Ch.01 General Principles

7/28/2019 Ch.01 General Principles

1/3

2/13/20

01. General Principles

HCM City Univ. of Technology, Faculty of Mechanical Engineering Nguyen Tan Tien

Engineering Mechanics Statics 1.01 General Principles

Chapter Objectives

To provide an introduction to the basic quantities and

idealizations of mechanics

To give a statement of Newtons Laws of Motion and

Gravitation

To review the principles for applying the SI system of units To examine the standard procedures for performing numerical

calculations

To present a general guide for solving problems


Engineering Mechanics Statics 1.02 Force Vectors

1. Mechanics

- Mechanics: the study how body react to the forces acting on

them

- Branches of mechanics

Statics deals with the equilibrium of bodies, that is, those thatare either at rest or move with a constant velocity

Dynamics is concerned with the accelerated motion of bodies



2. Fundamental Concepts

- Basic Quantities

Length, : used to locate the position of a point in space andthereby describe the size of a physical system

Time, : although the principles of statics are timeindependent, this quantity plays an importantrole

in the study ofdynamics

Mass, : mass is a measure of a quantity of matter

Force, : considered as a push orpull exerted by one bodyon another

A force is completely characterized by

- magnitude

- direction

- point of application




- Idealizations

Particle: particle has a mass, but it size can be

neglected

Rigid body: a rigid body can be considered as acombination of a large numberofparticles

Concentrated force: represents the effect of a loading which is

assumed to act at a point on a body



Three forces act

on the hook at .Since theseforces all meet at

a point, then for

any force analysis,we can assume

the hook to be

represented as a

particle

Steel is a common

engineering material

that does not deform

very much under

load. We can

consider this railroadwheel to be a rigid

body acted upon bythe concentrated

force of the rail


- Newtons Laws of Motion

First Law

A particle originally at rest, or moving in an

straight line with constant velocity, willremain in this state provided the particle is

not subjected to an unbalanced force

Second Law A particle acted upon by an unbalanced

force experiences an acceleration that has

the same direction as the force and a

magnitude that is directly proportional to the

force: =

Third Law The mutual forces of action and reaction

between two particles are equal, opposite

and collinear




2/3

2/13/20


- Newtons Law of Gravitational Attraction

Gravitational attraction between any two particles

=

: force of gravitationalbetween the two particles,

: universal constant of gravitation, /

From experiment: = 66.73 10/

: mass of each of the two particles,

: distance between the two particles,

- Mass

Mass is an absolute property of a body

The mass provides a measure of the resistance of a body to a

change in velocity, as defined by Newton's second law of

motion = /HCM City Univ. of Technology, Faculty of Mechanical Engineering Nguyen Tan Tien



- Weigh

Weight refers to the gravitational attractionof the earthon abody or quantity of mass

=

, = 9.8066/

The astronaut is weightless, for all practical purposes, since she is far removed from the

gravitational field of the earth



3. Units of Measurement

- SI units: The International System of units abbreviated SI after

the French Systme InternationaldUnits

- U.S. Customary (FPS)




- Conversion of Units




- Prefixes

- Rules for Use: when performing calculations, represent the

numbers in terms of theirbase orderived unitsby converting

all prefixes to powers of10. E.g. 50() = 50 10()



5. Numerical Calculations

- Dimensional Homogeneity: dimensions have to be the same

on both sides of the equal sign, (e.g. = )

- Significant Figures: the number of significant figures contained

in any number determines the accuracy of the number usean appropriate number of significant figures

E.g. 23400 might have three (234), four (2340), or five (23400)significant figures. To avoid these ambiguities, rewrite

23 400 23.400 1023.4 10- Rounding Off Numbers: be consistent when rounding off

greater than 5, round up: 3528 3530

smaller than 5, round down: 0.03521 0.0352

equal to 5

+ the digit preceding the 5 is an even number: 75.25 75.2

+ the digit preceding the 5 is an odd number: 0.175 0.18




3/3

2/13/20

6. General Procedure for Analysis

Problem Solving Strategy

- Interpret: read carefully and determine what is given and what

is to be found/delivered. Ask if not clear. If necessary,

make assumptions and indicate them

- Plan: think about major steps (or road map) that you will

take to solve a given problem. Think ofalternative/creative solutions and choose the best

one

- Execute: carry out your steps. Use appropriate diagrams and

equations. Estimate your answers, avoid simple

calculation mistakes, Reflect on/revise your work



Documents

Ch.01 General Principles