Angular Momentum and Conservation of Angular Momentum - NEET Physics Questions
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Angular Momentum and Conservation of Angular Momentum

Question 1:

A thin circular ring of mass M and radius R rotates about an axis through its centre and perpendicular to its plane, with a constant angular velocity . Four small spheres each of mass m (negligible radius) are kept gently to the opposite ends of two mutually perpendicular diameters of the ring. The new angular velocity of the ring will be

1. \[ \left( \frac{M}{M+4m} \right)\omega \]
2. \[ \left( \frac{M}{4m} \right)\omega \]
3. \[ \left( \frac{M+4m}{M} \right)\omega \]
4. \[ \left( \frac{M}{M-4m} \right)\omega \]
View Answer

If We take Ring and 4 small blocks as one system net Torque will be zero, Using Principal of conservation of angular momentum.

\[ I_{1}\omega= \left( I_{1}+ I_{2} \right)\omega_{1} \]

\[ MR^{2}\omega= \left( MR^{2}+ 4mR^{2} \right)\omega_{1} \]

\[ \omega_{1}= \left( \frac{M}{M+4m} \right)\omega \]

 

Question 2:

A particle is projected with a speed v at 45° with the horizontal. The magnitude of angular
momentum of the projectile about the point of projection when the particle is at its maximum height h is

1. Zero
2. mvh²/√2
3. mv²h/2
4. mvh/√2
View Answer

Angular Momentum = momentum × ( Perpendicular distance of momentum from axis of rotation )

Angular Momentum = mv cos (45º) × h = mvh/√2

 

Question 3:

What is moment of inertia in terms of angular momentum (L) and kinetic energy (K)

1. \[ \frac{L^{2}}{K} \]
2. \[ \frac{L^{2}}{2K} \]
3. \[ \frac{L}{2K^{2}} \]
4. \[ \frac{L}{2K} \]
View Answer

\[ L = I \omega \]

and

\[ K = \frac{1}{2} I \omega^{2} \]

Squaring L and Dividing it with K we get,

\[ I=  \frac{L^{2}}{2K} \]

Question 4:

A particle of mass m = 5 units is moving with a uniform speed v = 3√2 m in the XOY plane along the line Y = X + 4. The magnitude of the angular momentum about origin is

1. Zero
2. 60 unit
3. 7.5 unit
4. 40√2 unit
View Answer

Distance of line 

\[ ax+by+c=0 \]  from point (x1,y1) is given by 

\[ d = \left( \frac{ax_{1}+ by_{1}+c}{\sqrt{a^{2}+b^{2}}} \right) \]

So, distance of direction of velocity from origin is d= 2√2 

Angular momentum = Perpendicular distance of momentum × momentum = 2√2 × 5 ×3√2= 60 Unit

Question 5:

A conical pendulum consists of a simple pendulum moving in a horizontal circle as shown. C is the pivot, O the centre of the circle in which the pendulum bob moves and ω the constant angular velocity of the bob. If L is the angular momentum about point C, then

1. L is constant ( magnitude as well as direction is constant )
2. Only direction of L is constant
3. Only magnitude of L is constant
4. None of the above.
View Answer

As object is in circular motion angular momentum

\[ \vec L=I\vec\omega \]

Direction of omega is along the axis so, L will have direction along axis OC. So both magnitude

and direction of angular momentum L is constant.

Question 6:

A uniform rectangular plate of mass m which is free to rotate about the smooth vertical hinge passing through the centre and perpendicular to the plate, is lying on a smooth horizontal surface. A particle of mass m moving with speed ‘u’ collides with the plate and sticks to it as shown in figure. The angular velocity of the plate after collision will be

1. \[ \frac{12u}{5a} \]
2. \[ \frac{12u}{19a} \]
3. \[ \frac{3u}{2a} \]
4. \[ \frac{3u}{5a} \]
View Answer

Taking rectangle and the object as one system angular momentum is conserved.

so,  mva= ( m(√5a/2)² + m((2a)²+ a²)/12) ω

\[ \omega = \frac{3u}{5a} \]

Question 7:

A disc of mass M and radius R is rolling with angular speed ω on a horizontal plane as shown. The magnitude of angular momentum of the disc about the origin O is

1. (1/2)MR²ω
2. MR²ω
3. (3/2)MR²ω
4. 2MR²ω
View Answer

For Rolling L = MvR + Iω = M(ωR)R + (MR²/2)ω = (3/2)MR²ω

Question 8:

A thin circular ring of mass M and radius R is rotating about its axis with a constant angular velocity ω. Two objects of mass ‘m’ are attached gently to the ring. The wheel now rotates with an angular velocity.

1. \[ \frac{\omega M }{(M+m)} \]
2. \[ \frac{\omega(M-2m)}{(M+ 2m)}\]
3. \[ \frac{\omega M}{M+2m}\]
4. \[ \frac{\omega (M+2m)}{M}\]
View Answer

We can solve this problem using the principle of conservation of angular momentum since no external torque acts on the system.

