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Constant angular acceleration - class-XI

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A solid sphere of mass 0.5 kg and diameter 1 m rolls without sliding with a constant velocity of 5 m/s, the ratio of the rotational K.E. to the total kinetic energy of the sphere is :

  1. $\cfrac{7}{10}$

  2. $\cfrac{4}{9}$

  3. $\cfrac{2}{7}$

  4. $\cfrac{1}{2}$


Correct Option: B

Analogue of mass in rotational motion is

  1. Moment of inertia

  2. Angular momentum

  3. Gyration

  4. None of these


Correct Option: A
Explanation:

Analogue of mass in rotational motion is moment of inertia. It plays the same role as mass plays in translational motion.

Newton's second law of motion and work done in rotation of a rigid body can be expressed as 

  1. Newton's law cannot be expressed in rotation, work done in rotation is $W=\tau \ theta$

  2. Force and work done are expressed as $\tau = I \alpha$ and $W=\tau \ theta$

  3. Force can be expressed as $\tau = I \alpha$, while work done will be zero

  4. Force will be zero, since no net displacement is present


Correct Option: B
Explanation:

Newton's second law of motion in kinematics is F =  ma. To express the same in rotation, replace mass by moment of inertia I AND linear acceleration a by angular acceleration $\alpha$. Thus, Newton's law of motion becomes, Torque $\tau = I \alpha$

Similarly work done in kinematics is given by W = F.S. To express the same in rotation, replace Force by torque $\tau$ AND linear displacement by angular displacement $\theta$. Thus, Work done becomes, Torque $W= \tau \theta$

How do you express Newton's second law of motion in differential form

  1. $\tau=dL/dt$

  2. $\tau=dp/dt$

  3. $\tau=mdv/dt$

  4. $\tau=md\alpha/dt$


Correct Option: A

If Kinetic energy is expressed as $mv^2/2$ for a particle undergoing uniform velocity motion, How is the kinetic energy expressed in case of the same particle, if it was rotating:

  1. $m \omega^2/2$

  2. $I \omega^2/2$

  3. $m V^2/2$

  4. $I V^2/2$


Correct Option: B
Explanation:

In rotation, m is replaced by I and v by  $\omega$. Thus the expression for kinetic energy becomes $I \omega^2/2$

The correct option is thus option (b)

A sphere rolls down on an inclined plane of inclination $\theta$. What is the acceleration as the sphere reaches bottom?

  1. $\dfrac { 5 }{ 7 } g\sin \theta$

  2. $\dfrac { 3 }{ 5 } g\sin \theta$

  3. $\dfrac { 2 }{ 7 } g\sin \theta$

  4. $\dfrac { 2 }{ 5 } g\sin \theta$


Correct Option: A
Explanation:

Net force = $mg \sin \theta$

Friction force = $F (\uparrow)$
For linear motion, 
$mg \, \sin \theta = f = mg$ ...(1)
angular motion 
$fR = I \alpha $ .... (2)
$\therefore mg \, \sin \theta = ma + \dfrac{I \alpha}{R}$ ....(3)
$a = \alpha R$
$\therefore a = \dfrac{59}{7} \sin \theta$

What is the displacement of the point on the wheel initially in contact with the ground when the wheel rolls forwards half of revolution? Take the radius of the wheel as $'R'$ and the x-axis in the forward direction

  1. $R\sqrt{\pi^2 + 9}, Tan^{-1}\left(\dfrac{3}{\pi}\right)$ with x-axis

  2. $R\sqrt{\pi^2 + 4}$ and angle $Tan^{-1}\left(\dfrac{2}{\pi}\right)$ with x-axis

  3. $R\sqrt{\pi^2 + 16}, Tan^{-1}\left(\dfrac{4}{\pi}\right)$ with x-axis

  4. None


Correct Option: B

A solid cylinder rolls down a rough inclined plane without slipping. As it goes down, what will happen due to force of friction?

  1. Decrease its mechanical kinetic energy

  2. Increase its translational energy

  3. Increases its rotational kinetic energy

  4. Decreases its potential energy


Correct Option: C

A ball rolling off the top of a staicase of each step with height H and width W, with an initial velocity U will just hit nth step. Then n = 

  1. $\frac{2U^2H^2}{gW}$

  2. $\frac{2U^2H^2}{gW^2}$

  3. $\frac{2U^2H}{gW^2}$

  4. $\frac{2UH^2}{gW^2}$


Correct Option: C

A small charged ball of mass m and charge q is suspended from the highers point of a ring of radius R by means of an insulated code of negligible mass.The ring is made of a rigid wire of negligible cross-section and lies in a vertical plane.On the ring, there is uniformly distributed charge Q of the same as that of q .determine the length of the cord so as the equilibrium position of the ball lies on the symmetry axis ,perpendicular to the plane of the ring. 

