Thermodynamics

Chapter 4 | 199EXAMPLE 4–14 Burning Off Lunch CaloriesA 90-kg man had two hamburgers, a regular serving of french fries, and a200-ml Coke for lunch (Fig. 4–43). Determine how long it will take for himto burn the lunch calories off (a) by watching TV and (b) by fast swimming.What would your answers be for a 45-kg man?Solution A man had lunch at a restaurant. The times it will take for him toburn the lunch calories by watching TV and by fast swimming are to bedetermined.Assumptions The values in Tables 4–1 and 4–2 are applicable for food andexercise.Analysis (a) We take the human body as our system and treat it as a closedsystem whose energy content remains unchanged during the process. Then FIGURE 4–43the conservation of energy principle requires that the energy input into thebody must be equal to the energy output. The net energy input in this case A typical lunch discussed inis the metabolizable energy content of the food eaten. It is determined from Example 4–14.Table 4–1 to be© Vol. 30/PhotoDiscE in 2 E hamburger E fries E cola 2 275 250 87 887 CalThe rate of energy output for a 68-kg man watching TV is given in Table 4–2to be 72 Calories/h. For a 90-kg man it becomes72 Cal>hE out 190 kg2 95.3 Cal>h68 kgTherefore, it will take887 Cal¢t 95.3 Cal>h 9.3 hto burn the lunch calories off by watching TV.(b) It can be shown in a similar manner that it takes only 47 min to burn thelunch calories off by fast swimming.Discussion The 45-kg man is half as large as the 90-kg man. Therefore,expending the same amount of energy takes twice as long in each case:18.6 h by watching TV and 94 min by fast swimming.EXAMPLE 4–15Losing Weight by Switching to Fat-Free ChipsThe fake fat olestra passes through the body undigested, and thus adds zerocalorie to the diet. Although foods cooked with olestra taste pretty good, theymay cause abdominal discomfort and the long-term effects are unknown. A1-oz (28.3-g) serving of regular potato chips has 10 g of fat and 150 Calories,whereas 1 oz of the so-called fat-free chips fried in olestra has only 75Calories. Consider a person who eats 1 oz of regular potato chips every day atlunch without gaining or losing any weight. Determine how much weight thisperson will lose in one year if he or she switches to fat-free chips (Fig. 4–44).FIGURE 4–44Schematic for Example 4–15.