Physics Test

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Physics Test - 5

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Weekly Quiz Competition

Question 1
4 / -0.83

When a source of sound of frequency crosses a stationary observer with a speed v, ( «speed of sound v), the apparent change in frequency Δf is given by :

A

B

C

D

Solution

The correct answer is :
Question 2
4 / -0.83

In a stationary wave that forms as a result of reflection of wave from an obstacle the ratio of the amplitude at an antinode to the amplitude at node is n. The fraction of energy reflected is :

A

B

C

D

Solution

Let a_i , and a_r be the amplitudes of incident and reflected waves. Then

∴ Fraction of energy reflected is

The correct answer is:
Question 3
4 / -0.83

The displacement y of a particle executing periodic motion is given by :

The expression may be consider to be a result of the superposition of independent harmonic motions :

A
Four

B
Two

C
Three

D
Five

Solution

The given equation can be written as

y = 2(cost + 1) sin(1000t)
= 2cost sin 1000t + 2sin(1000t)
= sin(1001t) + sin(999t) + 2sin(1000t)
i.e. the given expression is a small of superposition of three independent harmonic motions of angular frequencies 999,1000 and 1001 rad/sec.
The correct answer is: Three
Question 4
4 / -0.83

The displacement of particles in a string stretched in the x-direction is represented by y. Among the following expressions for y, those describing wave motion are :

A

B

C

D

Solution

Only first equation is the equation of standing wave. The condition for a function x and t to represent a wave is :

Only first equation satisfies the condition.
The correct answer is:
Question 5
4 / -0.83

A transverse wave is described by the equation The maximum particle velocity is equal to four times the wave velocity if :

A

B

C

D

Solution

Wave velocity
Maximum particle velocity
Given,
or

The correct answer is:
Question 6
4 / -0.83

A transverse wave in metres, is propagating along +ve X- axis on a string. A light insect starts crawling on the string with the velocity of 5 cm/s at t = 0 along the +ve X- axis from a point where x = 5 cm . After 5 s the difference in the phase of its position is equal to :

A

B

C

D

Solution

At t = 0, x = 5 cm, initial phase of insect

After 5s insect will more a distance s = vt = 25 cm
So, it will beat x = 30 cm .
At this instant,
The correct answer is:
Question 7
4 / -0.83

Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the wavelength of reflected wave will be :

A

B

C

D

Solution

Apparent frequency for reflector (will act as an observer) would be :

where f = actual frequency of source.
After being reflected the apparent frequency will further change and the reflector will now behave as a source. The apparent frequency will now become :

Substituting value of f₁ , we get

The correct answer is:
Question 8
4 / -0.83

A train is moving with a constant speed along a circular track. The engine o f the train emits a sound of frequency f. The frequency heard by the guard at rear end of the train :

A
is equal to f

B
is greater than f

C
is less than f

D
may be greater than, less than or equal to f depending on the factors like speed of train, length of train and radius of circular track

Solution

Let v be the speed of sound and u the speed of train.

Then
and

The correct answer is: is equal to f
Question 9
4 / -0.83

A standing wave is maintained in a homogeneous string of cross-sectional area a and density p. It is formed by superposition of two waves travelling in opposite directions given by the equation.

The total mechanical energy confined between the sections corresponding to the adjacent antinodes is :

A

B

C

D

Solution

Distance between two adjacent antinodes is
Volume of string between two adjacent antinodes

Here, u is the energy density (energy per unit volume) which is equal to

The correct answer is :
Question 10
4 / -0.83

A travelling wave is partialy reflected partialy and partly transmitted from a rigid boundary. Let a_v a_r and a_t be the amplitudes of incident wave, reflected wave and transmitted wave and l_i , I_r and l_t be the corresponding intensities. Then choose the correct alternative:

A

B

C

D
All of these

Solution

only when other properties like density of medium, speed of wave etc, remains the same
In incident and reflected waves medium is same. Therefore, other properties are same. Hence,

The correct answer is: