Question

Question: A father lifts his child as shown inFigure. What force should the upper leg muscle exert to lift the child at a constant speed?

Short answer

The total force exerted by the upper leg muscle is 2254N.

Step-by-step solution

Define the force

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

Free-body diagram

The diagram for the leg muscles is shown below:

Calculation of the force on the upper leg muscle

The net torque about the pivot point which is knee, is zero in equilibrium. We can write

$F_Q\times 2=w_{leg}\times 20+w_{child}\times 38$

Here, the weight of the child is,

$\begin{array}{rcl}{w}_{child}& =& 10\times 9.8\\ & =& 98\text{N}\end{array}$

The weight of the leg is,

$$\begin{gathered} F_Q\times 2=39.2\times 20+98\times 38 \\ {F}_Q=\frac{39.2\times 20+98\times 38}{2} \\ {F}_Q=2254\text{N} \end{gathered}$$

Hence, the force is 2254N.