Answered: A person bending forward to lift a load…

Principles of Physics: A Calculus-Based Text

5th Edition

ISBN:9781133104261

Author:Raymond A. Serway, John W. Jewett

Publisher:Raymond A. Serway, John W. Jewett

Chapter10: Rotational Motion

Section: Chapter Questions

Problem 79P

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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_o = 195–N object. The spine and upper body are represented as a uniform horizontal rod of weight W_b = 290 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°.

In figure (a), a man bends his back forward to lift a set of weights. The hips are labeled as the pivot and the back muscle is also labeled to the right of the pivot. In figure (b), a horizontal rod is shown. There are two points on the rod. One of the points is on the left end of the rod and an arrow labeled R_x extends rightward from the point while an arrow labeled R_y extends upward from the point. The second point is slightly to the right of the center of the rod. An arrow labeled vector T originates from here and points up and to the left making and angle 12.0° with the horizontal. An arrow labeled W_b originates from the center of the rod and points down, it is shorter than vector T. An arrow labeled W_o originates from the right end of the rob and points down. It is shorter than W_b.

(a) Find the tension in the back muscle.

Your response 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.)

Your response differs from the correct answer by more than 100%. kN

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
= 290 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°.
Back muscle
R,
T 12.0°
Pivot
Rx
W.
W,
(a) Find the tension in the back muscle.
2.705
Your response 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.)
12.5
Your response differs from the correct answer by more than 100%. kN
еВook

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