مواضيع المحاضرة: Mechanics , Statics , Dynamics
قراءة
عرض

E ngineering Mechanics I

Statics
Asst.Lecturer :
Mohanad Nihad

Mohanad N Noaman Electronic Engineering College Ninevah University

2
General Principles
 Mechanics
Is the physical science which deals with the effects of forces on objects .
 The subject of mechanics is logically divided into two parts:
Statics:
which concerns the equilibrium of bodies under action of forces.
Dynamics: which concerns the motion of bodies.

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3
Basic Quantities
Length Time Mass Force
Isthe measure ofthe
succession ofevents
and isabasic quantity
of dynamics .Time is
not directly involved
in the analysis of
statics problems . Is
ameasure ofthe
inertia ofabody,
which isits resistance
to achange of
velocity .Mass can
also bethought ofas
the quantity ofmatter
in abody . Is
the action ofone
body onanother .A
force tends tomove a
body inthe direction
of its action .
Is
used tolocate the
position ofapoint in
space andthereby
describe thesize ofa
physical system.

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Idealizations
 Particle:
Isabody ofnegligible dimensions .In the mathematical sense,a
particle isabody whose dimensions areconsidered tobe near
zero sothat wemay analyse itas amass concentrated atapoint .
Rigid body:
Abody isconsidered rigidwhen thechange indistance between
any two ofits points isnegligible forthe purpose athand.
 Concentrated Force:
Aconcentrated forcerepresents theeffect ofaloading whichis
assumed toact atapoint onabody.

Mohanad N Noaman Electronic Engineering College Ninevah University
5
Newton’s Laws of Motion
LawI: particle remains atrest orcontinues tomove withuniform
velocity (inastraight linewith aconstant speed)ifthere is
no unbalanced forceacting onit.

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Law II: The acceleration of a particle is proportional to the vector sum of forces acting on
it, and is in the direction of this vector sum.
Law III: The forces of action and reaction between interacting bodies are equal in
magnitude, opposite in direction, and collinear (they lie on the same line).

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7
Newton’s Law of Gravitational Attraction
The law of gravitation is expressed by the equation:
Where:
F = force of gravitation between the two particles
G = universal constant of gravitation; according to
experimental evidence,
G = 66.73( ��������
−�������� ) m³ / (kg . s²)
���� ���� , ����

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Weight (w ):
Anytwoparticles orbodies haveamutual attractive (gravitational) forceacting between them.
• we can develop anapproximate expressionforfinding theweight Wofaparticle havinga
mass ����
1=m.
• Ifwe assume theearth tobe anon-rotating sphereofconstant densityandhaving amass
����
2 = Me ,then ifris the distance between theearth’s centreandtheparticle, wehave
• For most engineering calculations, however,gis determined atsea level andatalatitude of
45°, which isconsidered the“standard location.” :
g= 9.81 m/s²

Mohanad N Noaman Electronic Engineering College Ninevah University
9
Units of Measurement
SI units US Customary (FPS)
Length ( m)
Time ( S)
Mass ( kg)
Force ( N)
1 N = The force required to give ( 1 Kg) of mass an
acceleration of ( 1 m/s²) (N = kg . m/s²) (from F= m.a).
o body of mass 1 kg has a w of 9.81 N Length (
ft)
Time ( S)
Mass (Slug) (
�������� .������
��������
)
Force ( Ib)
g = 32.2 ft/s²

Mohanad N Noaman Electronic Engineering College Ninevah University
10
Common Conversion Factor
In the FPS system (Foot -Pound- Second):
1 ft = 12 in. (inches)
5280 ft= 1 mi (mile),
1000 lb = 1 kip (kilo- pound)
2000 lb = 1 ton.

Mohanad N Noaman Electronic Engineering College Ninevah University
11
SI units rules
•No plurals ( Ex. M=5kg not kgs)
• Use (.) dot to separate units ( Ex. Meter second = m.s)
• Most of units using small letter except Names ( Ex. m, s, kg and the exception are N,
Pa, M,G ).
• Exponential power apply ( Ex. cm² = cm.cm)
• Use Prefixes:

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• Withtheexception ofthe base unitthekilogram, ingeneral avoidtheuse ofaprefix
in the denominator ofcomposite units.For example, donot write N/mm, butrather
kN /m;also, m/mg should bewritten asMm/kg .
• When performing calculations, representthenumbers interms oftheir baseor
derived unitsbyconverting allprefixes topowers of10. The final result should then
be expressed usingasingle prefix.Also, aftercalculation, itis best tokeep
numerical valuesbetween 0.1and 1000; otherwise, asuitable prefixshould be
chosen .For example,
Base units
Prefix

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13
Numerical Calculations
 Dimensional Homogeneity
Theterms ofany equation usedtodescribe aphysical processmustbedimensionally
homogeneous ;that is,each term must beexpressed inthe same units.
ExDistance =speed *time m=
����
����
* ����
 Significant figures
Forlarge numbers, write:24500 24.5*10 3
For small numbers, write:0.000324 0.324* 10 −3
 Rounding OffNumbers
3.6675 =0.68
1 .2331 =1.23
0 .3762 =0.376
 Calculations
Do not round offcalculations untilexpressing thefinal result .

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 Readtheproblem carefully andtrytocorrelate theactual physical situation withthe
theory studied .
 Tabulate theproblem dataanddraw toalarge scale anynecessary diagrams.
 Apply therelevant principles, generallyinmathematical form.When writing any
equations, besure they aredimensionally homogeneous.
 Solve thenecessary equations, andreport theanswer withnomore thanthree significant
figures .
 Study theanswer withtechnical judgment andcommon sensetodetermine whetherornot
it seems reasonable.
General Procedure for Analysis


رفعت المحاضرة من قبل: Mohammed Aldhanuna
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