JJ205-Engineering Mechanics Unit 1

JJ205-ENGINEERING JJ205-ENGINEERING MECHANICSMECHANICS

Fundamental ConceptsWHAT IS MECHANICS?Study of what happens to a “thing” (the technical name is“body”) when FORCES are applied to it.Either the body or the forces could be large or small.

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An Overview of An Overview of MechanicsMechanics

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Particles vs. ObjectsParticles vs. ObjectsParticles: Point mass No geometry Rotation is not important

Objects Contain mass Have geometry Rotation is important

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Systems of UnitsSystems of Units

Four fundamental physical quantities: Length, mass, time, force (derived

unit)

One equation relates them, F=ma We use this equation to develop

systems of units Units are arbitrary names we give to

thephysical quantities

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Newton’s Law of MotionNewton’s Law of Motion First Law:

A particle originally at rest, or moving in a

straight line with constant velocity, willremain in this state provided the particle isnot subjected to an unbalanced force.

Second Law: Force is not balanced: F = ma Third Law:

The mutual forces of action and reactionbetween two particles are equal, oppositeand collinear.

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Mass and WeightMass and WeightMass is an absolute property of a body.

It is independent of the gravitational field in which it is measured. 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 (m = F/a).

The weight of a body is not absolute, since it depends on the gravitational field in which it is measured.Weight is defined as W = mgwhere g is the acceleration due to gravity (9.81m/s2)

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Newton’s Law of Newton’s Law of Gravitational AttractionGravitational Attraction

Any two particles or bodies have a mutually attractive gravitational force acting between them. Newton postulated the law governing this gravitational force as

F = G(m1m2/r2)where F = force of attraction

between the two bodies,G = universal constant of

gravitation ,m1, m2 = mass of each body, andr = distance between centers of

the two bodies.

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Common PrefixesCommon Prefixes

1012 Tera T 10-12 Pico p109 Giga G 10-9 Nano n106 Mega M 10-6 Micro μ103 Kilo k 10-3 Milli m102 Hecto h 10-2 Centi c101 Deka da 10-1 Deci d

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Rules for Using SI SymbolsRules for Using SI SymbolsNo Plurals (e.g., m = 5 kg not kgs )Separate Units with a • (e.g., meter

second = m • s )Most symbols are in lowercase ( some

exception are N, Pa, M and G)Exponential powers apply to units ,

e.g., cm2 = cm •cmOther rules are given in your textbook

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Numerical CalculationNumerical CalculationMust have dimensional “homogeneity.”

Dimensions have to be the same on both sides of the equal sign, (e.g. distance = speed × time.)

Use an appropriate number of significant figures (3 for answer, at least 4 for intermediate calculations).

Be consistent when rounding off.- greater than 5, round up (3528 3530)- smaller than 5, round down (0.03521 0.0352)

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Problem Solving StrategyProblem Solving Strategy1. Interpret: Read carefully and determine

what is givenand what is to be found/ delivered. Ask, if not clear. If necessary, make assumptions and indicate them.

2. Plan: Think about major steps (or a road map) thatyou will take to solve a given problem. Think of alternative/creative solutions and choose the best one.

3. Execute: Carry out your steps. Use appropriate

diagrams and equations. Estimate your answers. Avoid simple calculation mistakes. Reflect on / revise your work.

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