A person bending forward to lift a load "with his back" (Figure a) rather than "with his knees" can be injured by large forces exerted on the muscles and vertebrae. The spine pivots mainly at the fifth lumbar vertebra, with the principal supporting force provided by the erector spinalis muscle in the back. To see the magnitude of the forces involved, and to understand why back problems are common among humans, consider the model shown in Figure b, of a person bending forward to lift a W. 195-N object. The spine and upper body are represented as a uniform horizontal rod of weight W=305 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 12.0⁰. Back muscle Pivot R R₂ b T 12.0° W₂ W₂ ↑ (a).Find the tension in the back muscle. 1.114 x Enter a number. differs from the correct answer by more than 10%. Double check your calculations. kN (b) Find the compressional force in the spine. (Enter the magnitude.) 1.09 X Your response differs from the correct answer by more than 10%. Double check calculations, KN

Two children push on opposite sides of a door during play. Both push horizontally and perpendicular to the door. One child pushes with a force of 12.5 N at a distance of 0.750 m from the hinges, and the second child pushes at a distance of 0.550 m. What force must the second child exert to keep the door from moving? (assume friction is negligible)

What is the student’s weight?

The forearm shown below is positioned at an angle with respect to the upper arm, and a 5.6 kg mass is held in the hand. The total mass of the forearm and hand is 2.1 kg, and their center of mass is 16.1 cm from the elbow. (Assume the biceps muscle exerts a force on the forearm that acts vertically.) 5.6 kg B 37.5 cm 4.0 cm (a) What is the magnitude of the force (in N) that the biceps muscle exerts on the forearm for 0-60%7 N (b) What is the magnitude of the force on the elbow joint (in N) for the same angle? N (c) How do these forces depend on the angle 07 The force that the biceps muscle exerts increases and the force on the elbow joint decreases as increases. They both decrease as @ increases. They both increase as increases. They do not depend on 0. The force that the biceps muscle exerts decreases and the force on the elbow joint increases as increases.

A homeowner is trying to move a stubborn rock from his yard. By using a a metal rod as a lever arm and a fulcrum (or pivot point) the homeowner will have a better chance of moving the rock. The homeowner places the fulcrum a distance ?=0.288 mfrom the rock, which has a mass of 465 kg, and fits one end of the rod under the rock's center of weight.   If the homeowner can apply a maximum force of 671 N at the other end of the rod, what is the minimum total length ? of the rod required to move the rock? Assume that the rod is massless and nearly horizontal so that the weight of the rock and homeowner's force are both essentially vertical. The acceleration due to gravity is ?=9.81 m/s2.