1. Sara, who weighs 72 kg, is on a guilt trip after eating a 663-Calorie (663 kc

Question

1. Sara, who weighs 72 kg, is on a guilt trip after eating a 663-Calorie (663 kcal) breakfast of pancakes loaded with fruits, cream, and syrup. She decides to work out before going to her weekly weight-watchers' meeting. If the height of a single stair is 12 cm, and her body is only 31% efficient in converting chemical energy to mechanical energy, determine the number of stairs Sara must climb to cancel out the Calories consumed.

_____ STAIRS

2. The Sun radiates like a perfect blackbody with an emissivity of exactly 1.

(b) How much power does the Sun radiate per square meter of its surface?
W/m2

Explanation / Answer

mass of the woman m = 72 kg
energy equivalent of the doughnut E = 663 kcal = 663*10^3 cal
....................................................................................
energy equivalent of one jelly doughnut in joules is
E = (663*10^3 cal)(4.186 J / 1 cal)
   = 2775318 J

the height of a single stair h = 12 cm = 0.12 m
work done E = N(mgh)
number of stairs N = work done / mgh
= (2775318 J) / (72 kg)(9.8 m/s2)(0.12 m)
= 32777.3 stairs

mechnical energy E ' = 31 % of E = 0.31 E
mechnical energy E ' = N'(mgh)
N' = E' / mgh
= (0.31 E) / (mgh)
= (0.31)[E / mgh]
= (0.31)N
= (0.31)(32777.3 )
= 10160.9 stairs 10161 stairs

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