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Found in: Page 319

### College Physics (Urone)

Book edition 1st Edition
Author(s) Paul Peter Urone
Pages 1272 pages
ISBN 9781938168000

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

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

See the step by step solution

## Step 1: 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.

## Step 2: Free-body diagram

The diagram for the leg muscles is shown below:

## Step 3: 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

${F}_{Q}×2={w}_{leg}×20+{w}_{child}×38$

Here, the weight of the child is,

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

The weight of the leg is,

${F}_{Q}×2=39.2×20+98×38\phantom{\rule{0ex}{0ex}}{F}_{Q}=\frac{39.2×20+98×38}{2}\phantom{\rule{0ex}{0ex}}{F}_{Q}=2254\text{N}$

Hence, the force is 2254N.