Tag: physics

Questions Related to physics

What should be the angular momentum of an electron in Bohr's hydrogen atom whose energy is -0.544 eV?

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. $\large \frac{h}{\pi}$

  2. $\large \frac{3h}{2\pi}$

  3. $\large \frac{5h}{2\pi}$

  4. $\dfrac{2h}{2\pi}$

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Correct Option: C

Angular momentum in $UCM$ is directed:

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. Perpendicular to the plane of $UCM$

  2. Angular radius and away from centre

  3. Along radius and toward the centre

  4. Tangential to the $UCM$

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Correct Option: C

In absence of external forces on a rigid system, which of the following quantities must remain constant?

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. angular momentum

  2. linear momentum

  3. moment of inertia

  4. kinetic energy

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Correct Option: A,B

A particle of mass 2kg located at the  position $\hat{i}+\hat{j}$     has a velocty$2(\hat{i}-\hat{j}+\hat{k})$      it s angular momentum about the x-axis in kg 

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. Zero

  2. 8

  3. 12

  4. 4

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Correct Option: B

Which parameters of all the particles of rotating fan are same :-

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. Only angular $( \theta, \omega, \alpha)$

  2. Only linear (s,v,a)

  3. Both linear and angular

  4. None of them

Show answer
Correct Option: A
Explanation:

In a rotational motion, only the angular parameters ($\theta, w,\alpha$) are same but not the linear parameters.

A man, sitting firmly over a rotating stool has his arms stretched. If he folds his arms, the work done by the man is :

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. zero

  2. positive

  3. negative

  4. may be positive or negative

Show answer
Correct Option: B
Explanation:

When the arms are stretched the rotational inertia is higher. When the man folds his arm he is basically decreasing his rotational inertia thereby gaining more angular velocity using the principle of conservation of angular momentum. This implies more angular distance is covered in the same time giving us positive work.

A 40 Kg mass hanging at the end of a rope of length L oscillates in a vertical plane with an angular amplitude $ \theta _o $. If the breaking strength of the rope is 80 Kg. Wt What is the maximum angular amplitude so that the mass can oscillates without the rope breaking ?

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. $ 90^o $

  2. $ 60^o $

  3. $ 30^o $

  4. $ 45^o $

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Correct Option: A

Angular momentum of an electron having energy -0.85 eV in hydrogen atom, will be 

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. $h/2\pi $

  2. $h/\pi $

  3. $2h/\pi $

  4. $3h/\pi $

Show answer
Correct Option: A

Assertion (A): Even though a planet revolve around the sun in an elliptical orbit, the angular momentum of planet is constant.
Reason (R) : Any force other than mutual gravitational force is absent between the planet and the sun.

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. Both A and R are true and R is correct explanation of A

  2. Both A and R are true and R is not correct explanation of A

  3. A is true and R is false

  4. A is false and R is true

Show answer
Correct Option: A
Explanation:

Angular momentum of planet revolving around the sun is constant because the line of force of the gravitational force passes through the  sun . Therefore torque about the sun is zero.
$\therefore$  Angular momentum will remain constant.  And there is no other force acting on the planet, which results into zero tangential force.

The law of conservation of angular momentum is obtained from Newton's II law in rotational motion when:

physics rotational motion of a rigid body and moment of inertia angular momentum (l) and conservation of angular momentum angular momentum in case of rotation about a fixed axis law of conservation of angular momentum

  1. External torque is maximum

  2. External torque is minimum

  3. External torque is zero

  4. External torque is constant

Show answer
Correct Option: C
Explanation:

Newton's 2nd law says that External Force is directly proportional to the rate of change of momentum. Similarly, external torque is directly proportional to the rate of change of angular momentum. So if angular momentum is conserved i.e., the rate of change of angular momentum is zero then external torque is zero.