BIO Chin-ups.? While doing a chin-up, a man lifts his body 0.40 m. (a) How much work must the man do per kilogram of body mass? (b) The muscles involved in doing a chin-up can generate about 70 J of work per kilogram of muscle mass. If the man can just barely do a 0.40-m chin-up, what percentage of his body’s mass do these muscles constitute? (For comparison, the ?total percentage of muscle in a typical 70-kg man with 14% body fat is about 43%.) (c) Repeat part (b) for the man’s young son, who has arms half as long as his father’s but whose muscles can also generate 70 J of work per kilogram of muscle mass. (d) Adults and children have about the same percentage of muscle in their bodies. Explain why children can commonly do chin-ups more easily than their fathers.
Solution 64P Step 1 of 4: a)We know that w = mgh w The work per unit mass , m = gh The man does work , 9.8 m/s × 0.4 m = 3.92 J/kg Step 2 of 4: (3.92 J/kg) b) 70 J/kg× 100 = 5.6 %