Question

Consider the free-body diagram of a person holding a ball with their arm extended horizontally, as shown below on the left. The ball weighs WB 30 N, and is held at a distance d0.35 m from the elbow (E). The lower arm weighs WA 10N, with the center of mass distance 1- 0.15 m away from E. The biceps muscle applies a vertical upward force B to the lower arm at distance s - 0.03 m from E. a) What is the net moment generated about point E by the lower arm's and ball's weights? (Units: N-m) b) The lower arm is held steady at angle e -50° from the vertical, as shown below on the right. What are the vertical and horizontal components of the reaction force at the elbow? What is the force generated by the biceps? Are these forces dependent on θ? (Units: N) WA We WA WB

Answer 1

a) Free body diagram- net moment generated by E by weight 's of arm and balls = M M = w_A times 0 . 15 + w_B times 0. 35 = (10 times 0. 15 + 30 times 0. 35) N - m M = 12 N - m (in clock wise direction) moment about E = 0 or mu_E = 0 implies F_B times 0.03 sin 50 degree - 10 times 0. 15 sin 50 degree - 30 times 0 .35 sin 50 degree F_B = 400 N rightarrow force generated by biceps it does 't depend upon angle theta.