# NCERT Physics Class 11 Exemplar Ch 7 System of Particles and Rotational Motion Part 7 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q. 26. Two cylindrical hollow drums of radii and of a common height h, are rotating with angular velocities ω (anti-clockwise) and ω (clockwise) , respectively. Their axes, fixed are parallel and in a horizontal plane separated by . They are now brought in contact

(a) Show the frictional forces just after contact.

(b) Identify forces and torques external to the system just after contact.

Answer: where F and F ″ and external forces through support.

External torque , anticlockwise.

(c) What would be the ratio of final angular velocities when friction ceases?

Answer: Let ω1 and ω2 be final angular velocities (anticlockwise and clockwise respectively)

Finally there will be no friction.

Hence,

Q. 27. A uniform square plate S (side c) and a uniform rectangular plate R (sides b, a) have identical areas and masses (Fig.) .

Show that

Answer: Area of square = area of rectangle ➾

And