JEE MAIN - Physics (2021 - 17th March Morning Shift)

• 1

  A boy is rolling a 0.5 kg ball on the frictionless floor with the speed of 20 ms^-1. The ball gets deflected by an obstacle on the way. After deflection it moves with 5% of its initial kinetic energy. What is the speed of the ball now?

  პასუხი

  (C)

  4.47 ms^$$-$$1
• 2

  A polyatomic ideal gas has 24 vibrational modes. What is the value of $$\gamma$$?

  პასუხი

  (C)

  1.03
• 3

  Two ideal polyatomic gases at temperatures T₁ and T₂ are mixed so that there is no loss of energy. If F₁ and F₂, m₁ and m₂, n₁ and n₂ be the degrees of freedom, masses, number of molecules of the first and second gas respectively, the temperature of mixture of these two gases is :

  პასუხი

  (B)

  $${{{n_1}{F_1}{T_1} + {n_2}{F_2}{T_2}} \over {{n_1}{F_1} + {n_2}{F_2}}}$$
• 4

  If an electron is moving in the n^th orbit of the hydrogen atom, then its velocity (v_n) for the n^th orbit is given as :

  პასუხი

  (A)

  $${v_n} \propto {1 \over n}$$
• 5

  Two identical metal wires of thermal conductivities K₁ and K₂ respectively are connected in series. The effective thermal conductivity of the combination is :

  პასუხი

  (A)

  $${{2{K_1}{K_2}} \over {{K_1} + {K_2}}}$$
• 6

  A triangular plate is shown. A force $$\overrightarrow F $$ = 4$$\widehat i$$ $$-$$ 3$$\widehat j$$ is applied at point P. The torque at point P with respect to point 'O' and 'Q' are :
  
  

  პასუხი

  (D)

  $$-$$ 15 $$-$$ 20$$\sqrt 3 $$, 15 $$-$$ 20$$\sqrt 3 $$
• 7

  When two soap bubbles of radii a and b (b > a) coalesce, the radius of curvature of common surface is :

  პასუხი

  (D)

  $${{ab} \over {b - a}}$$
• 8

  A car accelerates from rest at a constant rate $$\alpha$$ for some time after which it decelerates at a constant rate $$\beta$$ to come to rest. If the total time elapsed is t seconds, the total distance travelled is :

  პასუხი

  (C)

  $${{\alpha \beta } \over {2(\alpha + \beta )}}{t^2}$$
• 9

  A mass M hangs on a massless rod of length l which rotates at a constant angular frequency. The mass M moves with steady speed in a circular path of constant radius. Assume that the system is in steady circular motion with constant angular velocity $$\omega$$. The angular momentum of M about point A is L_A which lies in the positive z direction and the angular momentum of M about point B is L_B. The correct statement for this system is :
  
  

  პასუხი

  (A)

  L_A is constant, both in magnitude and direction
• 10

  A solenoid of 1000 turns per metre has a core with relative permeability 500. Insulated windings of the solenoid carry an electric current of 5A. The magnetic flux density produced by the solenoid is : (permeability of free space = 4$$\pi$$ $$\times$$ 10^$$-$$7 H/m)

  პასუხი

  (A)

  $$\pi$$T
• 11

  The thickness at the centre of a plane convex lens is 3 mm and the diameter is 6 cm. If the speed of light in the material of the lens is 2 $$\times$$ 10⁸ ms^$$-$$1. The focal length of the lens is ____________.

  პასუხი

  (B)

  30 cm
• 12

  For what value of displacement the kinetic energy and potential energy of a simple harmonic oscillation become equal ?

