Chapter 9: Q19P (page 378)
A uniform-density disk of mass 13 kg, thickness 0.5 m. and radius 0.2 m make one complete rotation every 0.6 s. What is the rotational kinetic energy of the disk?
Short Answer
The rotational kinetic energy is,14.26 J .
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Chapter 9: Q19P (page 378)
A uniform-density disk of mass 13 kg, thickness 0.5 m. and radius 0.2 m make one complete rotation every 0.6 s. What is the rotational kinetic energy of the disk?
The rotational kinetic energy is,14.26 J .
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A rod of length Land negligible mass is attached to a uniform disk of mass Mand radius R (Figure 9.64). A string is wrapped around the disk, and you pull on the string with a constant force F . Two small balls each of mass mslide along the rod with negligible friction. The apparatus starts from rest, and when the center of the disk has moved a distance d, a length of string shas come off the disk, and the balls have collided with the ends of the rod and stuck there. The apparatus slides on a nearly frictionless table. Here is a view from above:

(a) At this instant, what is the speed vof the center of the disk? (b) At this instant the angular speed of the disk is . How much internal energy change has there been?
A solid uniform-density sphere is tied to a rope and moves in a circle with speed . The distance from the center of the circle to the center of the sphere is , the mass of the sphere is , and the radius of the sphere is . (a) What is the angular speed ? (b) What is the rotational kinetic energy of the sphere? (c) What is the total kinetic energy of the sphere?
Two people with different masses but equal speeds slide toward each other with little friction on ice with their arms extended straight out to the slide (so each has the shape of a 鈥鈥). Her right hand meets his right hand, they hold hands and spin 90掳, then release their holds and slide away. Make a rough sketch of the path of the center of mass of the system consisting of the two people, and explain briefly. (It helps to mark equal time intervals along the paths of the two people and of their center of mass.)
Show that moment of inertia of a disk of mass M and radius R is . Divide the disk into narrow rings, each of radius r and width dr. The contribution I of by one of these rings is r2dm, where dm is amount of mass contained in that particular ring. The mass of any ring is the total mass times the fraction of the total area occupied by the area of the ring. The area of this ring is approximately . Use integral calculus to add up all the calculations.
A runner whose mass is accelerates from a stop to a speed of in . (A good sprinter can run in about , with an average speed of .) (a) What is the average horizontal component of the force that the ground exerts on the runner鈥檚 shoes? (b) How much displacement is there of the force that acts on the sole of the runner鈥檚 shoes, assuming that there is no slipping? Therefore, how much work is done on the extended system (the runner) by the force you calculated in the previous exercise? How much work is done on the point particle system by this force? (c) The kinetic energy of the runner increases鈥攚hat kind of energy decreases? By how much?
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