Additional Problems (4 problems): 1) A weightlifter squats motionless on a scale

Question

Additional Problems (4 problems): 1) A weightlifter squats motionless on a scale with a barbell held on her shoulders. First, the weightlifter stands up and then she lifts the barbell up over her head and ultimately holding the barbell motionless. If the woman is 120 lb and the barbell's total weight is 70 lb, answer the following: What does the scale read when the weightlifter is initially squatting on the scale? What does the scale read when the weightlifter, after standing, moves the barbells up with an acceleration of 9 ft/s2? What does the scale read when the weightlifter has the barbell up over her head? a. b. c.

Explanation / Answer

(a) Initially when barbell is held on shoulder motionelss, reading on scale would be the sum of the two weights = 120+70 = 190 lbs

(b) When the barbell is moved up with an accleration of 9 ft2/s, the relative acceleration of the barbell to the earth = g+9 = 32.173+9 = 41.173 ft/sec2

so the new weight = 120 + 70*41.173/32.173 = 209.581 lbs.

(c) When barbell is motionless above the head, again there is no other relative force, so scale reads = 120+70 = 190 lbs.

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