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Chapter 7 : System of Particles and Rotational Motion

NCERT

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questions: 45
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Question 33
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Separation of Motion of a system of particles into motion of the centre of mass and motion about the centre of mass : 
(a) Show
where  is the momentum of the th particle (of mass ) and . Note  is the velocity of the th particle relative to the centre of mass. 
Also, prove using the definition of the center of mass
(b) Show
where  is the total kinetic energy of the system of particles,  is the total kinetic energy of the system when the particle velocities are taken with respect to the centre of mass and is the kinetic energy of the translation of the system as a whole (i.e. of the centre of mass motion of the system). The result has been used in Sec. 7.14. 
(c) Show
where  is the angular momentum of the system about the centre of mass with velocities taken relative to the centre of mass. Remember ; rest of the notation is the standard notation used in the chapter. Note  and  can be said to be angular momenta, respectively, about and of the centre of mass of the system of particles.
(d) Show
Further, show that 

where  is the sum of all external torques acting on the system about the centre of mass.
(Hint : Use the definition of centre of mass and Newtons Third Law. Assume the internal forces between any two particles act along the line joining the particles.)
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