Chapter Four: Statics

1. Chapter 4: Statics
(‫)علم السكون‬
Torques and Equilibrium
(‫)العزوم و التزان‬

2. Outline
 Torques
(‫)العزوم‬
 Equilibrium of Rigid Bodies
(‫) توازن الجسام الجاسئة-الصلبة‬
Center of Gravity (‫)مركز الثقل‬
 Stability and Balance (‫)الستقرار و التوازن‬
 Levers; Mechanical Advantage
(‫)الروافع: الفائدة اللية‬
 Muscles (‫)العضلت‬
 Levers in the Body (‫الروافع في جسم الكائنات‬
‫)الحية‬
 The

3. 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(‫ ,)اطراف‬or backbones(‫.)العمود الفقاري‬
 Engineering, comparative anatomy(‫تشريح‬
‫ ,)مقارن‬physical therapy(‫ ,)العلج الطبيعي‬and
orthodonture (‫.)تقويم السنان‬

4. Torques

5. Torque

6. Examples

7. Conditions for a rigid body equilibrium:

8. 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

9. Example

10. Example 2

11. Stability and Balance

12. 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
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.

13. A person clenching a bullet
between his teeth

14. Solution

15. Lever in Human Body

16. Lever in Human Body

17. Lever in Human Body

18. Lever in Human Body

19. Lever in Human Body

20. Lever in Human Body

21. 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

22. 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.

24. Solution

25. 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 (‫)العضلت المامية‬produce
forward motions of the head. What class of
levers does their action represent? Class 3