ch11 - CHAPTER 11 FLUIDS CONCEPTUAL QUESTIONS 1 REASONING...

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CHAPTER 11 FLUIDS CONCEPTUAL QUESTIONS ______________________________________________________________________________ 1. REASONING AND SOLUTION A pile of empty aluminum cans has a volume of 1.0 m 3 . This volume includes the inner volume of each can which contains air. It also includes the air in the spaces between the cans. Therefore, most of the volume is occupied by air, not by aluminum. The mass of the pile of cans is not ρ A l V = (2700 kg/m 3 )(1.0 m 3 ) = 2700 kg , because the volume of aluminum in the pile of cans is not 1.0 m 3 . ____________________________________________________________________________________________ 2. REASONING AND SOLUTION Initially, the net force on the plunger is zero. The pressure in the bottle is equal to atmospheric pressure; therefore, the magnitude of the force per unit area pushing outward on the plunger is equal to the magnitude of the force per unit area that pushes inward on the plunger. After the first pull-and-release cycle, the force per unit area pushing outward on the plunger is less than the force per unit area pushing inward, because there is less pressure inside the bottle. Therefore, a larger external force is required to pull the plunger. On the 15th pull, the force per unit area pushing outward is very close to zero. The net force on the plunger is inward and due mostly to atmospheric pressure. Therefore, on the 15th pull, a much greater force will be required than on previous pulls. ____________________________________________________________________________________________ 3 . REASONING AND SOLUTION A person could not balance her entire weight on the pointed end of a single nail, because it would penetrate her skin. According to Equation 11.3, the pressure exerted by the nail is P = F / A where F represents the weight of the person, and A is the area of the tip of the nail. Since the tip of the nail has a very small radius, its area is very small; therefore, the pressure that the nail exerts on the person is large. The reason she can safely lie on a "bed of nails" is that the effective area of the nails is very large if the nails are closely spaced. Thus, the weight of the person F is distributed over all the nails so that the pressure exerted by any one nail is small. ____________________________________________________________________________________________ 4 . REASONING AND SOLUTION As you climb a mountain, your ears "pop" because of the change in atmospheric pressure. a. As you climb up, the outside pressure becomes lower than the pressure in your inner ear. The outward force per unit area on your eardrum is greater than the inward force per unit area; therefore, your eardrum moves outward. b. As you climb down, the outside pressure becomes greater than the pressure in your inner ear. The outward force per unit area on your eardrum is less than the inward force per unit area on your eardrum; therefore, your eardrum moves inward. ____________________________________________________________________________________________
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548 FLUIDS 5 . REASONING AND SOLUTION The bottle of juice is sealed under a partial vacuum. Therefore, when the seal is intact, the button remains depressed, because the pressure inside the bottle is less than the atmospheric pressure outside of the bottle. The force per unit area pushing up on the button from the inside is significantly smaller than the force per unit area pushing down on the outside. When the seal is broken, air rushes inside the bottle. The force pushing up on the button increases and offsets the force pushing down. The natural
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This note was uploaded on 05/09/2008 for the course PHYS 23 taught by Professor Holland during the Spring '08 term at Pacific.

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ch11 - CHAPTER 11 FLUIDS CONCEPTUAL QUESTIONS 1 REASONING...

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