Class 11 Physics Rotational Motion Linear Momentum of System of Particles

Linear Momentum of System of Particles

  • The total momentum of a system of particles is equal to the product of the total mass of the system and the velocity of its COM.
  • P = p1 + p2 + …. + pn

          = m1v1 + m2v2 + …. + mnvn

  • P = MV


  • pi = momentum of ith particle ,
  • P = momentum of system of particles ,
  • V = velocity of COM


  • Newton’s Second Law extended to system of particles: dP/dt = Fext .
  • When the total external force acting on a system of particles is zero (Fext = 0), the total linear momentum of the system is constant (dP/dt = 0 => P = constant). Also the velocity of the centre of mass remains constant (Since P = mv = Constant ).
  • If the total external force on a body is zero, then internal forces can cause complex trajectories of individual particles but the COM moves with a constant velocity.
  • Example: Decay of Ra atom into He atom & Rn Atom
  • Case I – If Ra atom was initially at rest, He atom and Rn atom will have opposite direction of velocity, but the COM will remain at rest.

Case II – If Ra atom is having an uniform velocity before , then He and Rn can have complex trajectories but COM will have the same VELOCITY as of Ra atom.


Example - A bullet of mass m is fired at a velocity of v1, and embeds itself in a block of mass M, initially at rest and on a frictionless surface. What is the final velocity of the block?

Solution: Now if we take bullet and block as a system, then no external force is acting on it. So we can conserve momentum. 
(m1v1 + Mv2)before = (m1+ M)vafter
m1v1/( m1+ M) , as the initial velocity of block M is zero.

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