Chapter 4: Statics ( السكون (علم

Torques and Equilibrium

( االتزان و (العزوم

Outline

Torques (العزوم) Equilibrium of Rigid Bodies

( - الصلبة الجاسئة االجسام ( توازن The Center of Gravity ( الثقل (مركز Stability and Balance ( التوازن و (االستقرار Levers; Mechanical Advantage

( االلية: الفائدة (الروافع Muscles (العضالت) Levers in the Body ( الكائنات جسم في الروافع(الحية

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),

limbs(اطراف), or backbones( الفقاري .(العمود Engineering, comparative anatomy( تشريح)physical therapy ,(مقارن ال طبيعيالعالج ), and orthodonture ( االسنان .(تقويم

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( الماضغة (العضلةin the jaw, as illustrated in next slide, is 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 lies

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 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

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? Class 1

(b)The anterior muscles ( العضالت .produce forward motions of the head(االماميةWhat class of levers does their action represent? Class 3