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NEET physics

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Question
CBSEENPH11020837
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A solid sphere is rotating freely about its symmetry axis in free space. The radius of the sphere is increased keeping its mass same. Which of the following physical quantities would remain constant for the sphere?

  • Angular velocity

  • Moment of inertia

  • Angular momentum

  • Rotational Kinetic energy

Solution

C.

Angular momentum

Angular momentum remains conserved until the torque acting on sphere remains zero. τex = 0

So, dLdt = 0

i.e. L = constant

So angular momentum remains constant.

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Question
CBSEENPH11020839
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If the mass of the Sun were ten times smaller and the universal gravitational constant were ten times larger in magnitude, which of the following is not correct?

  • Raindrops will fall faster

  • Walking on the ground would become more difficult

  • 'g' on the Earth will not change

  • Time period of a simple pendulum on the Earth would decrease

Solution

C.

'g' on the Earth will not change

If universal gravitational constant becomes tem times then G' = 10 G

Acceleration due to gravity, g = GMR2

So, Acceleration due to gravity increases.

Question
CBSEENPH11020849
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A body initially at rest and sliding along a frictionless track from a height h (as shown in the figure) just completes a vertical circle of diameter AB = D. The height h is equal to

  • 3D/2

  • D

  • 5D/4

  • 7D/5

Solution

C.

5D/4

As track is frictionless, so total mechanical energy will remain constant,

i.e., 0 + mgh = 12mvL2 + 0using v2 - u2 = 2gh,h = vL22g (∵ u = 0)For completing the vertical circle VL ≥5gRor, h = 5gR2g = 52R = 54D

Question
CBSEENPH11020850
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Three objects, A : (a solid sphere), B : (a thin circular disk) and C : (a circular ring), each have the same mass M and radius R. They all spin with the same angular speed ω about their own symmetry axes. The amounts of work (W) required to bring them to rest, would satisfy the relation

  • WC > WB> WA

  • WA > WB > WC

  • WA>WC>WB

  • WB>WA>WC

Solution

A.

WC > WB> WA

B.

WA > WB > WC

Work done required to bring them rest

∆W = ∆KE∆W =12Iω2∆W ∝ I for same ωWA: WB: WC = 25MR2: 12MR2 :MR2 = 25:12 :1= 4:5:10⇒ WC > WB> WA

Question
CBSEENPH11020851
Wired Faculty App

Which one of the following statements is incorrect?

  • Rolling friction is smaller than sliding friction.

  • Limiting value of static friction is directly proportional to normal reaction.

  • Coefficient of sliding friction has dimensions of length.

  • Frictional force opposes the relative motion.

Solution

C.

Coefficient of sliding friction has dimensions of length.

Coefficient of friction or sliding friction has no dimension.

f = μsN⇒ μs = fN = [M0L0T0]

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