221.
Two rotating bodies $$A$$ and $$B$$ of masses $$m$$ and $$2\,m$$ with moments of inertia $${I_A}$$ and $${I_B}\left( {{I_B} > {I_A}} \right)$$ have equal kinetic energy of rotation. If $${L_A}$$ and $${L_B}$$ be their angular momenta respectively, then
As we know that, the kinetic energy of a rotating body,
$$KE = \frac{1}{2}I{\omega ^2} = \frac{1}{2}\frac{{{I^2}{\omega ^2}}}{I} = \frac{{{L^2}}}{{2I}}$$
Also, angular momentum, $$L = I\omega $$
Thus, $${K_A} = {K_B}$$
$$\eqalign{
& \Rightarrow \frac{1}{2}\frac{{L_A^2}}{{{I_A}}} = \frac{1}{2}\frac{{L_B^2}}{{{I_B}}} \cr
& \Rightarrow {\left( {\frac{{{L_A}}}{{{L_B}}}} \right)^2} = \frac{{{I_A}}}{{{I_B}}} \Rightarrow \frac{{{L_A}}}{{{L_B}}} = \sqrt {\frac{{{I_A}}}{{{I_B}}}} \cr
& L \propto \sqrt I \cr
& \therefore {L_A} < {L_B}\,\,\left[ {\because {I_B} > {I_A}} \right] \cr} $$
222.
There is a flat uniform triangular plate $$ABC$$ such that $$AB = 4\,cm,BC = 3 cm$$ and angle $$ABC = {90^ \circ }.$$ The moment of inertia of the plate about $$AB,BC$$ and $$CA$$ as axis is respectively $${I_1},{I_2}$$ and $${I_3}.$$ Which one of the following is true?
Moment of Inertia depend upon mass and distribution of masses as $$I = \Sigma m{r^2}.$$
Further, as the distance of masses is more, more is the moment of Inertia.
If we choose $$BC$$ as axis. Distance is maximum.
Hence, Moment of Inertia is maximum.
$$\therefore {I_2} > {I_1},{I_2} > {I_3}$$
223.
A particle of mass $$2\,kg$$ is on a smooth horizontal table and moves in a circular path of radius $$0.6\,m.$$ The height of the table from the ground is $$0.8\,m.$$ If the angular speed of the particle is $$12\,rad\,{s^{ - 1}},$$ the magnitude of its angular momentum about a point on the ground right under the centre of the circle is
$$\tau = r \times F,$$ where $$r =$$ position vector
$$F = {\text{force}} \Rightarrow \tau = \left| r \right| \cdot \left| F \right|\sin \theta $$
Torque is perpendicular to both $$r$$ and $$F.$$ So, dot product of two vectors will be zero.
$$\therefore \tau \cdot r = 0 \Rightarrow F \cdot \tau = 0$$