A physics student spends part of her day walking between classes or for recreation, during which time she expends energy at an average rate of 280 W. The remainder of the day she is sitting in class, studying, or resting; during these activities, she expends energy at an average rate of 100 W. If she expends a total of 1.1 X 107 J of energy in a 24-hour day, how much of the day did she spend walking?

Solution 93P Step 1: Rate of energy during walking, P walking 280 W That is, P walking 280 J/s 1 hour = 3600 s 1 s = 1/3600 hr Converting the unit from J/s to J/hr Pwalking280 J / 1/3600 hr = 1008000 J/hr Total energy expenditure for 24 hours, E = 1.1 × 10 J 7 Similarly, P other 100 W = 100 J / (1/3600) hr = 360000 J/ hr