Question

A force $$F = \alpha \hat i + 3\hat j + 6\hat k$$    is acting at a point $$r = 2\hat i - 6\hat j - 12\hat k.$$    The value of $$\alpha $$ for which angular momentum about origin is conserved is

A. $$-1$$  
B. $$2$$
C. zero
D. $$1$$
Answer :   $$-1$$
Solution :
When the resultant external torque acting on a system is zero, the total angular momentum of a system remains constant. This is the principle of the conservation of angular momentum.
Given, force $$F = \alpha \hat i + 3\hat j + 6\hat k$$    is acting at a point $$r = 2\hat i - 6\hat j - 12\hat k$$
As, angular momentum about origin is conserved.
i.e. $$\tau = {\text{constant}}$$
$$ \Rightarrow {\text{Torque,}}\,\tau = 0 \Rightarrow r \times F = 0$$
\[\left| {\begin{array}{*{20}{c}} {\hat i}&{\hat j}&{\hat k}\\ 2&{ - 6}&{ - 12}\\ \alpha &3&6 \end{array}} \right| = 0\]
$$\eqalign{ & \Rightarrow \left( { - 36 + 36} \right)\hat i - \left( {12 + 12\alpha } \right)\hat j + \left( {6 + 6\alpha } \right)\hat k = 0 \cr & \Rightarrow 0\hat i - 12\left( {1 + \alpha } \right)\hat i + 6\left( {1 + \alpha } \right)\hat k = 0 \cr & \Rightarrow 6\left( {1 + \alpha } \right) = 0 \cr & \Rightarrow \alpha = - 1 \cr} $$
So, value of $$\alpha $$ for angular momentum about origin is conserved, $$\alpha = - 1$$

Releted MCQ Question on
Basic Physics >> Momentum

Releted Question 1

Two particles of masses $${m_1}$$ and $${m_2}$$ in projectile motion have velocities $${{\vec v}_1}$$ and $${{\vec v}_2}$$ respectively at time $$t = 0.$$  They collide at time $${t_0.}$$ Their velocities become $${{\vec v}_1}'$$ and $${{\vec v}_2}'$$ at time $$2{t_0}$$ while still moving in the air. The value of $$\left| {\left( {{m_1}{{\vec v}_1}' + {m_2}{{\vec v}_2}'} \right) - \left( {{m_1}{{\vec v}_1} + {m_2}{{\vec v}_2}} \right)} \right|$$        is

A. zero
B. $$\left( {{m_1} + {m_2}} \right)g{t_0}$$
C. $$\frac{1}{2}\left( {{m_1} + {m_2}} \right)g{t_0}$$
D. $$2\left( {{m_1} + {m_2}} \right)g{t_0}$$
Releted Question 2

Two blocks of masses $$10kg$$  and $$4kg$$  are connected by a spring of negligible mass and placed on a frictionless horizontal surface. An impulse gives a velocity of $$14 m/s$$  to the heavier block in the direction of the lighter block. The velocity of the centre of mass is

A. $$30 m/s$$
B. $$20 m/s$$
C. $$10 m/s$$
D. $$5 m/s$$
Releted Question 3

A ball of mass $$0.2kg$$  rests on a vertical post of height $$5m.$$  A bullet of mass $$0.01kg,$$  traveling with a velocity $$V m/s$$  in a horizontal direction, hits the centre of the ball. After the collision, the ball and bullet travel independently. The ball hits the ground at a distance of $$20m$$  and the bullet at a distance of $$100m$$  from the foot of the post. The velocity $$V$$ of the bullet is
Momentum mcq question image

A. $$250 m/s$$
B. $$250\sqrt 2 \,m/s$$
C. $$400 m/s$$
D. $$500 m/s$$
Releted Question 4

A particle of mass $$m$$ is projected from the ground with an initial speed $${u_0}$$ at an angle $$\alpha $$ with the horizontal. At the highest point of its trajectory, it makes a completely inelastic collision with another identical particle, which was thrown vertically upward from the ground with the same initial speed $${u_0}.$$ The angle that the composite system makes with the horizontal immediately after the collision is

A. $$\frac{\pi }{4}$$
B. $$\frac{\pi }{4} + \alpha $$
C. $$\frac{\pi }{2} - \alpha $$
D. $$\frac{\pi }{2}$$

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