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Chapter 9: Q13 CQ (page 315)

Why are the forces exerted on the outside world by the limbs of our bodies usually much smaller than the forces exerted by muscles inside the body?

Short Answer

Expert verified

The limbs are the third-class lever, so the forces exerted on the outside world by the limbs of our bodies are usually much smaller than the forces exerted by muscles inside the body

Step by step solution

01

The given information

The forces exerted on the outside world by the limbs of our bodies.

The forces exerted by muscles inside the body.

02

The concept of Mechanical Advantage

For a lever, the mechanical advantage is,

FoFi=IiIo

HereF∘is the output force,Fi input force, l∘is the distance of output force from the joint, and liis the distance of the input from the joint.

03

Analysis of the forces

Let the load is on the palm of a person’s hand. The fulcrum is the elbow joint, then. The biceps muscles exert the required input force.

In the case of the human hand, holding an object, the input force is at a closer distance from the fulcrum than the output force; we can write,

li<I∘

So,

Fi>Fo

Hence, the input force is greater than the output force.

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Most popular questions from this chapter

(a) What force must be exerted by the wind to support a \({\rm{2}}{\rm{.50}}\;{\rm{kg}}\)chicken in the position shown inFigure\({\rm{9}}{\rm{.33}}\)? (b) What is the ratio of this force to the chicken’s weight? (c) Does this support the contention that the chicken has a relatively stable construction?

Suppose a \({\rm{900}}\;{\rm{kg}}\)car is on the bridge inFigure\({\rm{9}}{\rm{.34}}\)with its center of mass halfway between the hinges and the cable attachments. (The bridge is supported by the cables and hinges only.) (a) Find the force in the cables. (b) Find the direction and magnitude of the force exerted by the hinges on the bridge.

Two children push on opposite sides of a door during play. Both pushes horizontally and perpendicular to the door. One child push with a force of 17.5 N at a distance of 0.600 m from the hinges, and the second child pushes at a distance of 0.450 m. What force must the second child exert to keep the door from moving? Assume friction is negligible.

InFigure 9.21, the cg of the pole held by the pole vaulter is \(2.00\;{\rm{m}}\)from the left hand, and the hands are \(0.700\;{\rm{m}}\) apart. Calculate the force exerted by (a) his right hand and (b) his left hand. (c) If each hand supports half the weight of the pole inFigure 9.19, show that the second condition for equilibrium(net\(\tau = 0\))is satisfied for a pivot other than the one located at the center of gravity of the pole. Explicitly show how you follow the steps in the Problem-Solving Strategy for static equilibrium described above.

A \({\rm{17}}{\rm{.0}}\;{\rm{m}}\)high and \({\rm{11}}{\rm{.0}}\;{\rm{m}}\)long wall under construction and its bracing are shown in Figure \({\rm{9}}{\rm{.32}}\). The wall is in stable equilibrium without the bracing but can pivot at its base. Calculate the force exerted by each of the 10 braces if a strong wind exerts a horizontal force of \({\rm{650}}\;{\rm{N}}\)on each square meter of the wall. Assume that the net force from the wind acts at a height halfway up the wall and that all braces exert equal forces parallel to their lengths. Neglect the thickness of the wall.
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