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Q14 CQ

College Physics (Urone)
Found in: Page 315

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Short Answer

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?

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

See the step by step solution

Step by Step Solution

Step 1: 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 force FM upward, at a distance l1from E. The center of mass of the forearm and the load are at distances l2and l3 from E.

Step 2: Calculation of force

Under the equilibrium of forces, we can write,


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

Under the equilibrium of the moments about the point E,

FMl1=Wl2+Ll3 FM=W I2l1+Ll3l1........2

We can write from equation (1) that,

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

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


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