Chapter 4: Statics

(علم السكون)

Torques and Equilibrium Torques and Equilibrium

(العزوم و االتزان)

Outline

� Torques (العزوم)

� Equilibrium of Rigid Bodies ( الصلبة - توازن االجسام الجاسئة )

� The Center of Gravity (مركز الثقل)� The Center of Gravity (مركز الثقل)

� Stability and Balance (االستقرار و التوازن)

� Levers; Mechanical Advantage ( الفائدة االلیة: الروافع )

� Muscles (العضالت)

� Levers in the Body (الروافع في جسم الكائنات الحیة)

Introduction

� Statics is the study of the forces acting on an

object that is in equilibrium and at rest.

� Engineering structures: Bridges or buildings

Biological structures : Jaws (الفك), � Biological structures : Jaws (الفك), limbs(اطراف), or backbones(العمود الفقاري).

� Engineering, comparative anatomy( تشریح)physical therapy ,(مقارن طبیعيالعالج ال ), and

orthodonture (تقویم االسنان).

Torques

�

Torque

Examples

Conditions for a rigid body equilibrium:

�

Types of equilibrium:

� static equilibrium: a state of equilibrium in

which the net external force and torque acting

on a system is zero

� dynamic equilibrium: a state of equilibrium in

which the net external force and torque on a

system moving with constant velocity are

zero

Example

Example 2

Stability and Balance

Example

� Unlike most of the other muscles in our

bodies, the masseter muscle(العضلة الماضغة) in

the jaw, as illustrated in next slide, is

attached relatively far from the joint, enabling attached relatively far from the joint, enabling

large forces to be exerted by the back teeth.

(a) Using the information in the figure,

calculate the force exerted by the teeth on

the bullet.

(b) Calculate the force on the joint.

A person clenching a bullet

between his teeth

Solution

�

Lever in Human Body

Lever in Human Body

Lever in Human Body

Lever in Human Body

Lever in Human Body

Lever in Human Body

Glossary

� Torque: turning or twisting effectiveness of a force

� Perpendicular lever arm: the shortest distance from the pivot point to the line along which F lieslies

� SI units of torque: newton times meters, usually written as N·m

� Center of gravity: the point where the total weight of the body is assumed to be concentrated

Exercise 4-13

� Find the force F1 and F2 on the tooth in fig.

4.36 (In orthodonture, forces applied to the

teeth lead to forces on the supporting bones,

Gradually the bone tissue breaks down and Gradually the bone tissue breaks down and

permits the tooth to rotate or translate. New

bone tissue grows in the space left behind.

The forces must be small enough to avoid

damaging the root of the tooth.

Solution

�

Types of Levers

Exercise 37

� The head pivots about the atlanto-occipital

joint (Fig.4.46). The splenius muscles

attached behind the joint support the head.

(a) What class of lever does this represent? (a) What class of lever does this represent?

Class 1

(b) The anterior muscles (العضالت االمامیة)produce

forward motions of the head. What class of

levers does their action represent? Class 3