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1

### JEE Main 2021 (Online) 27th August Morning Shift

Numerical
Two cars X and Y are approaching each other with velocities 36 km/h and 72 km/h respectively. The frequency of a whistle sound as emitted by a passenger in car X, heard by the passenger in car Y is 1320 Hz. If the velocity of sound in air is 340 m/s, the actual frequency of the whistle sound produced is .................. Hz.

## Explanation

Image

Vx = 36 km/hr = 10 m/s

Vy = 72 km/hr = 20 m/s

by doppler's effect

$$F' = {F_0}\left( {{{V \pm {V_0}} \over {V \pm {V_s}}}} \right)$$

$$1320 = {F_0}\left( {{{340 + 20} \over {340 - 10}}} \right) \Rightarrow {F_0} = 1210$$ Hz
2

### JEE Main 2021 (Online) 26th August Evening Shift

Numerical
Two waves are simultaneously passing through a string and their equations are :

y1 = A1 sin k(x $$-$$ vt), y2 = A2 sin k(x $$-$$ vt + x0). Given amplitudes A1 = 12 mm and A2 = 5 mm, x0 = 3.5 cm and wave number k = 6.28 cm$$-$$1. The amplitude of resulting wave will be ................ mm.

## Explanation

y1 = A1 sin k(x $$-$$ vt)

y1 = 12 sin 6.28 (x $$-$$ vt)

y2 = 5 sin 6.28 (x $$-$$ vt + 3.5)

$$\Delta \phi = {{2\pi } \over \lambda }(\Delta x)$$

$$= K(\Delta x)$$

$$= 6.28 \times 3.5 = {7 \over 2} \times 2\pi = 7\pi$$

$${A_{net}} = \sqrt {A_1^2 + A_2^2 + 2{A_1}{A_2}\cos \phi }$$

$${A_{net}} = \sqrt {{{(12)}^2} + {{(5)}^2} + 2(12)(5)\cos (7\pi )}$$

$$= \sqrt {144 + 25 - 120}$$
3

### JEE Main 2021 (Online) 26th August Morning Shift

Numerical
Two travelling waves produces a standing wave represented by equation,

y = 1.0 mm cos(1.57 cm$$-$$1) x sin(78.5 s$$-$$1)t.

The node closest to the origin in the region x > 0 will be at x = .............. cm.

## Explanation

For node

cos(1.57 cm$$-$$1)x = 0

(1.57 cm$$-$$1)x = $${\pi \over 2}$$

x = $${\pi \over {2(1.57)}}$$ cm = 1 cm
4

### JEE Main 2021 (Online) 26th August Morning Shift

Numerical
A source and a detector move away from each other in absence of wind with a speed of 20 m/s with respect to the ground. If the detector detects a frequency of 1800 Hz of the sound coming from the source, then the original frequency of source considering speed of sound in air 340 m/s will be ............... Hz.

## Explanation

Image

$$f' = f\left( {{{C - {V_0}} \over {C + {V_s}}}} \right)$$

$$\Rightarrow$$ $$1800 = f\left( {{{340 - 20} \over {340 + 20}}} \right)$$

f = 2025 Hz

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