Conservation of Linear Momentum

The linear momentum which is the product of mass and velocity also remains constant if the total force acting on it is zero. Consider two particles A and B. We can call the group of these two particles as System of Particles. The force exerted by A on B and exerted by B on A are called Internal Forces. Force applied on A or B by any external particle is called External Forces.We define the total linear momentum of the system as sum of linear momenta of A and B.If the two particles moving along the same straight same line, the momentum of the system is arithematic sum of their momenta, i.e. $m_1v_1:+:m_2v_2$ , where $m_1$ , $m_2$ are the masses of the particles and $v_1 , v_2$ are their velocities.

Examples of law of conservation of linear momentum:

1. Bomb shell explosion: A bomb shell suddenly explodes in two fragments having masses in the ratio 2 : 1. As there is no external force involved in the process of explosion the momentum of the system should be conserved.In this case the bomb is initially at rest the initial momentum of the system is zero. The conservation of momentum demands that the final momentum should also be zero. The two fragments should carry equal and opposite momentum to make total momentum zero.

2. Recoil of the gun: As the initial momentum before the firing of bullet is zero .So according to the conservation of momentum the final momentum of the system also needs to be zero.The final momentum could be zero only if the gun has a momentum equal and opposite to that of the bullet.

3. Jet engines and Rockets: A rocket at the launching pad has zero momentum.When the chemicals inside the rocket burn a high velocity blast of hot gases is produced.Thses gaes pass out the tail nozzle of the rocket in downward direction on the form of the jet with tremendous velocity.Therefore the rocket moves up with such a velocity so as to make the momentum of the system(rocket +emited gas) zero again.

Sample Questions

(More Questions for each concept available in Login)

Question : 1

Two balls of mass 20 kg and 30 kg are moving with speed 5 m/s and 10 m/s in opposite directions towards each other and collide. What will be the magnitude of total momentum of the system before the collision ?
A. 400 kg m/s	B. 100 kg m/s	C. 200 kg m/s	D. 300 kg m/s
Right Option : A
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Question : 2

A body with mass M and velocity V strikes the wall and rebounds with the same speed. Its change of momentum is
A. zero	B. MV	C. 2MV	D. - MV
Right Option : C
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Question : 3

Two balls of mass 20 Kg and 30 kg are moving with speed 5 m/s in opposite directions towards each other and collide. What will be the mangnitude of total momentum of the system before the collision?
A. -50 Kg m/sec	B. 100 Kg m/sec	C. 200 Kg m/sec	D. 300 Kg m/sec
Right Option : A
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