Engineering Questions with Answers - Multiple Choice Questions

# Stationary Waves

1 - Question

A train is approaching with velocity 25m/s towards a pedestrian standing on the track, frequency of horn of train is 1 kHz. Frequency heard by the pedestrian is (v=350m/s).
a) 1077Hz
b) 1167Hz
c) 985Hz
d) 945Hz

Explanation: γ‘=v/(v-vs)×γ=350/(350-25)×1000Hz
=1077Hz.

2 - Question

A person is standing on a railway platform and a train is approaching to him, what is maximum wavelength of sound he can hear? Wavelength of whistle=1m, speed of sound in air=330m/s, speed of train=36km/h.
a) 1m
b) 32/322m
c) 33/32m
d) 12/13m

Explanation: As γ‘=v/(v-vs)×γ
vs=36km/h=10m/s
ʎ‘=(v-vs)/v×ʎ=(330-10)/330×1=32/33 m.

3 - Question

The source of sound generating of a frequency of 3 kHz reaches an observer with a speed of 0.5 times the velocity of sound in air. The frequency heard by the observer is?
a) 1 kHz
b) 3 kHz
c) 4 kHz
d) 6 kHz

Explanation: γ‘=v/(v-vs)×γ
=v/(v-0.5γ)×3kHz=6kHz.

4 - Question

A car sounding its horn at 480Hz moves towards a high wall at a speed of 20m/s, the frequency of the reflected sound heard by the man sitting in the car will be nearest to __________
a) 480Hz
b) 510Hz
c) 540Hz
d) 570Hz

Explanation: After reflection from the wall, the sound moves towards observer in the car,
γ‘=(v+v0)/(v-vs)×γ=(340+20)/(340-20)×480=540Hz.

5 - Question

A racing car moving towards a cliff sound its horn. The driver observes that the sound reflected from the cliff has a pitch one octave higher than the actual sound of the horn. If v is the velocity of sound, the velocity of the car is?
a) v/√2
b) v/2
c) v/3
d) v/4

Explanation: γ‘/γ=(v+vs)/(v-vs)=2
v+vs=2v-2vs
3vs=v or vs=v/3.

6 - Question

If a particle travelling with a speed of 0.9 of the speed of sound and is emitting radiations of frequency if 1kHz and moving towards the observer, what is the apparent frequency?
a) 1.1 kHz
b) 0.8 kHz
c) 0.4 kHz
d) 10 kHz

Explanation: γ‘=v/(v-vs)×γ
=v/(v-0.9v)×1kHz=10kHz.

7 - Question

Two trains, each moving with a velocity of 30m/s, cross each other. One of the trains gives a whistle whose frequency is 600Hz. If the speed of sound is 330m/s, the apparent frequency for passengers sitting in the other train before crossing would be __________
a) 600Hz
b) 630Hz
c) 930Hz
d) 720Hz

Explanation: γ‘=v/(v-vs)×γ
=(330+30)/(330-30)×600
360/300×600=720Hz.

8 - Question

The phenomenon by which stars recede from each other is explained by __________
a) Black hole theory
b) Neutron star theory
c) White dwarf
d) Red shift

Explanation: Red shift confirms that stars are continuously receding away from each other.

9 - Question

Speed of recession of a galaxy is proportional to its distance __________
a) Directly
b) Inversely
c) Exponentially
d) Linearly

Explanation: According to Hubble’s law, the speed of recession (v) of a galaxy is directly proportional to its distance (r) from us.
Where H is Hubble’s constant.

10 - Question

The loudness and pitch of a sound note depends on __________
a) Intensity and frequency
b) Frequency and number of harmonics
c) Intensity and velocity
d) Frequency and velocity