53. A bicyclist consumes 9.0 L of water over the span of 3.5 hours during a race. Making the approximation that 80 % of the cyclist’s energy goes into evaporating this water (at 20°C) as sweat, how much energy in kcal did the rider use during the ride? [ Hint : See page 399.] 54.If coal gives off when burned, how much coal isneeded to heat a house requiring for the wholewinter? Assume that 30%of the heat is lost up the chimney. 55. A 15-g lead bullet is tested by firing it into a fixed block of wood with a mass of 35 kg. The block and imbedded bullet together absorb all the heat generated. After thermal equi- librium has been reached, the system has a temperature rise measured as 0.020 C°. Estimate the bullet’s entering speed. 56. A 310-kg marble boulder rolls off the top of a cliff and falls a vertical height of 120 m before striking the ground. Estimate the temperature rise of the rock if 50 % of the heat generated remains in the rock. 57.A 2.3-kg lead ball is placed in a 2.5-L insulated pail ofwater initially at 20.0°C. If the final temperature of thewater–lead combination is 32.0°C, what was the initialtemperature of the lead ball? 58. A microwave oven is used to heat 250 g of water. On its maxi- mum setting, the oven can raise the temperature of the liquid water from 20°C to 100°C in ( a ) At what rate does the oven put energy into the liquid water? ( b ) If the power input from the oven to the water remains constant, determine how many grams of water will boil away if the oven is operated for 2 min (rather than just 1 min 45 s). 1 min 45 s A = 105 s B . 2.0 * 10 5 MJ 30 MJ kg 1.5 m 2 . General Problems 39. (II) How long does it take the Sun to melt a block of ice at 0°C with a flat horizontal area and thickness 1.0 cm? Assume that the Sun’s rays make an angle of 35° with the vertical and that the emissivity of ice is 0.050. 40. (II) Heat conduction to skin . Suppose 150 W of heat flows by conduction from the blood capillaries beneath the skin to the body’s surface area of If the temperature differ- ence is 0.50 C°, estimate the average distance of capillaries below the skin surface. 41.(II) Two rooms, each a cube 4.0 m per side, share a 14-cm-thick brick wall. Because of a number of 100-W lightbulbsin one room, the air is at 30°C, while in the other room it isat 10°C. How many of the 100-W bulbs are needed tomaintain the temperature difference across the wall?
- Spring '14