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S E C T I O N 1 2 . 4 • Questions 377 1. Stand with your back against a wall. Why can’t you put your heels firmly against the wall and then bend forward 2. Can an object be in equilibrium if it is in motion? Explain. 3. Can an object be in equilibrium when only one force acts upon it? If you believe the answer is yes, give an example 4. (a) Give an example in which the net force acting on an object is zero and yet the net torque is nonzero. (b) Give 5. Can an object be in equilibrium if the only torques acting on it produce clockwise rotation? 6. If you measure the net force and the net torque on a system to be zero, (a) could the system still be rotating with respect 7. The center of gravity of an object may be located outside the object. Give a few examples for which this is the case. 8. Assume you are given an arbitrarily shaped piece of plywood, together with a hammer, nail, and plumb bob. How could 9. For a chair to be balanced on one leg, where must the cen- ter of gravity of the chair be located? 10. A girl has a large, docile dog she wishes to weigh on a small bathroom scale. She reasons that she can determine her 11. A tall crate and a short crate of equal mass are placed side by side on an incline, without touching each other. As the 13. When you are lifting a heavy object, it is recommended that you keep your back as nearly vertical as possible, lift- 14. What kind of deformation does a cube of Jell-O exhibit when it jiggles? 15. Ruins of ancient Greek temples often have intact vertical columns, but few horizontal slabs of stone are still in place. 12. Q U E S T I O N S A rigid object is in equilibrium if and only if the resultant external force acting on it is zero and the resultant external torque on it is zero about any axis: (12.1) (12.2) The first condition is the condition for translational equilibrium, and the second is the con- The gravitational force exerted on an object can be considered as acting at a single point called the center of gravity. The center of gravity of an object coincides with its cen- ter of mass if the object is in a uniform gravitational field. We can describe the elastic properties of a substance using the concepts of stress and strain. Stress is a quantity proportional to the force producing a deformation; strain is a measure of the degree of deformation. Strain is proportional to stress, and the constant of elastic modulus: (12.5) Three common types of deformation are represented by (1) the resistance of a solid to elongation under a load, characterized by Young’s modulus Y; (2) the resistance of a solid to the motion of internal planes sliding past each other, characterized by the shear modu- lus S; and (3) the resistance of a solid or fluid to a volume change, characterized by the Elastic modulus $ stress strain !
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