Tag: physics

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Multiple choice 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

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

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

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

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

  4. $\dfrac{2h}{2\pi}$
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Energy E = -13.6 / n^2 eV. Given E = -0.544 eV, n^2 = 13.6 / 0.544 = 25, so n = 5. Bohr's quantization condition is L = n * h / (2 * pi). For n = 5, L = 5h / (2 * pi).

Multiple choice 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

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

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

  2. Only linear (s,v,a)

  3. Both linear and angular

  4. None of them
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

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

Multiple choice 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

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

  1. zero

  2. positive

  3. negative

  4. may be positive or negative
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

Multiple choice 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

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 ?

  1. $ 90^o $

  2. $ 60^o $

  3. $ 30^o $

  4. $ 45^o $
Reveal answer Fill a bubble to check yourself
A Correct answer
Multiple choice 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

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.

  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
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

Multiple choice 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

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

  1. External torque is maximum

  2. External torque is minimum

  3. External torque is zero

  4. External torque is constant
Reveal answer Fill a bubble to check yourself
C Correct answer
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.