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Chapter 9: Q14 CQ (page 315)

Explain why the forces in our joints are several times larger than the forces we exert on the outside world with our limbs. Can these forces be even greater than muscle forces?

Short Answer

Expert verified

The joints in human bodies act as a third-class lever. The mechanical advantage is less than 1.

Step by step solution

01

Human hand as lever

The action of muscles when an object is placed on human hand palm is depicted as below:

Human hand as lever

The arm is represented by EP, Elbow at E, palm at P. The load of weight L is at palm.

The muscles exert a forceFMupward, at a distance l1from E. The center of mass of the forearm and the load are at distances l2and l3from E.

02

Calculation of force

Under the equilibrium of forces, we can write,

FM=FE+W+L..鈥�(1)

Here, W is the weight of the fore-arm.

Under the equilibrium of the moments about the point E,

FMl1=Wl2+Ll3FM=WI2l1+Ll3l1........2

We can write from equation (1) that,

FE=FM-W+L=Wl2l1+Ll3l1-W+L=Wl2l1-1+Ll3l1-1

As the factors,I2l1-1andI3l1-1are greater than unity,

FE>W+L

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