. A thin string is replaced by another made of the same
material, but with a double diameter. How many times should
the tension of the string be changed to retain the previous
frequency of asci11atlons?
650. Find the frequencies of natural oscillations of a steel
string with a length of l = 50 em and 8
diameter of d = 1 mm
if the tension of the string is T = 2,450 dynes. The density of
steel is p = 7.8 g/cm".
651. Find the frequencies of the natural oscillations of an air
column in a pipe with a length of l = 3.4 metres closed at both
ends.
652. A tuning fork with a frequency of natural oscillations
of v = 340 5- 1 sounds above a cylindrical vessel one metre high.
Water is slowly poured into the vessel. At what level of the
water in the vessel will the sound of the tuning fork be appreciably
intensified?
653. What is the shape of the front of the shock wave produced
in air when a bullet flies at a supersonic velocity?
654. A jet airplane flies at a velocity of 500 mls at a distance
of 6 km from a man. At what distance from the man was
the plane when the man heard its sound?
655. If a source of sound and a man are about at the same
altitude the sound is heard better in the direction of the wind
than in the opposite direction. How can you explain this phenomenon?
656. Why can TV programmes be seen only within the range
of direct visibility?
657. A radio direction finder operates in pulse duty. The pulse
frequency is f = 1,700 cps and its duration is 1: ~ 0.8 J.LS. Determine
the maximum and minimum range of this finder.
658. Besides the wave transmitted directly from the station
(point A), a TV aerial (point C in Fig. 232) receives the wave
reflected from the iron roof of a building (point B). As a result
a double image appears. How many centimetres are the images
shifted with respect to each other if the aerial and the roof are