2. Assume a person bends forward to lift a load "with his back" as shown in Figure P12.42a. The person's spine piv- ots mainly at the fifth lumbar vertebra, with the principal supporting force provided by the erector spinalis muscle in the back. To estimate the magnitude of the forces involved, consider the model shown in Figure P12.42b for a person bending forward to lift a 200-N object. The per- son's spine and upper body are represented as a uniform horizontal rod of weight 350 N, pivoted at the base of the spine. The erector spinalis muscle, attached at a point two thirds of the way up the spine, maintains the position of the back. The angle between the spine and this muscle is 12.0. Find (a) the tension in the back muscle and (b) .the compressional force in the spine. (c) Is this method a good way to lift a load? Explain your answer, using the results of parts (a) and (b). It can be instructive to com- pare a human to other animals. Can you suggest a better method to lift a load? Back muscle Pivot (a) R R 350 NV (b) Figure P12.42 12.0 200 NV 1. A crane of mass 3 000 kg supports a load of 10 000 kg as shown in Figure P12.41. The crane is pivoted with a fric- tionless pin at A and rests against a smooth support at B. Find the reaction forces at A and B. 1.00 m A (3 000 kg) g B 2.00 m 6.00 m Figure P12.41 10 000 kg A 10 000-N shark is supported by a cable attached to a 4.00 m rod that can pivot at the base. Calculate the tension in the tie rope between the rod and the wall, assuming the tie rope is holding the system in the position shown in Fig- ure P12.43. Find the horizontal and vertical forces exerted on the base of the rod. Ignore the weight of the rod. 20.0 60.0 10 000 N Figure P12.43