  1. $\left( \cfrac { 2kQqR }{ mg } \right) ^{ 1/3 }$

  2. $\left( \cfrac { kQqR }{ mg } \right) ^{ 1/3 }$

  3. $\left( \cfrac { kQqR }{ 2mg } \right) ^{ 1/3 }$

  4. $\left( \cfrac { kQqR }{ mg } \right) ^{ 3 }$


Correct Option: D

If a spherical ball rolls on a table without slipping the fraction of its total energy associated with rotation is:

  1. $3/5$

  2. $2/7$

  3. $2/5$

  4. $3/7$


Correct Option: C

A coin placed on a rotating turn table just slips if it is at a distance of $40$ cm from the centre if the angular velocity of the turntable is doubled, it will just slip at a distance of 

  1. 10 cm

  2. 20 cm

  3. 40 cm

  4. 80 cm


Correct Option: C

The minimum coefficient of friction for which the sphere will have pure rolling after some time, for $\theta ={ 45 }^{ 0 }$ is

  1. $\frac { 2 }{ 7 } $

  2. $\frac { 1 }{ 7 } $

  3. $\frac { 2 }{ 5} $

  4. none of these


Correct Option: C

A wheel whose radius is $r$ and moment of inertia about its-own axis is $I$, can rotate freely about its own horizontal axis. A rope is wrapped on the wheel. A boy of mass $m$ is suspended from the free end of the rope. The body is released from rest. The velocity of the body after falling a distance $h$ would be- 

  1. $\left(\dfrac{mgh}{I}\right)^{{1}/{2}}$

  2. $\left(\dfrac{2mgh}{m+I}^{{1}/{2}}\right)$

  3. $\left(\dfrac{2mgh}{m+I/r^2}\right)^{{1}/{2}}$

  4. $\left(\dfrac{m +I}{mgh}\right)^{{1}/{2}}$


Correct Option: C

A solid cylinder (SC),Hollow cylinder (HC)& solid sphere (SS)of same mass & radii are released simultaneously from the same height on an incline. The order in which they will reach the bottom is (From least time to most time order)

  1. SC,HC,SS

  2. SS,SS,HC

  3. SS,SC,HC

  4. HC,SC,SS


Correct Option: C

A solid homogeneous cylinder of height h and base radius r is kept vertically on a conveyer belt moving horizontally with an increasing velocity $v=a+{ bt }^{ 2 }$. If the cylinder is not allowed to slip then the time when the cylinder is about to topple, will be equal to

  1. $\dfrac { 2rg }{ bh } $

  2. $\dfrac { rg }{ bh } $

  3. $\dfrac { 2bg }{ rh } $

  4. $\dfrac { rg }{ 2bh } $


Correct Option: B

A solid cylinder of mass 2 kg rolls down an inclined plane from a height of 4 cm. Its rotational kinetic energy when it reaches the foot of the plane is (g = 10 ${ m/s }^{ 2 })$

  1. 20 J

  2. 40 J

  3. (80/3) J

  4. 80 J


Correct Option: C

A disc of radius R rolls on a horizontal surface with linear velocity $ \overrightarrow {v} = v \hat {i} $ and angular velocity $ \overrightarrow {\omega} = - \omega \hat k $ there is a particle P on the circumference of the disc which has velocity in vertical direction. the height of that particle from the ground will be

  1. $ R + \dfrac {v}{ \omega} $

  2. $ R - \dfrac {v}{ \omega} $

  3. $ R + \dfrac {R}{ 2} $

  4. $ R - \dfrac {R}{ 2} $


Correct Option: B

A wheel of mass M and radius a and M.I. $I _G$ (about centre of mass) is set rolling with angular velocity $\omega$ up a rough inclined plane of inclination $\theta$. The distance travelled by it up the plane is :

  1. $\dfrac{I _G \omega^2}{2Mgsin\theta}$

  2. $\dfrac{\omega^2(Ma^2 + I _G}{2Mgsin\theta}$

  3. $\dfrac{I _G \omega}{Mgsin\theta}$

  4. $\dfrac{I _G \omega}{2Mgsin\theta}$


Correct Option: A

A disc of mass $m$ of radius $r$ is placed on a rough horizontal surface. A cue of mass $m$ hits the disc at a height $h$ from the axis passing through centre and parallel to the surface. The cue stop and falls down after impact. The disc starts pure rolling for

  1. $h < \dfrac{r}{3}$

  2. $h = \dfrac{r}{2}$

  3. $h > \dfrac{r}{2}$

  4. $h\ge \dfrac{r}{2}$


Correct Option: A

A uniform rigid rod has length $L$ and mass $m$. It lies on a horizontal smooth surface, and is rotated at a uniform angular velocity $\omega$ about a vertical axle passing through one of its ends. The force exerted by the axle on the rod will be

  1. $m \omega^2 L$ outward

  2. $m \omega^2 L$ inward

  3. $\dfrac{1}{2} m\omega^2 L$ outward

  4. $\dfrac{1}{2} m\omega^2 L$ inward


Correct Option: A

A disc and a sphere of same radius but different masses roll off on two inclined planes of the same altitude and length. Which one of the two objects gets to the bottom of the plane first?

  1. Both reach at the same time

  2. Depends on their masses

  3. Disc

  4. Sphere


Correct Option: D

A body is given translational velocity and kept on a surface that has sufficient friction. Then:

  1. Body will move forward before pure rolling

  2. Body will move backward before pure rolling

  3. Body will start pure rolling immediately

  4. None of these


Correct Option: A
Explanation:

since the body is given initial translational velocity so it will move forward while coming in pure rolling condition.

so the answer is A.

(a) A child stands at the center of a turntable with his two arms outstretched. The turntable is set rotating with an angular speed of $40$ rev/min. How much is the angular speed of the child if he folds his hands back and thereby reduces his moment of inertia to $2/5$ times the initial value? Assume that the turntable rotates without friction. (b) Show that the child’s new kinetic energy of rotation is more than the initial kinetic energy of rotation. How do you account for this increase in kinetic energy?

  1. $300\ rev/min\ and 2.5 E _1$

  2. $100\ rev/min\ and 2.5 E _1$

  3. $500\ rev/min\ and 7.5 E _1$

  4. none of the above


Correct Option: B
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