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Motion of rigid body - class-XI

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A uniform sphere is placed on a smooth horizontal surface and a horizontal force $F$ is applied on it at a distance $'h'$ above the surface. The acceleration of the centre

  1. Is maximum when $h=0$

  2. Is maximum when $h=\dfrac { 2R }{ 5 }$

  3. Is maximum when $h=\dfrac { 7 }{ 5 } R$

  4. Is independent of $h$


Correct Option: D

A block of mass $m$ slides down a smooth vertical circular track. During the motion, the block is in.

  1. Vertical equilibrium

  2. Horizontal equilibrium

  3. Radial equilibrium

  4. None of these.


Correct Option: A

A heavy seesaw (i.e., not massless) is out of balance. A light girl sits on the end that is tilted downward, and a heavy body sits on the other side so that the seesaw now balances. If they both move forward so that they are one-half their original distance from the pivot point (the fulcrum) what will happen to the seesaw?

  1. The side the body is sitting on will tilt downward

  2. The side the girl is sitting on will once again tilt downward

  3. Nothing; the seesaw will still be balanced.

  4. It is impossible to say without knowing the masses and the distances.


Correct Option: B
Explanation:

Let the mass distribution on the left of seesaw be x kg and that on the right be y kg such that $x>y$. And let the mass of the girl be $m _g$ and that of heavy object be $m _h$ such that $m _h>m _g$
Now during the equilibrium position we have the moments about the fulcrum as
$m _h\times \displaystyle\frac{l}{2}+y\times \displaystyle\frac{l}{4}=m _g\times \displaystyle\frac{l}{2}+x\times \displaystyle\frac{l}{4}$
or
$m _h-m _g=\displaystyle\frac{x-y}{2}$
Now when both the girl and the heavy object move half the distance we see that the moment about the pivot by the girl and the heavy object are $m _g\times \displaystyle\frac{l}{4}+x\times \displaystyle\frac{l}{4}$ and $m _h\times \displaystyle\frac{l}{4}+y\times \displaystyle\frac{l}{4}$

Subtracting moment of the girl from the moment of the heavy object
$(m _h+y)\displaystyle\frac{l}{4}-(m _g+x)\displaystyle\frac{l}{4}$
or
$\displaystyle\frac{l}{4}(m _h-m _g+y-x)$
or
$\displaystyle\frac{l}{4}(\displaystyle\frac{x}{2}-\displaystyle\frac{y}{2}+y-x)$
or
$\displaystyle\frac{l}{4}(\displaystyle\frac{y-x}{2})$
We get this value as negative because $x>y$
Thus the moment applied by the girl is more than that of the heavy object implying that the side of the girl tilts downwards

Three weight $W, 2W$ and $3W$, are connected to identical springs suspended from rigid horizontal rod. The assembly of the rod and the weights fall freely. The positions of the weights from the rod are such that

  1. $3W$ will be farthest

  2. $W$ will be farthest

  3. all will be at the same distance

  4. $2W$ will be farthest


Correct Option: C
Explanation:

Since all the weights fall freely under gravity along with the assembly of the rod, the acceleration of all objects is equal to $g$. Hence all the weights have same relative distance between them at all points, and hence no stretching of spring occurs and thus the position(distance of falling) of all the weights would be the same.

Assertion (A) : A wheel may be rotated with uniform angular velocity even though the tangential forces are applied on it.
Reason (R) : Angular acceleration of wheel is zero when tangential force and frictional force produce torques equal in magnitude and opposite in direction.

  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


Correct Option: A
Explanation:

A wheel is rotated with uniform angular velocity even if the tangential force is applied it happens only if a counter torque is acted on the wheel to balance the torque of the applied force. The $ angular$ $ acceleration $ of the wheel is $ zero$ if the torque of the tangential force is balanced by any other force $(i.e. friction)$.

A uniform rule is pivoted at its mid point. A weight of $50\ gf$ is suspended at one end of it. Where should a weight of $100\ gf$ be suspended, to keep the rule horizontal?

  1. $10\ cm$ from mid point

  2. $25\ cm$ from mid point

  3. $20\ cm$ from mid point

  4. $15\ cm$ from mid point


Correct Option: B
Explanation:

To keep the rod horizontal, total torque should be zero

$\Rightarrow \tau = 50gf (0.5) - 100gf (x) = 0$
$\Rightarrow 50gf (0.5) = 100gf (x) $
$\Rightarrow x= 0.25 $
So weight should suspended from $25 cm$ from mid point.
Therefore, B is correct option.

