Question

Two bodies have their moments of inertia $$I$$ and $$2I$$ respectively about their axis of rotation. If their kinetic energies of rotation are equal, their angular momenta will be in the ratio

A. $$1:2$$
B. $$\sqrt 2 :1$$
C. $$2:1$$
D. $$1:\sqrt 2 $$  
Answer :   $$1:\sqrt 2 $$
Solution :
As for linear motion $$KE = \frac{{{p^2}}}{{2m}}$$
Similarly, for rotational motion $$K{E_{rot}} = \frac{{{L^2}}}{{2I}}$$
As said, $${\left( {KE} \right)_{rot}}$$   remains same.
i.e. $$\frac{1}{2}{I_1}\omega _1^2 = \frac{1}{2}{I_2}\omega _2^2$$
$$\eqalign{ & \Rightarrow \frac{1}{{2{I_1}}}{\left( {{I_1}{\omega _1}} \right)^2} = \frac{1}{{2{I_2}}}{\left( {{I_2}{\omega _2}} \right)^2} \cr & \Rightarrow \frac{{L_1^2}}{{{I_1}}} = \frac{{L_2^2}}{{{I_2}}} \cr & \Rightarrow \frac{{{L_1}}}{{{L_2}}} = \sqrt {\frac{{{I_1}}}{{{I_2}}}} \cr & {\text{but}}\,{I_1} = I,\,{I_2} = 2I \cr & \therefore \frac{{{L_1}}}{{{L_2}}} = \sqrt {\frac{I}{{2I}}} = \frac{1}{{\sqrt 2 }} \cr & {\text{or}}\,\,{L_1}:{L_2} = 1:\sqrt 2 \cr} $$

Releted MCQ Question on
Basic Physics >> Rotational Motion

Releted Question 1

A thin circular ring of mass $$M$$ and radius $$r$$ is rotating about its axis with a constant angular velocity $$\omega ,$$  Two objects, each of mass $$m,$$  are attached gently to the opposite ends of a diameter of the ring. The wheel now rotates with an angular velocity-

A. $$\frac{{\omega M}}{{\left( {M + m} \right)}}$$
B. $$\frac{{\omega \left( {M - 2m} \right)}}{{\left( {M + 2m} \right)}}$$
C. $$\frac{{\omega M}}{{\left( {M + 2m} \right)}}$$
D. $$\frac{{\omega \left( {M + 2m} \right)}}{M}$$
Releted Question 2

Two point masses of $$0.3 \,kg$$  and $$0.7 \,kg$$  are fixed at the ends of a rod of length $$1.4 \,m$$  and of negligible mass. The rod is set rotating about an axis perpendicular to its length with a uniform angular speed. The point on the rod through which the axis should pass in order that the work required for rotation of the rod is minimum, is located at a distance of-

A. $$0.42 \,m$$  from mass of $$0.3 \,kg$$
B. $$0.70 \,m$$  from mass of $$0.7 \,kg$$
C. $$0.98 \,m$$  from mass of $$0.3 \,kg$$
D. $$0.98 \,m$$  from mass of $$0.7 \,kg$$
Releted Question 3

A smooth sphere $$A$$  is moving on a frictionless horizontal plane with angular speed $$\omega $$  and centre of mass velocity $$\upsilon .$$  It collides elastically and head on with an identical sphere $$B$$  at rest. Neglect friction everywhere. After the collision, their angular speeds are $${\omega _A}$$  and $${\omega _B}$$  respectively. Then-

A. $${\omega _A} < {\omega _B}$$
B. $${\omega _A} = {\omega _B}$$
C. $${\omega _A} = \omega $$
D. $${\omega _B} = \omega $$
Releted Question 4

A disc of mass $$M$$  and radius $$R$$  is rolling with angular speed $$\omega $$  on a horizontal plane as shown in Figure. The magnitude of angular momentum of the disc about the origin $$O$$  is
Rotational Motion mcq question image

A. $$\left( {\frac{1}{2}} \right)M{R^2}\omega $$
B. $$M{R^2}\omega $$
C. $$\left( {\frac{3}{2}} \right)M{R^2}\omega $$
D. $$2M{R^2}\omega $$

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Rotational Motion


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