  პასუხი

  (C)

  x = $$\pm$$ $${A \over {\sqrt 2 }}$$
• 13

  An AC current is given by I = I₁ sin$$\omega$$t + I₂ cos$$\omega$$t. A hot wire ammeter will give a reading :

  პასუხი

  (C)

  $$\sqrt {{{I_1^2 + I_2^2} \over 2}} $$
• 14

  A modern grand - prix racing car of mass m is travelling on a flat track in a circular arc of radius R with a speed v. If the coefficient of static friction between the tyres and the track is $$\mu$$_s, then the magnitude of negative lift F_L acting downwards on the car is : (Assume forces on the four tyres are identical and g = acceleration due to gravity)
  
  

  პასუხი

  (C)

  $$m\left( {{{{v^2}} \over {{\mu _s}R}} - g} \right)$$
• 15

  The vernier scale used for measurement has a positive zero error of 0.2 mm. If while taking a measurement it was noted that '0' on the vernier scale lies between 8.5 cm and 8.6 cm, vernier coincidence is 6, then the correct value of measurement is ___________ cm. (least count = 0.01 cm)

  პასუხი

  (B)

  8.54 cm
• 16

  A current of 10A exists in a wire of cross-sectional area of 5 mm² with a drift velocity of 2 $$\times$$ 10^$$-$$3 ms^$$-$$1. The number of free electrons in each cubic meter of the wire is ___________.

  პასუხი

  (A)

  625 $$\times$$ 10²⁵
• 17

  The output of the given combination gates represents :
  
  

  პასუხი

  (C)

  NAND Gate
• 18

  Which level of the single ionized carbon has the same energy as the ground state energy of hydrogen atom?

  პასუხი

  (B)

  6
• 19

  An electron of mass m and a photon have same energy E. The ratio of wavelength of electron to that of photon is : (c being the velocity of light)

  პასუხი

  (B)

  $${1 \over c}{\left( {{E \over {2m}}} \right)^{1/2}}$$
• 20

  The radius in kilometer to which the present radius of earth (R = 6400 km) to be compressed so that the escape velocity is increased 10 times is ___________.

  პასუხი

  64
• 21

  The angular speed of truck wheel is increased from 900 rpm to 2460 rpm in 26 seconds. The number of revolutions by the truck engine during this time is _____________. (Assuming the acceleration to be uniform).

  პასუხი

  728
• 22

  A parallel plate capacitor whose capacitance C is 14 pF is charged by a battery to a potential difference V = 12 V between its plates. The charging battery is now disconnected and a porcelin plate with k = 7 is inserted between the plates, then the plate would oscillate back and forth between the plates with a constant mechanical energy of _____________ pJ. (Assume no friction)

  პასუხი

  864
• 23

  The following bodies,
  
  (1) a ring
  
  (2) a disc
  
  (3) a solid cylinder
  
  (4) a solid sphere,
  
  of same mass 'm' and radius 'R' are allowed to roll down without slipping simultaneously from the top of the inclined plane. The body which will reach first at the bottom of the inclined plane is ___________. [Mark the body as per their respective numbering given in the question]
  
  

  პასუხი

  4
• 24

  Four identical rectangular plates with length, l = 2 cm and breadth, b = $${3 \over 2}$$ cm are arranged as shown in figure. The equivalent capacitance between A and C is $${{x{\varepsilon _0}} \over d}$$. The value of x is ______________. (Round off to the Nearest Integer)
  
  

  პასუხი

  2
• 25

  Consider two identical springs each of spring constant k and negligible mass compared to the mass M as shown. Fig. 1 shows one of them and Fig. 2 shows their series combination. The ratios of time period of oscillation of the two SHM is T_b/T_a = $$\sqrt x $$, where value of x is ___________. (Round off to the Nearest Integer)
  
  

  პასუხი

  2
• 26

  The equivalent resistance of series combination of two resistors is 's'. When they are connected in parallel, the equivalent resistance is 'p'. If s = np, then the minimum value for n is ____________. (Round off to the Nearest Integer)

  პასუხი

  4
• 27

  Two blocks (m = 0.5 kg and M = 4.5 kg) are arranged on a horizontal frictionless table as shown in figure. The coefficient of static friction between the two blocks is $${3 \over 7}$$. Then the maximum horizontal force that can be applied on the larger block so that the blocks move together is ___________ N. (Round off to the Nearest Integer) [Take g as 9.8 ms^$$-$$2]
  
  

  პასუხი

  21
• 28

  If 2.5 $$\times$$ 10^$$-$$6 N average force is exerted by a light wave on a non-reflecting surface of 30 cm² area during 40 minutes of time span, the energy flux of light just before it falls on the surface is ___________ W/cm². (Round off to the Nearest Integer)
  
  (Assume complete absorption and normal incidence conditions are there)

  პასუხი

  25