State whether true or false.
A cone resting on its side is an example of a body in neutral equilibrium.

  1. True

  2. False


Correct Option: A
Explanation:

Since, The energy of cone resting on its side is minimum, it will be in neutral equilibrium.

Therefore,statement is true.

State whether true or false.
Self balancing toys have curved and heavy bases.

  1. True

  2. False


Correct Option: A
Explanation:

Since curved and heavy base provide stability to the toys so they have curved and heavy bases.

Therefore, Given statement is true.

Calculate the force required to lift a load of $60\ N$, placed at a distance of $3\ m$, from the fulcrum if the effort force is applied at a distance of $6\ cm$ from the fulcrum.

  1. $300\ N$

  2. $3000\ N$

  3. $1500\ N$

  4. $30\ N$


Correct Option: B
Explanation:

In order to lift the load we have to apply force such that it can balance the torque by load.

Or, Total torque shold be zero.
We know  $\tau = r \times F$
$\Rightarrow \tau _{total} = (3)60 - (0.06)F$... where F is force required.
$\Rightarrow (3)60 = (0.06)F$

$\Rightarrow F = 3000N$
Therefore, B is correct option.

A body is in pure rotation. The linear speed $v$ of the particle, the distance $r$ of the particle from the axis and the angular velocity $\omega$ of the body are related as $\omega=\dfrac{v}{r}$. Thus

  1. $\omega \propto \dfrac{1}{r}$

  2. $\omega \propto r$

  3. $v$

  4. $\omega$ $is\ independent\ of$ $r$


Correct Option: A

When a spinning top slows down, it begins to wobble. This phenomenon can be explained by

  1. gyroscopic precession

  2. inertia of motion

  3. force of gravity

  4. more complicated types of motion are coming into play


Correct Option: D
Explanation:

When the top slows down, air bouancy, gravitational force, inertial movement and other complicated motion concepts act on it resulting into wobbling of the top.

The COM of body in pure rotation does-

  1. translation

  2. rotation

  3. remain steady

  4. none of these


Correct Option: C
Explanation:

Net external force acting on a pure rotating body is zero i.e.  $F _{ext} = 0$

Thus acceleration of COM  $a _{cm} = \dfrac{F _{ext}}{M} = 0$
Hence the COM of body in pure rotation remain steady.

Find the new coordinates of the point (3, 4), if the origin is shifted to the point (1, 3).

  1. (2, 1)

  2. (1, 2)

  3. (1, -2)

  4. (2, -1)


Correct Option: A
Explanation:

Initially the origin was (0,0) and the point was (3,4). If the origin is now shifted to a new point (1,3), then the coordinates of the new point will be (X,Y)  with respect to the new origin(h=1, k=3).  Thus, (3,4) = (X+1, Y+3). Solving, we get, X=2 and Y=1

A bicycle tyre in motion has :

  1. linear motion only

  2. rotatory motion only

  3. linear and rotatory motion

  4. vibratory motion only


Correct Option: C
Explanation:

A bicycle tyre rotates around the center which causes forward motion of the bicycle. Therefore, both rotation and linear motion of tyre is involved. 

Which of the following statements can be suitable?

  1. Rotational motion is a type of circular motion.

  2. Circular motion is similar to rotational motion.

  3. Rotational and circular motion are fundamentally unrelated.

  4. None of these


Correct Option: B
Explanation:

In circular motion, the body revolves around a certain axis. So that body posses a certain angular velocity, angular acceleration and frequency of rotation where these terms are used in rotational motion. Thus circular motion is said to be similar to rotational motion.

Which of the following cannot be considered an example of precession?

  1. A spinning top

  2. A spinning wheel

  3. Rolling ball

  4. Spinning of the earth


Correct Option: C
Explanation:

Spinning of top, spinning wheel and spinning of the earth are purely rotational motion whereas rolling ball exhibits both rotational motion as well as translational motion. As precession is described by rotational motion only, thus rolling of ball cannot be considered as an example of precession.

