College Physics:
11th Edition
ISBN: 9781305965515
Author: SERWAY, Raymond A.
Publisher: Brooks/Cole Pub Co
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Question
Chapter 8, Problem 23P
(a)
To determine
The tension in the back muscle.
(b)
To determine
The compressional force in the spine.
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A person bending forward to lift a load "with his back" (Fig. P8.15a) 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 Fig. PB.15b of a person
bending forward to lift a Wo= 185 N object. The spine and upper body are represented as a uniform horizontal rod of weight W, 355 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 the tension in the back muscle and the compressional force in the spine.
tension in the back muscle
KN
compressional force in the spinel
Pivot
Back muscle
Figure PB.15
(A)
KN
(b)
12.0
A 35 N forearm (we are ignoring the hand for this problem) are held at a 45 deg angle to the vertically oriented humerus. The COM of the forearm is located at a distance of 15 cm from the joint center at the elbow,
and the elbow flexor muscles have a 3 cm moment arm. How much force must be exerted by the elbow flexor muscles to maintain this position?
Hide answer choices A
A 35 N
C
81.7 N
123.7 N
D 371 N
Fm
45°
*
--Wt₂
As a swimmer pulls his arm through the water, various muscles exert forces on the upper arm. The figure below shows a force F exerted on the humerus (upper arm bone) by the pectoral muscle. The muscle is connected to the bone d = 7.30 cm from the center point O of the shoulder joint. Find the magnitude of F, if this force's torque on the arm provides half of the total torque balancing the torque produced by the water pushing against the hand. (Let Fhand = 119and D = 33.0 cm. )
Chapter 8 Solutions
College Physics:
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