91Ó°ÊÓ

A force is an external factor that can change the rest or motion of a body. It has a size and a general direction.

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

Human hand

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 $F_M$upward, at a distance $l_1$from E. The center of mass of the forearm and the load are at distances $l_2$and$l_3$ from E.

Under the equilibrium of forces, we can write,

Under the equilibrium of the moments about the point E,

$\begin{array}{l}{F}_M{l}_1=W{l}_2+L{l}_3\\ F_M=W\left(\frac{l_2}{l_1}\right)+L\left(\frac{l_3}{l_1}\right)….\left(2\right)\end{array}$

We can write from equation (2) that $\left(\frac{I_2}{I_1}\right)$and $\left(\frac{I_3}{I_1}\right)$are greater than 1. So, the force,

$F_M>L$

Hence, the maximum force the muscle can exert is$1000\mathrm{N}$ , it can lift only less than $1000\mathrm{N}$.