A solid sphere is given angular velocity $\omega _0 $ and then kept on rough horizontal surface gently. When sphere starts pure rolling its angular velocity $\omega$ is

  1. $
    \dfrac{2}
    {5}\omega _0
    $

  2. $
    \dfrac{3}
    {5}$u

  3. $
    \dfrac{5}
    {7}
    $u

  4. $
    \dfrac{3}
    {7}\omega _0
    $


Correct Option: C

A man stands at the centre of a turn table it extended horizontally, with a $5 kg$ mass hand. He is set into rotation with an angular of one revolution in $2s$. His new angular is he drops his hands to his sides is (Assume moment of inertia of the man is $6 \ kgm^2$. The distance of the wavelength from the axis is $1 m$and final distance is $0.2 m$)

  1. $2.5 \ rev/s$

  2. $1.25 \ rev/s$

  3. $5 \ rev/s$

  4. None of these


Correct Option: A

For a particle showing motion under the force $F=-5{ \left( x-2 \right)  }^{ 2 },$ the motion is

  1. Translatory

  2. Oscillatory

  3. SHM

  4. All of these


Correct Option: B

For a particle showing motion under the force $F=-5{ \left( x-2 \right) },$ the motion is

  1. Translatory

  2. Oscillatory

  3. SHM

  4. Both (2) & (3)


Correct Option: D

Ram says, $'A$ body may be in pure rotation in the presence of a single external force, 'Shyam says, 'This is possible only for a rigid body', then:

  1. Ram's statement is correct

  2. both statements are correct in different situations

  3. both statements are wrong

  4. both statements are stated by physicists


Correct Option: C

What torque will increase angular velocity of a solid disc of mass $16kg$ and diameter $1m$ from zero to $2$rpm in $8s$?

  1. $\cfrac { \pi }{ 4 } N-m$

  2. $\cfrac { \pi }{ 2 } N-m$

  3. $\cfrac { \pi }{ 3 } N-m$

  4. $\pi N-m$


Correct Option: D
Explanation:

Torque $\left( \tau  \right) =I\alpha $
$\tau =\cfrac { 1 }{ 2 } \times M{ R }^{ 2 }\times \cfrac { 2\pi \left( { n } _{ 2 }-{ n } _{ 1 } \right)  }{ t } $
$\therefore$ $\tau =16\times { \left( \cfrac { 1 }{ 2 }  \right)  }^{ 2 }\times \pi \cfrac { \left( 2-0 \right)  }{ 8 } =\pi N-m$

The translation distances (dx, dy) is called as

  1. Translation vector

  2. Shift vector

  3. Both A and B

  4. Neither A nor B


Correct Option: C
Explanation:

Basic theory ,$(dx,dy) $ is called as transitional or shift vector.

Rotation as well as translation motion is an example of

  1. constrained motion

  2. unconstrained motion

  3. perpetual motion

  4. None of these


Correct Option: A
Explanation:

Rotational motion and translational motion both are constrained motion.

Any point on the circumference of a rigid body which is rolling without slipping undergoes :

  1. a circular path

  2. an elleptic path

  3. a cycloid path

  4. an parabolic path


Correct Option: C
Explanation:

Any point on the rigid body, undergoing rolling without slipping, will have linear and angular speed, such that

$linear\ speed = distance\ from\ CoM \times angular\ speed$

Such particles follows cycloid path.

A common example of precession is

  1. a spinning top

  2. a ball bearing

  3. a ball rolling down an inclined plane

  4. a ball sliding down an inclined plane


Correct Option: A
Explanation:

A spinning top is a common example of precession.

Rolling without slipping is an example of

  1. Rotation

  2. Translation

  3. Rotation with translation

  4. None of these


Correct Option: C
Explanation:

When a body is rolling without slipping on the ground, its center of mass exhibits translational motion whereas the body exhibits rotational motion in its center of mass frame. Thus rolling body exhibits both translational motion as well as rotational motion.

A pendulum having a bob of mass m is hanging in a ship sailing along the equator from east to west. When the ship is stationary with respect to water, the tension in the string is $T _0$. Find the difference between tension when the ship is sailing with a velocity $v$.

  1. $m v\omega$

  2. $2 mv \omega$

  3. $\displaystyle \frac{mv \omega}{2}$

  4. $\sqrt{2} m v \omega$


Correct Option: B
Explanation:

 Additional force is Coriolis force which acts perpendicular to the plane of motion. so $\Delta T=2mv\omega $ . 

 This force causes moving objects on the surface of the Earth to be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to the left in the Southern Hemisphere.

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