A weightlifter works out at the gym each day. Part of her routine is to lie on h

Question

A weightlifter works out at the gym each day. Part of her routine is to lie on her back and lift a 45 kg barbell straight up from chest height to full arm extension, a distance of 0.50 m.

Part A: How much work does the weightlifter do to lift the barbell one time?

Part B: If the weightlifter does 25 repetitions a day, what total energy does she expend on lifting, assuming a typical efficiency for energy use by the body (25%).

Part C: How many 420 Calorie donuts can she eat a day to supply that energy?

Explanation / Answer

(A) Work = force * distance
Weight = 45 * 9.8
Work = 45 * 9.8 * 0.5 = 220.5 J

(B) Work = Energy
Energy =220.5 J /rep * 25 rep = 5512.5 J if 100% efficiency
Since she is only 25% efficient, she has to expend 4 * 5512.5 J = 22050 J of energy to do 5512.5 J of work.

(C) 4.18 J = 1 calorie
22050 J ÷ 4.18 J/cal = 5275.12 calories ÷ 420 cal /donut = 12.56 donuts

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