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Class 11
Physics :-NCERT Solutions - SYSTEM OF PARTICLES AND ROTATIONAL MOTION

Page No 181:
Question 7.33:
Separation of Motion of a system of particles into motion of the centre of mass and motion about the centre of mass:
(a) Show pi = pmiV
Where pi is the momentum of the ith particle (of mass mi) and p′ i = mi v′ i. Note v′ i is the velocity of the ith particle relative to the centre of mass.
Also, prove using the definition of the centre of mass 
(b) Show K′ + ½MV2
Where K is the total kinetic energy of the system of particles, K′ is the total kinetic energy of the system when the particle velocities are taken with respect to the centre of mass and MV2/2 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 L′ + × MV
Where is the angular momentum of the system about the centre of mass with velocities taken relative to the centre of mass. Remember rr– R; rest of the notation is the standard notation used in the chapter. Note L′ and M× V 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 τext 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 Newton’s Third Law. Assume the internal forces between any two particles act along the line joining the particles.)


Answer:

(a)Take a system of moving particles.

Mass of the ith particle = mi

Velocity of the ith particle = vi

Hence, momentum of the ith particle, pmi vi

Velocity of the centre of mass = V

The velocity of the ith particle with respect to the centre of mass of the system is given as:

vv– … (1)

Multiplying mi throughout equation (1), we get:

mi vmi v– mi V

p’p– ­mi V

Where,

pi’ = mivi’ = Momentum of the ith particle with respect to the centre of mass of the system

pp’i ­+ mi V

We have the relation: p’mivi

Taking the summation of momentum of all the particles with respect to the centre of mass of the system, we get:

(b) We have the relation for velocity of the ith particle as:

vv’V

… (2)

Taking the dot product of equation (2) with itself, we get:

Where,

K =  = Total kinetic energy of the system of particles

K’ =  = Total kinetic energy of the system of particles with respect to the centre of mass

 = Kinetic energy of the translation of the system as a whole

(c) Position vector of the ith particle with respect to origin = ri

Position vector of the ith particle with respect to the centre of mass = ri

Position vector of the centre of mass with respect to the origin = R

It is given that:

rr– R

rr’R

We have from part (a),

pp’i ­+ mi V

Taking the cross product of this relation by ri, we get:

(d) We have the relation:

We have the relation:


SET

NCERT Physics Class 11