Physics Test 139

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Physics Test 139

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Question 1
4 / -1

When a train is at a distance of 2km, its engine sounds a whistle. A man near the railway track hears the whistle directly and by placing his ear against the track of the train. If the two sounds are heard at an interval of 5.2 s, find the speed of the sound in iron (material of the rail track). given that velocity of sound in air is 330 ms^-1.

A
2,325.6 ms^-1

B
1,932.4 ms^-1

C
2,132.5 ms^-1

D
3,213.2 ms^-1

Solution

Here, S = 2 km = 2,000 m ; v_air = 330 m s^-1

Therefore, time taken by sound to travel through air,

As the two sounds (through air and iron rails) are heard at an interval of 5.2 s, time taken by sound to travel through iron rails,

t_iron = t_air - 5.2

= 6.06 - 5.2 = 0.86 s

Therefore, velocity of sound in iron,

= 2,325.6 m s^-1
Question 2
4 / -1

Dispersive powers of materials used in lenses of an achromatic doublet are in the ratio 5:3. If the focal length of concave lens is 15 cm, then the focal length of the other lens will be

A
-9 cm

B
+9 cm

C
-12 cm

D
+12 cm

Solution
Question 3
4 / -1

As per the diagram a point charge +q is placed at the origin O. Work done in taking another point charge -Q from the point A [coordinates (0, a)] to another point B [coordinates (a, 0)] along the straight path AB is

A
Zero

B

C

D

Solution

Work done is equal to zero because the potential of A and B are the same

= 1 /4πε₀ q/a

No work is done if a particle does not change its potential energy.

i.e., initial potential energy = final potential energy.
Question 4
4 / -1

A man crosses the river perpendicular to river flow in time t seconds and travels an equal distance down the stream in T-seconds. The ratio of man's speed in still water to the speed of river water will be :

A

B

C

D

Solution

Let velocity of man in still water be v and that of water with repect to ground be u. Velocity of man perpendicular to river
Question 5
4 / -1

The mean lives of a radioactive substance are 1620 years and 405 years for α - emission and β - emission respectively. Find the time during which three-fourth of a sample will decay if it is decaying both by α – emission and β – emission simultaneously.

A
449 years

B
450 years

C
365 years

D
439 years

Solution
Question 6
4 / -1

An uncalibrated spring balance is found to have a period of oscillation of 0.314 s, when a 1 kg weight is suspended from it, How much does the spring elongate, when a 1 kg weight is suspended from it ? Take π = 3.14

A
2.45 cm

B
2.40 cm

C
2.30 cm

D
2.10 cm

Solution

Here. T = 0.314 s ; m = 1 kg
Question 7
4 / -1

A point object is moving with a speed v in front of an arrangement of two mirrors as shown in figure. If the velocity of image in mirror M₁ with respect to image in mirror M₂ is n V sinθ, n =

A
2

B
2.5

C
3

D
3.5

Solution
Question 8
4 / -1

A point source of heat of power P is placed at the centre of a spherical shell of mean radius R. The material of the shell has thermal conductivity k. Calculate the thickness of the shell if temperature difference between the outer and inner surfaces of the shell in steady state is T.

A

B

C

D

Solution

Consider a concentric spherical shell of radius r and thickness dr as shown in diagram. The radial rate of flow of heat through this shell in steady state will be,

Now in steady state as no heat is absorbed, rate of loss of heat by conduction is equal to that of supply i.e., H = P and here
Question 9
4 / -1

A body falling freely from a given height ‘H’ hits an inclined plane in its path at a height ‘h’. As a result of this impact the direction of the velocity of the body becomes horizontal. For what value of (h/H) the body will take maximum time to reach the ground?

A
h/H = √2

B
h/H = 2

C
h/H = 1/√2

D
h/H = 1/2

Solution
Question 10
4 / -1

Two radio antennas radiating waves in phase are located at point A and B, 200 m part (Figure). The radio waves have a frequency of 6 MHz. A radio receiver is moved out from point B along a line perpendicular to the line connecting A and B (line BC shown in figure). At what distances from B will there be destructive interference?

A
787.5, 229m, 97.5m, 26.7m

B
789.5, 230m, 98.5m, 27.7m

C
788.5, 228m, 98.5m, 28.7m

D
777.5, 239m, 87.5m, 36.7m

Solution