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The rate at which work is done by an external force in rotating a rigid object about a fixed axis, or the power delivered, is (10.23) If work is done on a rigid object and the only result of the work is rotation about a fixed axis, the net work done by external forces in rotating the object equals the (10.24) The total kinetic energy of a rigid object rolling on a rough surface without slip- ping equals the rotational kinetic energy about its center of mass, , plus the translational kinetic energy of the center of mass, : (10.28) K " 1 2
I CM &
2 ) 1 2
Mv CM
2 1 2
Mv CM
2 1 2
I
CM &
2 % W " 1 2
I& f
2 % 1 2
I& i
2 ! " 2& Questions 321 What is the angular speed of the second hand of a clock? 2. One blade of a pair of scissors rotates counterclockwise in the xy plane. What is the direction of "? What is the direc- 3. Are the kinematic expressions for !, &, and ( valid when the angular position is measured in degrees instead of in 4. If a car’s standard tires are replaced with tires of larger out- side diameter, will the reading of the speedometer 5. Suppose a " b and M - m for the system of particles de- scribed in Figure 10.8. About which axis (x, y, or z) does the 6. Suppose that the rod in Figure 10.10 has a nonuniform mass distribution. In general, would the moment of inertia 2 /12? If not, could the moment of inertia be calculated without knowledge of the 7. Suppose that just two external forces act on a stationary rigid object and the two forces are equal in magnitude and 8. Suppose a pencil is balanced on a perfectly frictionless table. If it falls over, what is the path followed by the center 9. Explain how you might use the apparatus described in Ex- ample 10.12 to determine the moment of inertia of the .) 10. Using the results from Example 10.12, how would you cal- culate the angular speed of the wheel and the linear speed 1 2
MR 2 1. 11. If a small sphere of mass M were placed at the end of the rod in Figure 10.24, would the result for & be greater than, 12. Explain why changing the axis of rotation of an object changes its moment of inertia. 13. The moment of inertia of an object depends on the choice of rotation axis, as suggested by the parallel-axis theorem. 14. Suppose you remove two eggs from the refrigerator, one hard-boiled and the other uncooked. You wish to deter- 15. Which of the entries in Table 10.2 applies to finding the moment of inertia of a long straight sewer pipe rotating 16. Is it possible to change the translational kinetic energy of an object without changing its rotational energy? 17. Must an object be rotating to have a nonzero moment of inertia? 19. Can a (momentarily) stationary object have a nonzero an- gular acceleration? 20. 18. Q U E S T I O N S |