General Principles 1 Engineering Mechanics: Statics in SI Units, 12e Copyright © 2010 Pearson Education South Asia Pte Ltd

Chapter Objectives Basic quantities and idealizations of mechanics Newton ’ s Laws of Motion and Gravitation Principles for applying the SI system of units Standard procedures for performing numerical calculations General guide for solving problems

Copyright © 2010 Pearson Education South Asia Pte Ltd Chapter Outline 1.Mechanics 2.Fundamental Concepts 3.Units of Measurement 4.The International System of Units 5.Numerical Calculations 6.General Procedure for Analysis

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.1 Mechanics Mechanics can be divided into 3 branches: - Rigid-body Mechanics - Deformable-body Mechanics - Fluid Mechanics Rigid-body Mechanics deals with - Statics - Dynamics

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.1 Mechanics Statics – Equilibrium of bodies  At rest  Move with constant velocity Dynamics – Accelerated motion of bodies

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Basic Quantities 1.Length - locate the position of a point in space 2.Mass - measure of a quantity of matter 3.Time - succession of events 4.Force - a “push” or “pull” exerted by one body on another

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Idealizations 1.Particles - has a mass and size can be neglected 2.Rigid Body - a combination of a large number of particles 3.Concentrated Force - the effect of a loading

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Newton ’ s Three Laws of Motion First Law “ A particle originally at rest, or moving in a straight line with constant velocity, will remain in this state provided that the particle is not subjected to an unbalanced force ”

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Newton ’ s Three Laws of Motion Second Law “ A particle acted upon by an unbalanced force F experiences an acceleration a that has the same direction as the force and a magnitude that is directly proportional to the force ”

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Newton ’ s Three Laws of Motion Third Law “ The mutual forces of action and reaction between two particles are equal and, opposite and collinear ”

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.2 Fundamentals Concepts Newton ’ s Law of Gravitational Attraction Weight: Letting yields F = force of gravitation between two particles G = universal constant of gravitation m 1,m 2 = mass of each of the two particles r = distance between the two particles

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.3 Units of Measurement SI Units Stands for Système International d’Unités F = ma is maintained only if – 3 of the units, called base units, are defined – 4 th unit is derived from the equation SI system specifies length in meters (m), time in seconds (s) and mass in kilograms (kg) Force unit, Newton (N), is derived from F = ma

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.3 Units of Measurement NameLengthTimeMassForce International Systems of Units (SI) Meter (m)Second (s)Kilogram (kg)Newton (N)

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.3 Units of Measurement At the standard location, g = m/s 2 For calculations, we use g = 9.81 m/s 2 Thus, W = mg (g = 9.81m/s 2 ) Hence, a body of mass 1 kg has a weight of 9.81 N, a 2 kg body weighs N

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.4 The International System of Units Prefixes For a very large or small numerical quantity, units can be modified by using a prefix Each represent a multiple or sub-multiple of a unit Eg: 4,000,000 N = 4000 kN (kilo-newton) = 4 MN (mega- newton) 0.005m = 5 mm (milli-meter)

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.4 The International System of Units

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.5 Numerical Calculations Dimensional Homogeneity Each term must be expressed in the same units Regardless of how the equation is evaluated, it maintains its dimensional homogeneity All terms can be replaced by a consistent set of units

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.5 Numerical Calculations Significant Figures Accuracy of a number is specified by the number of significant figures it contains A significant figure is any digit including zero e.g and have four significant numbers When numbers begin or end with zero, we make use of prefixes to clarify the number of significant figures e.g. 400 as one significant figure would be 0.4(10 3 )

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.5 Numerical Calculations Rounding Off Numbers Accuracy obtained would never be better than the accuracy of the problem data Calculators or computers involve more figures in the answer than the number of significant figures in the data Calculated results should always be “ rounded off ” to an appropriate number of significant figures

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.5 Numerical Calculations Calculations Retain a greater number of digits for accuracy Work out computations so that numbers that are approximately equal Round off final answers to three significant figures

Copyright © 2010 Pearson Education South Asia Pte Ltd 1.6 General Procedure for Analysis To solve problems, it is important to present work in a logical and orderly way as suggested: 1.Correlate actual physical situation with theory 2.Draw any diagrams and tabulate the problem data 3.Apply principles in mathematics forms 4.Solve equations which are dimensionally homogenous 5.Report the answer with significance figures 6.Technical judgment and common sense

Copyright © 2010 Pearson Education South Asia Pte Ltd Example Convert to 2 km/h to m/s. Solution Remember to round off the final answer to three significant figures.