Question

A bullet of mass $$10\,g$$  moving horizontal with a velocity of $$400\,m/s$$   strikes a wood block of mass $$2\,kg$$  which is suspended by light inextensible string of length $$5 m.$$  As result, the centre of gravity of the block found to rise a vertical distance of $$10\,cm.$$  The speed of the bullet after it emerges of horizontally from the block will be

A. $$100\,m/s$$
B. $$80\,m/s$$
C. $$120\,m/s$$  
D. $$160\,m/s$$
Answer :   $$120\,m/s$$
Solution :
According to the law of conservation of momentum, $${p_i} = {p_f}$$
$$ \Rightarrow \left( {0.01} \right) \times 400 + 0 = 2v + \left( {0.01} \right)v'\,......\left( {\text{i}} \right)$$
Also velocity $$v$$ of the block just after the collision is
$$v = \sqrt {2gh} = \sqrt {2 \times 10 \times 0.1} = \sqrt 2 \,.......\left( {{\text{ii}}} \right)$$
$$ \Rightarrow $$ From Eqs. (i) and (ii), we have
$$v' \approx 120\,m/s$$

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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