Question

A particle of mass $$1\,kg$$  is thrown vertically upwards with speed $$100\,m/s.$$   After $$5\,s,$$  it explodes into two parts. One part of mass $$400\,g$$  comes back with speed $$25\,m/s,$$  what is the speed of other part just after explosion?

A. $$100\,m/s\,{\text{upwards}}$$  
B. $$600\,m/s\,{\text{upwards}}$$
C. $$100\,m/s\,{\text{downwards}}$$
D. $$300\,m/s\,{\text{upwards}}$$
Answer :   $$100\,m/s\,{\text{upwards}}$$
Solution :
According to 1st equation of motion, velocity of particle after $$5\,s$$
$$\eqalign{ & v = u - gt \cr & v = 100 - 10 \times 5 \cr & = 100 - 50 = 50\,m/s\,\,\left( {{\text{upwards}}} \right) \cr} $$
Applying conservation of linear momentum gives
$$Mv = {m_1}{v_1} + {m_2}{v_2}\,.......\left( {{\text{i}}} \right)$$
Taking upward direction positive, the velocity $${v_1}$$ will be negative.
$$\eqalign{ & \therefore {v_1} = - 25\,m/s, \cr & v = 50\,m/s \cr} $$
$$\eqalign{ & {\text{Also,}}\,\,M = 1\,kg,\,{m_1} = 400\,g = 0.4\,kg \cr & {\text{and}}\,\,\,{m_2} = \left( {M - {m_1}} \right) = 1 - 0.4 = 0.6\,kg \cr} $$
Thus, Eq. (i) becomes,
$$\eqalign{ & 1 \times 50 = 0.4 \times \left( { - 25} \right) + 0.6{v_2} \cr & {\text{or}}\,\,50 = - 10 + 0.6{v_2} \cr} $$
$$\eqalign{ & {\text{or}}\,\,0.6{v_2} = 60 \cr & {\text{or}}\,\,{v_2} = \frac{{60}}{{0.6}} \cr & = 100\,m/s \cr} $$
As $${v_2}$$ is positive, therefore the other part will move upwards with a velocity of $$100\,m/s.$$

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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