Suppose we replace the 4.0-kg book in Exercise 9.31 of the biceps muscle with an

Question

Suppose we replace the 4.0-kg book in Exercise 9.31 of the biceps muscle with an elastic exercise rope that obeys Hooke's Law. Assume its force constant k = 600 N/m. (a) How much is the rope stretched (past equilibrium) to provide the same force F_B as in this example? Assume the rope is held in the hand at the same location as the book. (b) What force is on the biceps muscle if the exercise rope is puled straight up so that the forearm makes an angle of 25 degree with the horizontal? Assume the biceps muscle is still perpendicular to the forearm. (a) What force should the woman in Figure 9.45 exert on the floor with each hand to do a push-up? Assume that she moves up at a constant speed. (b) The triceps muscle at the back of her upper arm has an effective lever arm of 1.75 cm. and she exerts force on the floor at a horizontal distance of

Explanation / Answer

a.sum of moments M = 0

F*1.5m - 50kg * 9.8m/s² * 0.9m = 0

F = 294 N

That's 147 N for each hand.

b. F = (20/1.75)*147N = 1680 N

1680N / (50kg * 9.8m/s²) = 3.42 times her weight

c. mgh = 50kg * 9.8m/s² * 0.24m = 117.6 J

d. power = work / time = 25 * 117.6J / 60s = 49 W

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