A racing car moving towards a cliff, sounds its horn. The driver

Subject

Physics

Class

JEE Class 12

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 Multiple Choice QuestionsMultiple Choice Questions

1.

A very large number of balls are thrown vertically upwards in quick successions in such a way that the next ball is thrown when the previous one is at the maximum height. If the maximum height is 5 m, the number of balls thrown per minute is (take g = 10 m/s2)

  • 80

  • 120

  • 40

  • 60


2.

The resultant of two forces, one double the other in magnitude, is perpendicular to the smaller of the two forces. The angle between the two forces is

  • 120°

  • 60°

  • 90°

  • 150°


3.

A projectile is moving at 20 ms-1 at its highest point, where it breaks into equal parts due to an internal explosion. One part moves vertically up at 30 ms-1 with respect to the ground. Then the other part will move at

  • 20 ms-1

  • 1031 ms-1

  • 50 ms-1

  • 30 ms-1


4.

A body is projected vertically upwards from the surface of a planet of radius R with a velocity equal to half the escape velocity for that planet. The maximum height attained by the body is

  • R/2

  • R/3

  • R/5

  • R/4


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5.

From the top of a tower a stone is thrown up which reaches the ground in a time t1. A second stone thrown down with the same speed reaches the ground in a time t2. A third stone released from rest from the same between reaches the ground in a time t3. Then

  • 1t3 = 1t2 - 1t1

  • t32 = t12 - t22

  • t3 = t1 + t22

  • t3 = t1 t2


6.

The energy spectrum of a black body exhibits a maximum around a wavelength λ0. The temperature of the black body is now changed such that the energy is maximum around a wavelength 3λ04. The power radiated by the black body will now increase by a factor of

  • 64/27

  • 256/81

  • 4/3

  • 16/9


7.

n identical bulbs, each designated to draw a power P from a certain voltage supply. The total power which they will draw is

  • Pn

  • Pn2

  • nP

  • P


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8.

A racing car moving towards a cliff, sounds 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, then the velocity of the car is

  • v/2

  • v/2

  • v/4

  • v/3


D.

v/3

Let f be the frequency of horn emitted by car.

        f' = observed frequency

Since, pitch ∝ f

∴      f' = f + f

One octave = frequency of instrument

                f' = v - vov + vs f

vo = velocity of observer, let it be u

vs = velocity of source

Because here source and observer (car) both are same and moving with same velocity.

∴      vs = vo = u

         v = velocity of sound

AB = source and sound are in same direction

Hence (v − u) = relatice velocity

BA = Observer and sound are in opposite direction hence

            v + u = relative velocity

       f' = v - uv + u f          2f = v - uv + u f2v + 2u = v - u            v = 3u            u = v3


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9.

A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer beside the railway track. Which of the following graphs best represent the variation of frequency of the sound n heard by the observer with the time t ?


10.

The tension of a stretched string is increased by 69 %. In order to keep its frequency of vibration constant, its length must be increased by

  • 30 %

  • 20 %

  • 69 %

  • 69 %


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