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A person is using both hands to hold a box on its two handles.
A person is using both hands to hold a box on its two handles. The posture of the person is straight up with arms bent downwards at a 30 degree angle from the shoulder, and upwards at a 60 degree angle at the elbow. Elbow to handle length is 14in, Shoulder to elbow length is 14.5in. The mass of the box is 5 kg (the mass center is located in the middle of the box), the mass of a lower arm plus a hand is 1.5 kg (the mass center is located in the middle of the segment), an upper arm weighs 2 kg (the mass center is located in the middle of this segment), and the total mass of the head and torso is 27 kg. Assuming g = 10 N/kg, and 1 inch = 2.54 cm. How large is the force exerted by the back extensor muscles at the L5/S1 in Newtons? How large is the compression force at the L5/S1? How large is the shear force at the L5/S1?