Step 1: Initial Angular Momentum

The moment of inertia of a thin circular ring about its axis is:

 

Iinitial=MR2I_{\text{initial}} = M R^2

 

The initial angular momentum is given by:

 

Linitial=Iinitialω=(MR2)ωL_{\text{initial}} = I_{\text{initial}} \cdot \omega = (M R^2) \cdot \omega

 

Step 2: Final Moment of Inertia

When two objects of mass m are attached to the ring, assuming they are symmetrically placed on the ring, their contribution to the moment of inertia is:

 

Iadded=2mR2I_{\text{added}} = 2m R^2

 

Thus, the new total moment of inertia becomes:

 

Ifinal=MR2+2mR2=(M+2m)R2I_{\text{final}} = M R^2 + 2m R^2 = (M + 2m) R^2

 

Step 3: Applying Conservation of Angular Momentum

Since no external torque acts on the system:

 

Linitial=LfinalL_{\text{initial}} = L_{\text{final}}

 

(MR2)ω=(M+2m)R2ω(M R^2) \cdot \omega = (M + 2m) R^2 \cdot \omega'

 

Canceling

R2R^2

from both sides:

 

Mω=(M+2m)ωM \omega = (M + 2m) \omega'

 

Solving for the new angular velocity

ω\omega'

:

 

ω=MωM+2m\omega' = \frac{M \omega}{M + 2m}

 

Final Answer:

 

ω=MωM+2m\omega' = \frac{M \omega}{M + 2m}

 

This shows that the angular velocity decreases after attaching the masses, as expected due to an increase in the moment of inertia.

Question 9:

A particle of mass 5g is moving with a uniform
speed of 3 √2 cm/s in the x–y plane along the line
y= 2 √5 cm. The magnitude of its angular
momentum about the origin in g-cm²/s is

1. 0 (Zero)
2. 30
3. 30√2
4. 30√10
View Answer

The angular momentum LL of a particle about the origin is given by:

L=mvrsinθL = m v r \sin\theta

where:

  • m=5m = 5 g (mass of the particle),
  • v=32v = 3\sqrt{2} cm/s (speed of the particle),
  • r=25r = 2\sqrt{5} cm (perpendicular distance from the origin),
  • θ=90\theta = 90^\circ (since the velocity is along a straight line parallel to the x-axis, the perpendicular distance is directly used).

Since sin90=1\sin 90^\circ = 1, the equation simplifies to:

L=mvrL = m v r

Substituting the given values:

L=(5)×(32)×(25)L = (5) \times (3\sqrt{2}) \times (2\sqrt{5}) L=5×3×2×10L = 5 \times 3 \times 2 \times \sqrt{10} L=3010 g-cm²/sL = 30 \sqrt{10} \text{ g-cm²/s}

Thus, the magnitude of the angular momentum is:

3010 g-cm²/s\mathbf{30\sqrt{10} \text{ g-cm²/s}}

Question 10:

A particle of mass m is projected with a velocity
v making an angle 45° with the horizontal. The
magnitude of the angular momentum of the
projectile about the point of projection when the
particle is at its maximum height h, is

1. zero
2. \[ \frac{mv^{3}}{4 \sqrt{2}g}\]
3. \[ \frac{mv^{3}}{\sqrt{2}g}\]
4. \[ m^{2} \sqrt{2gh^{3}}\]
View Answer

The angular momentum of a projectile about the point of projection when it reaches its maximum height is given by:

 

L=mvxrL = m v_x r

 

where:


  • mm
     

    = mass of the projectile


  • vxv_x
     

    = horizontal component of velocity


  • rr
     

    = perpendicular distance from the point of projection (which is the maximum height hh 

    )

Step 1: Horizontal Component of Velocity

The initial velocity components are:

 

vx=vcos45=v2v_x = v \cos 45^\circ = \frac{v}{\sqrt{2}}

 

vy=vsin45=v2v_y = v \sin 45^\circ = \frac{v}{\sqrt{2}}

 

Since there is no acceleration in the horizontal direction (ignoring air resistance),

vxv_x

remains constant throughout the motion.

Step 2: Maximum Height

Using the kinematic equation:

 

vy2=uy22ghv_y^2 = u_y^2 - 2 g h

 

At maximum height, the vertical velocity becomes zero, so:

 

0=(v2)22gh0 = \left(\frac{v}{\sqrt{2}}\right)^2 - 2 g h

 

Solving for

hh

:

 

h=v22g12=v24gh = \frac{v^2}{2 g} \cdot \frac{1}{2} = \frac{v^2}{4g}

 

Step 3: Angular Momentum Calculation

The angular momentum at maximum height is:

 

L=mvxhL = m v_x h

 

Substituting values:

 

L=m(v2)(v24g)L = m \left(\frac{v}{\sqrt{2}}\right) \left(\frac{v^2}{4g}\right)

 

L=mv2v24gL = m \cdot \frac{v}{\sqrt{2}} \cdot \frac{v^2}{4g}

 

L=mv34g2L = \frac{m v^3}{4g \sqrt{2}}

 

Thus, the magnitude of the angular momentum about the point of projection at maximum height is:

 

mv34g2\frac{m v^3}{4g \sqrt{2}}