Friday, August 20, 2010

HOMEWORK SET 4

Chapter 15: Problems 1, 3, 9, 13, 19, 25, 29, 31, 39, on pp 530-534.

1. Find the weight (in Newtons) of the air in your physics class. Assume that the room is a cube of 4 meters on a side, and that the density of air is rair = 1.29 kg/m3.

3. You buy a gold ring at a pawn shop, and run a “test” to determine if it's pure gold. The ring's mass is m = 0.014 grams, and has a volume of 0.022 cm3. The density of gold is rAu = 19.3 kg/m3.

9. Crutches will often have rubber tips at the bottom, so that contact with a flooring surface will decrease scratching of the floor, increase friction, and its broader tip spreads the pressure out over a larger area. Assume the radius of the tip of the cane to be 1.2 cm, and that the radius of the rubber tip is 2.5 cm. What is the ratio of the pressure of the cane alone, P1, to the pressure with the rubber tip, P2: P1/P2.

13. Two drinking glasses, G1 and G2, are filled with water to the same level of 5.0 cm. The diameter of the base of G1 is twice the diameter of the base of G2, but otherwise the two glasses are identical.
a. Is the weight of the water in G1 greater, less, or the same as weight in G2?
b. Is the water pressure at the bottom of G1 greater, less, or the same as the water pressure at the bottom of G2?

19. A circular wine barrel is shown in figure 15-26 on page 530. The barrel has a diameter of 75 cm. If a net upward force exceeds 643 Newtons it will burst. A tube of 1.0 cm in diameter extends into the barrel through a hole in the top. Initially the barrel is filled to the top and the tube is empty above that level. What weight of water must be poured into the tube in order to burst the barrel? And why would anyone want to do that anyway?

25. Referring to example 15-4 on page 507 in the book, suppose that some vegetable oil has been added to both sides of the U-tube. Not YouTube. On the right side of the tube the depth of oil is 5.0 cm as before. On the left side of the tube the depth is 3.0 cm. Find the difference in fluid level between the two sides of the tube.

29. On Wednesday, August 15, 1934, William Beebe and Otis Barton made history by descending in the Bathysphere, a steel sphere 4.75 feet in diameter, to 3028 feet below sea level.
a. as the Bathysphere was lowered, was the buoyant force exerted on it at a depth of 10 feet greater, less, or equal to it at 50 feet?
b. Choose the best explanation from below.
I Magic
II Miracle
III Mayonnaise.

31. A fish called Wanda is carrying a pebble in its mouth swims with a small constant velocity in a small bowl. When the fish drops the pebble to the bottom of the bowl, does the water level rise, fall or stay the same?
39. A hydrometer is a device for measuring fluid density. It's constructed as shown in Figure 15-30 on page 532. If the hydrometer samples fluid 1, the small float inside the tube is submerged to level 1. When fluid 2 is sampled the float is submerged to level 2. Is the density of fluid1 greater, less, or the same as fluid 2's density? [This is how auto mechanics test the antifreeze level in your car engine. Since antifreeze (ethylene glycol) is denser than water, the higher the density of coolant in your radiator the more antifreeze protection the engine has.]

Chapter 16: Problems 1, 3, 9, 13, 19, 25, 29, 31, 39, on pp 566-567.

1. The official record for lowest temperature ever recorded on Earth was set at Vostok, Antarctica, on July 21, 1983. The temperature on that day fell to T = ( – 89.2° C), which is well below the temperature of dry ice (frozen CO2). Give this temperature in degrees Fahrenheit.

3.Normal body temperature for humans is T = 98.6° F. Convert this temperature to degrees:
a. Celsius (aka Centrigrade)
b. Kelvin

9. A world record for the greatest change in temperature was set in Spearfish, North Dakota on January 22, 1943. At 7:30 AM, the temperature was T = ( - 4° F); two minutes later, at 7:32 AM, the temperature was T =+ 45° F. Calculate the mean rate of temperature change, per second, using Celsius or Kelvin scales (they use the same units); Hint: find DT.

13. Referring to Table 16 - 1 on page 545, which would be more accurate for an all-season, outdoor use: a tape measure made of steel or one made of aluminum?

19. It is desired to slip an aluminum ring over a steel bar as shown in Figure 16-14 on page 567. At T = 10.00° C, the inside diameter of the ring is d = 4.000 cm. The bar maintains a constant temperature of T = 10.0° C.
a. In order for the ring to slip over the bar, should the ring be heated, or cooled? Explain.
b. Find the temperature, T, of the ring at which it fits over the over the bar.
c. Who is the Lord of the Rings?

25. An aluminum saucepan with a diameter of d = 25.0 cm and a height of h = 6.0 cm, is filled “to the brim” with H2O. The initial temperature of the pan and water is T = 19° C. But then the pan with the water in it is placed on a range where it is heated to T = 88° C.
a. Will the water overflow from the pan, or will it decrease? Explain.
b. Calculate the volume of water that either overflows or decreases.
c. Go make some popcorn. Turn off the stove.

29. Consider the apparatus the Joule used in his experiments on the mechanical equivalent of heat, shown in Figure 16-8 on page 550. Suppose both blocks have a mass of m1 = m2 = 0.95 kg and that they fall through a distance of h = 0.48 meter.
a. Find the expected rise in temperature, DT, of the water, given that 6200 Joules are needed for every increase of DT = 1.0° C. Give your answer in degrees Celsius (°C).
b. Would the temperature rise in degrees Fahrenheit (°F) be greater than, or less than, the result in part (a)? Explain.
c. Find the rise in temperature, +DT, in Fahrenheit degrees (°F).

31. Two objects are made of the same material, but have different temperatures, T1 and T2. Object 1 has a mass of “m” while object two has a mass of “2m.” If the objects are brought into thermal contact,
a. Is the temperature change of object 1, DT1, greater, lesser, or equal to the temperature change of object 2, DT2?
b. Choose the best explanation from among the following three choices:
I The larger object gives up more heat and therefore its temperature change is greatest
II The heat given up by one object is taken up by the other. Since the objects have the same heat capacity the temperature changes are the same
III One object loses heat of magnitude Q the other gains heat of magnitude Q. With the same magnitude of heat involved the smaller object has the greater temperature change.

39. If 2200 Joules of heat are added to a 190-gram object, its temperature increases by +DT = 12° C.
a. Find the heat capacity of the object.
b. Find the specific heat of the object.

END

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