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Unformatted text preview: C HAPTER 9 Quick Quizzes 1. (a) 2. (c). The blood pressure measured at the calf would be larger than that measured at the arm. If we imagine the vascular system of the body to be a vessel containing a liquid (blood), the pressure in the liquid will increase with depth. The blood at the calf is deeper in the liquid than that at the arm and is at a higher pressure. Blood pressures are normally taken at the arm because that is approximately the same height as the heart. If blood pressures at the calf were used as a standard, adjustments would need to be made for the height of the person, and the blood pressure would be different if the person were lying down. 3. (c). The level of the ice is unaffected by the motion. The acceleration of the elevator is equivalent to a change in the gravitational field. If the elevator accelerates downward, you might be tempted to say that the effect is the same as if gravity decreases—the weight of the ice cube decreases, causing it to float higher in the liquid. Recall, however, that the magnitude of the buoyant force is equal to the weight of the liquid displaced by the ice cube. The weight of the liquid also decreases with the effectively decreased gravity. Because both the weight of the ice cube and the buoyant force decrease by the same factor, the level of the ice cube in the liquid is unaffected. 4. (c). The level of floating of a ship is unaffected by the atmospheric pressure. The buoyant force results from the pressure differential in the fluid. On a highpressure day, the pressure at all points in the water is higher than on a lowpressure day. Because water is almost incompressible, however, the rate of change of pressure with depth is the same, resulting in no change in the buoyant force. 5. (b). The level of the pond falls. This is because the anchor displaces more water while in the boat. A floating object displaces a volume of water whose weight is equal to the weight of the object. A submerged object displaces a volume of water equal to the volume of the object. Because the density of the anchor is greater than that of water, a volume of water that weighs the same as the anchor will be greater than the volume of the anchor. 6. (c). 7. (a). 279 C H A P T E R 9 Problem Solutions 9.1 If wires of diameter 1 mm are wrapped together so that all support nearly equal stress, the number if wires in the cable should be 20.0 kN 100 0.200 kN N = = The approximate crosssectional area of the cable is then ( ) 2 2 1 0.100 cm 100 100 0.785 cm 4 A A π ⎡ ⎤ = = = ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ , and its approximate diameter is ( ) 2 4 0.785 cm 4 1.00 cm A d π π = = = . Diameter ~1 cm 9.2 (a) ( ) ( ) 2 2 3 8 5.00 10 m 4.00 10 N m A stress π − ⎡ ⎤ = × × = ⎢ ⎥ ⎣ ⎦ 4 3.14 10 N × F = ⋅ (b) The area over which the shear occurs is equal to the circumference of the hole times the thickness of the plate. Thus, ( ) ( ) ( 3 3 4 2 2 2 5.00 10 m 5.00 10 m 1.57 10 m A r t π π − − − = ⎡ ⎤ = × × ⎣...
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 Spring '08
 Chilton
 Buoyancy, Force, kg

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