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Questions 353 1. Is it possible to calculate the torque acting on a rigid ob- ject without specifying an axis of rotation? Is the torque in- 2. Is the triple product defined by A # (B " C) a scalar or a vector quantity? Explain why the operation (A # B) " C has 3. Vector A is in the negative y direction, and vector B is in the negative x direction. What are the directions of 4. If a single force acts on an object and the torque caused by the force is nonzero about some point, is there any other 5. Suppose that the vector velocity of a particle is completely specified. What can you conclude about the direction of its 6. If a system of particles is in motion, is it possible for the total angular momentum to be zero about some origin? Explain. 7. If the torque acting on a particle about a certain origin is zero, what can you say about its angular momentum about 8. A ball is thrown in such a way that it does not spin about its own axis. Does this mean that the angular momentum is 9. For a helicopter to be stable as it flies, it must have at least two propellers. Why? 10. A particle is moving in a circle with constant speed. Locate one point about which the particle’s angular Why does a long pole help a tightrope walker stay 12. Often when a high diver wants to turn a flip in midair, she draws her legs up against her chest. Why does this make 13. In some motorcycle races, the riders drive over small hills, and the motorcycle becomes airborne for a short time. If 14. Stars originate as large bodies of slowly rotating gas. Be- cause of gravitation, these clumps of gas slowly decrease in If global warming occurs over the next century, it is likely 16. A mouse is initially at rest on a horizontal turntable mounted on a frictionless vertical axle. If the mouse 15. 11. Q U E S T I O N S The net external torque acting on a system is equal to the time rate of change of its angular momentum: (11.13) The z component of angular momentum of a rigid object rotating about a fixed z axis is L z # I) (11.14) where I is the moment of inertia of the object about the axis of rotation and ) is its an- The net external torque acting on a rigid object equals the product of its moment of inertia about the axis of rotation and its angular acceleration: (11.16) If the net external torque acting on a system is zero, then the total angular momen- tum of the system is constant: L i # L f (11.18) Applying this law of conservation of angular momentum to a system whose moment of inertia changes gives I i ) i # I f ) f # constant (11.19) #
! ext # I* #
! ext # d L tot dt |