engineering mechanic

8/3/2019 Engineering Mechanic

http://slidepdf.com/reader/full/engineering-mechanic 1/5

ENGINEERING

MECHANIC

JJ205

1.0 BASIC CONCEPT ON STATICS

1.1 Understand the basic measurement quantities

1.2 Understanhe basic concept but vital to the statics

1.3 Understand scalars and vectors

1.4 Understand the Newton¶s Laws of Motion

1.5 Apply the principles of SI system and unit



1.1 UNDERSTAND THE BASIC MEASUREMENT QUANTITIES

Fundamental Quantities. Four fundamental quantities commonly used in mechanics are length ,

time , mass , and force. In general , the magnitude of each of these quantities is defined by an

arbitrarily chosen unit or standrand.

Length. The concept of length is needed to locate the position of a point in space and thereby to

describe the size of a physical system. The standard unit of length measurement is the meter (m),

which is represented by 1650 763.73 wavelength of l ight produced from the orange red line

Of the spectrum of krypton 86. All other unit of length are defined in term of this standard. For

example , 1 foot (ft) is equal to 0.3048 m.

Time. The concept of time is conceived by a succession of events. The standard unit used for its

measurement is the second (s), which is based on the duration of 9 192 631 770 cycles of vibration

of an isotope cesium 133.

Mass. The mass of a body is regarded as a quantitative property of matter used to measure the

resistance of matter to a change in velocity . The standard unit of mass is the kilogram (kg), defined

by a bar of platinum iridium alloy kept at the international Bureau of Weights and Measures in

Sevres,France.

Force. In general,force is considered as a push or pull exerted by one body on another. This

interaction can occur when there is either direct contact between bodies , such as a person pushing

on a wall ,or it can occur through a distance by which the bodies are physically separated. Example

of the latter type include gravitational, electrical,and magnetic forces . In any case, a force is

completely characterized by its magnitude, direction and point of application. Most often, engineers

define the standard unit of force using either the newton (N) or the pound (Ib).

System of Unit. The four fundamental quantities-length, time, mass, and force-are not all

independent from one another; in fact, they are related by Newtons second law of motion,

F =ma.hence, the unit use to define force, mass, length, and time cannot all be selected arbitrarily.

The equality F=ma is maintained only if three of the for units, called base units, are arbitrarily

defined and the forth unit is derived from the equation.

Table 1-1 systems of units

Name Length Time Mass Force

International meter second kilogram Newton

system of units (m) (s) (KG) (N)

(SI) 120 ( kg. m/ s2)

U.S customary foot second slug* Pound

(FPS) (s) ( Ib . s2/ft) (Ib)



SCALAR AND VECTORS

SCALAR

A quantity possessing only a magnitude is called scalar . Mass, volume and length are scalar

quantities often used in statics. Scalars are indicating by letters in italic type, such as the scalar

A. The mathematical operations involving scalars follow the same rules as those of element

algebra.

VE CTOR

Vector a quantity that has both magnitude and direction and add according to the parallelogram law.

Vector quantities commonly used in statics are position, force and moment vectors.

For handwritten work, vector is generally represented by a letter with an arrow written over it, such as

. The magnitude is designated by , or simply A.

A vector is represented graphically by an arrow, which is used to define its magnitude, line of action, and

direction. The magnitude of vector is indicated by the length of arrow, and its direction is indicated by

an arrow head.

For example, the vector A shown below has a magnitude of 4 units and is directed upward,

along its line of action, which is above the horizontal. The point 0 is called the tail of the vector; the

point P is the tip.



The International System of Units

The SI system of units is used in this book since it will eventually become the worldwide stardard for

measurement. Consequently, the rules for its use and some of its terminology relevant to mechanics will

now be presented.

Prefixes. When a numerical quantity is either very large or very small the units used to define its size

may be modified by using a prefix. Each represents a multiple or submultiple of a unit which, if applied

successively, moves the deimal point of a numerical quantity to every third place. *For

example,4000000N=4000kN (kilo-newton)=4MN(mega-newton), or0.005m=5mm(milli-meter). Notice

that the SI system does not include the multiple deca(10) or the submultiple centi(0.01), which form

part of the old metric system. Except for some volume and area measurements, the use of these

prefixes is to be avoided in science and engineering.

Rules for Use. The following rules are given for the proper use of the various Si symbols:

1. A symbol is never written with a plural s ,since it ay be confused with the unit for second(s).2. Quantities defined by several units which are multiples of one another are separated by a dot to

avoid confusion with prefix notation, as illustrated by N=kg·m/s²=kg·m·s².

3. Physical constants or numbers having several digits on either side of the decimal point should be

reported with a space between every three digits rather than with a comma,eg; 73 569.213 427.

In the case of four digits on either side of the decimal, the spacing is optional,eg; 8537 or 8 537.

Furthermore,always try to use decimals and avoid fractions; that is,write 15.25,not 15¼.

When learning to use SI units,it is generally agreed that one should not think in terms of conversion

factors between systems. Instead,it is better to think only in terms of SI units. A feeling for these

units can only be gained through experience. As a memory aid,it might be helpful to recall that a

standard flashlight battery or a small apple weighs about 1 newton. Your body is a suitable

reference for small distances.

Mass and Weight

The mass of a body is an absolute quantity since its measurement can be made at any location. The

weight of a body,however,is not absolute since it is measured in a gravitational field and hence its

magniture depends upon where the measurement is made.

The mass and weight of a body are measured differently in the SI and FPS systems,and the

method of defining the units should be thoroughly understood.

SI System of Units. In the SI system the mass of the body is specified in ksg and weight must be

calculated using the equation F=ma. Hence,if a body has a mass of m(kg)and is located at a point



where the acceleration due to gravity is g(m/s²),then the weight is expressed in newtons as W-

mg(N). In particular, if the body is located on the earth at sea level and at a latitude of

45°(considered the standard location),the acceleration due to gravity is g=9.806 65m/s². For

calculations, the value g=9.81m/s² will be used,so that

W=mg(N) (g=9.81m/s²)

Therefore, a body of mass 1kg has a weight of 9.81N;a 2-kg body weighs 19.62N; and so on.

FPS System of Units. In the FPS system the weight of the body is specified in lb and the mass must

be calculated from F=ma. Hence,if a body has a weight of W(lb) and is located at a point where the

acceleration due to gravity is g(ft/s²),then the mass is expressed in slugs as m=W/g(slug). Since the

acceleration of gravity at the stardard location is approximately 32.2ft/s² (=9.81m/s²),the mass of

the body measured in slugs is

Therefore, a body weighing 32.2lb has a mass of 1 slug;a 64.4-lb body has a mass of 2 slug;and so

on.

engineering mechanic

Documents