NEW
New Website Launch
Experience the best way to solve previous year questions with mock tests (very detailed analysis), bookmark your favourite questions, practice etc...
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.
Your Input ________

Answer

Correct Answer is 1210

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.
Your Input ________

Answer

Correct Answer is 7

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.
Your Input ________

Answer

Correct Answer is 1

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.
Your Input ________

Answer

Correct Answer is 2025

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

Joint Entrance Examination

JEE Main JEE Advanced WB JEE

Graduate Aptitude Test in Engineering

GATE CSE GATE ECE GATE EE GATE ME GATE CE GATE PI GATE IN

Medical

NEET

CBSE

Class 12