Tag: rotational motion of a rigid body and moment of inertia

Questions Related to rotational motion of a rigid body and moment of inertia

A unit mass at position vector $\vec { r } = ( 3 \hat { i } + 4 \hat { j } )$ is moving with a velocity $\vec { v } = ( 5 \hat { i } - 6 \hat { j } )$ What is the angular momentum of the body about the origin

  1. 2 units along $z$ -axis

  2. 38 units along $x$ -axis

  3. 38 units along $y$ -axis

  4. 38 units along $Z$ -axis


Correct Option: C

A man standing on a platform holds weights in his outstretched arms. The system rotates freely about a central vertical axis. If he now draws the weights inwards close to his body, 

  1. the angular velocity of the system will increase.

  2. the angular momentum of the system will decrease.

  3. the kinetic energy of the system will increase.

  4. he will have to expend some energy to draw the weights in.


Correct Option: A,C,D
Explanation:

When the person contracts his hands he is decreasing his moment of inertia thereby increasing his angular velocity using the principle of conservation of angular momentum $I _1\omega _1=I _2\omega _2$
As the angular velocity increases, its kinetic energy also increases. Also energy would be required to draw the weights inwards from its position.

A particle performs uniform circular motion with an angular momentum L. If the frequency of particle's motion is doubled and its kinetic energy halved, the angular momentum becomes:

  1. $2L$

  2. $4L$

  3. $ \mathrm{L} / 2 $

  4. $ \mathrm{L} / 4 $


Correct Option: C

What is the magnitude of vertical force required to produced a moment of 20 Nm at point A (1 m, 1 m) if the force is acting at point B (2m,2m)

  1. 40 N

  2. 30 N

  3. 20 N

  4. 10 N


Correct Option: C

A stone tied to one end of the string is revolved round a rod in such a way that the string winds over the rod and get shortened. In this process which of the following quantities remain constant?

  1. Moment of inertia

  2. Momentum

  3. Angular momentum

  4. Kinetic energy


Correct Option: C
Explanation:

As the distance between the stone and the rotational axis is changing the moment of inertia thus changes. Momentum keeps on changing around the circular path. As the moment of inertia  changes we see that the kinetic energy changes too. The angular momentum is the only conserved quantity since there is no external torque acting on the stone.

A swimmer while jumping into water from a height easily forms a loop in air, if

  1. He pulls his arms and leg in

  2. He spreads legs and his arms

  3. He keeps himself straight

  4. His body is so formed


Correct Option: A
Explanation:

by pulling his arms and legs in he decreases his moment of inertia, thereby increasing his angular velocity using the principle of conservation of angular momentum.

If no external force acts on a system

  1. Velocity of centre of mass remains constant

  2. Total kinetic energy of the system remains constant

  3. Angular momentum of system may change

  4. Total mechanical energy of the system remains constant


Correct Option: A
Explanation:

No force means no acceleration implying constant velocity. Also as velocity is constant kinetic energy remains constant. The angular momentum may change due to the change in moment of inertia. Also the total mechanical energy may change due the change in potential energy.

Two loops P and Q are made from a uniform wire. The radii of P and Q are $r _1$ and $r _2$ respectively, and their moments of inertia about their own axises are $I _1$ and $I _2$ respectively. If $I _2 = 4I _1$, the $r _1/r _2$ equals:

  1. $4^{2/3}$

  2. $4^{1/3}$

  3. $4^{7/3}$

  4. $4^{-1/3}$


Correct Option: C

A particle moves on a circular path with decreasing speed. Choose the correct statement.

  1. Angular momentum remains constant.

  2. Acceleration $\vec{a}$ is towards the centre.

  3. Particle moves in a spiral path with decreasing radius.

  4. The direction of angular momentum remains constant.


Correct Option: C

A particle of mass 2 kg located at the position $ \left( \hat { i } +\hat { j }  \right) m $ has a velocity $ 2\left( \hat { i } -\hat { j } +\hat{ k } \right) ms^{-1} $ . Its angular momentum along z-axis in $ kgm^2 s^{-1}  $ is

  1. $-8$

  2. $+8$

  3. $-4$

  4. zero


Correct Option: A