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18. Assuming the lower arm has a mass of 2.8 kg and its center of gravity is 12 cm from the elbow-joint pivot, how much force must the extensor muscle in the upper arm exert on the lower arm to hold a 7.5 kg shot put? 2.5 cm 0 cm- Elbow joint A) 100 N B)500 N C)750 N D) 1000 N E)1500 N 19. A boy and a girl are riding on a merry-go-round that is turning. The boy is twice as far as the girl from the merry-go-round's center. If the boy and girl are of equal mass which statement is true about the boy's moment of inertia with respect to the axis of rotation? A) His moment of inertia is 4 times the girl's. B) His moment of inertia is twice the girl's. C) The moment of inertia is the same for both. D) The boy has a greater moment of inertia, but it is impossible to say exactly how mucih more B) The boy has a smaller moment of inertia, but it is impossible to say exactly how much smaller 20. The rotating systems shown in the figure below differ only in that the two identical movable masses are positioned a distance r from the axis of rotation (left), or a distance r/2 from the axis of rotation (right). If you release the hanging blocks simultaneously from rest, A) the block at left lands first. B) the block at right lands first. C) both blocks land at the same time. D) it is impossible to tell which block reaches the bottom first. 12

Answer 1