Problems Reference Book: Properties of Matter – Dr. Tofazzal Hossain 1. A mass of 1.0 kg is being rotated in a circle at the end of a string 0.5 m long, the other end of which is held in the hand. If the mass makes 50 revolutions per second, what is its angular momentum? If the number of revolutions decreases by 2 in 30 sec, calculate the mean value of the torque on the system. 2. A circular disc of 49 kg weights and of radius 50 cm is rotating. Calculate the kinetic energy possesses when executing 120 rotations per minute. 3. Assume the earth to be sphere of uniform density. [a] What is its rotational kinetic energy? [b] Suppose this energy could be harnessed for our use. For how long could the earth supply 1.0 kW of power to each of the 4.2 x 10 9 persons on earth? Take the radius of the earth to be 6.4 x 10 3 km and the mass of the earth to be 6.0 x 10 24 kg. 4. A circular disc of radius 0.06 m can rotate freely about a horizontal axis passing through its centre. A thread of negligible mass is wound over its circumference and a mass of 0.3 kg is tied to the free end
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