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| 55423 | For many electronic applications, such as in computer chips, it is desirable to make components as small as possible to keep the temperature of the components low and to increase the speed of the device. Thin metallic coatings (films) can be used instead of wires to make electrical connections. Gold is especially useful because it does not oxidize readily. Its atomic mass is 197 u. A gold film can be no thinner than the size of a gold atom. Calculate the minimum coating thickness, assuming that a gold atom occupies a cubical volume in the film that is equal to the volume it occupies in a large piece of metal. This geometric model yields a result of the correct order of magnitude. |
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| 55424 | A rectangular plate has a length of (21.3 ± 0.2) cm and a width of (9.8 ± 0.1) cm. Calculate the area of the plate, including its uncertainty. |
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| 55425 | Assume that an object covers an area A and has a uniform height h. If its cross-sectional area is uniform over its height, then its volume is given by V = Ah. (a) Show that V = Ah is dimensionally correct. (b) Show that the volumes of a cylinder and of a rectangular box can be written in the form V = Ah, identifying A in each case. (Note that A, sometimes called the “footprint” of the object, can have any shape and the height can be replaced by average thickness in general.) |
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| 55426 | A woman is reported to have fallen 144 ft from the 17th floor of a building, landing on a metal ventilator box, which she crushed to a depth of 18.0 in. She suffered only minor injuries. Neglecting air resistance, calculate (a) The speed of the woman just before she collided with the ventilator, (b) Her average acceleration while in contact with the box, and (c) The time it took to crush the box. |
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| 55427 | Within a complex machine such as a robotic assembly line, suppose that one particular part glides along a straight track. A control system measures the average velocity of the part during each successive interval of time ∆t0 =t0 - 0, compares it with the value vc it should be, and switches a servo motor on and off to give the part a correcting pulse of acceleration. The pulse consists of a constant acceleration am applied for time interval ∆tm=tm-0 within the next control time interval ∆t0. As shown in Fig. P2.35, the part may be modeled as having zero acceleration when the motor is off (between tm and t0). A computer in the control system chooses the size of the acceleration so that the final velocity of the part will have the correct value vc . Assume the part is initially at rest and is to have instantaneous velocity vc at time t0.
(a) Find the required value of am in terms of vc and tm.
(b) Show that the displacement ∆x of the part during the time interval ∆t0 is given by ∆x=vc (t0 - 0.5tm). For specified values of vc and t0,
(c) What is the minimum displacement of the part?
(d) What is the maximum displacement of the part?
(e) Are both the minimum and maximum displacements physically attainable? |
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| 55428 | Why should the amalgam used in dental fillings have the same average coefficient of expansion as a tooth? What would occur if they were mismatched? |
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| 55429 | Why is a person able to remove a piece of dry aluminum foil from a hot oven with bare fingers, while a burn results if there is moisture on the foil? |
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| 55430 | Why is it more comfortable to hold a cup of hot tea by the handle rather than by wrapping your hands around the cup itself? |
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| 55431 | Why does a diatomic gas have a greater energy content per mole than a monatomic gas at the same temperature? |
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| 55432 | Why do heavy draperies over the windows help keep a home cool in the summer, as well as warm in the winter? |
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| 55433 | Why can potatoes be baked more quickly when a metal skewer has been inserted through them? |
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| 55434 | Which is denser, dry air or air saturated with water vapor? Explain. |
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| 55435 | When the metal ring and metal sphere in Figure Q19.18 are both at room temperature, the sphere can just be passed through the ring. After the sphere is heated, it cannot be passed through the ring. Explain. What If? What if the ring is heated and the sphere is left at room temperature? Does the sphere pass through the ring? |
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| 55436 | When the 5-kg block is 6 m from the wall, it is sliding at v1 = 14 m/s. if the coefficient of kinetic friction between the block and the horizontal plane is uk = 0.3, determine the impulse of the wall on the block necessary to stop the block. Neglect the friction impulse acting on the block during the collision. |
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| 55437 | When a sealed Thermos bottle full of hot coffee is shaken, what are the changes, if any, in (a) the temperature of the coffee (b) the internal energy of the coffee? |
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| 55438 | When camping in a canyon on a still night, one notices that as soon as the sun strikes the surrounding peaks, a breeze begins to stir. What causes the breeze? |
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| 55439 | When alcohol is rubbed on your body, it lowers your skin temperature. Explain this effect. |
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| 55440 | What is wrong with the following statement? “Given any two objects, the one with the higher temperature contains more heat.” |
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| 55441 | What is the maximum possible coefficient of performance of a heat pump that brings energy from outdoors at "3.00°C into a 22.0°C house? Note that the work done to run the heat pump is also available to warm up the house. |
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| 55442 | What is the coefficient of performance of a refrigerator that operates with Carnot efficiency between temperatures "3.00°C and &27.0°C? |
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