chapt01_Introduction理论力学 第一章 英文

1.1. What is Mechanics? The science which describes and predicts the conditions of rest or motion of bodies under the action of forces 1. Mechanics of rigid bodies (GE 204, 205) 2. Mechanics of deformable bodies (GE 206) 3. Mechanics of fluids (CE, ME 308)

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1-1

Mechanics Statics (dealing with bodies at rest) and Dynamics (bodies in motion) Assumed to be perfectly rigid for statics and dynamics Fluid mechanics; compressible and incompressible flow (hydraulic, or low velocity aerodynamics) Mechanics uses mathematics, but applied science for engineering applications© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1-2

1.2 Fundamental Concepts and Principles Study of Mechanics goes back to Aristotle (384322 B.C.) and Archimedes (287-212 B.C.) Isaac Newton (1642-1727) D’Alembert, Lagrange and Hamilton Einstein; theory of relativity (1905)

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1-3

Aristotle Fourth century B.C.E. Mechanical Problems; collections of questions and answers; in physics, mathematics and engineering Among 35 mechanical problems posed by Aristotle

– Why are larger balances…more accurate than smaller ones? – Why are pieces of timber weaker the longer they are, and why do they bend more easily when raised?

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1-4

Renaissance shipbuilders found that their large timber ships were breaking under their own weight Galileo (1638) prefaced his seminal study of strength of material by reciting the breakup of ships, etc. Still there are failures of heavy steel ships and large missiles Factor of safety or factor of ignorance?

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1-5

Galileo’s seminal work on strength of materials and dynamics Dialogues Concerning Two New Sciences© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1-6

Marble lying on the ground can be soiled, discolored and hard to lift again. Inclined against wall can cause crack or it may fall. Best way is put on the support.

Galileo’s illustration of two failure modes© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1-7

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1-8

Failed Liberty ship, c.1940© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1-9

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1 - 10

Galileo’s PostulationS

h WL S bh 2 2 Sbh W 2L© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

W

1 - 11

Galileo, 1638 b XL S bh 2 Sbh2 Shb2 W , X 2L 2L W b X h© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1 - 12

Correcting the Error Can you tell what is wrong? 17th century Hooke’s law 1729 Bernard Forest de Balidor following the earlier lead of

Leibniz and P. Varignon found that

h WL S bh 3 2 Sbh W 3L

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1 - 13

Forest’s PostulationS

h WL S bh 3 2 Sbh W 3L© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

W

1 - 14

Edme Mariotte’s experiments; while designing pipelines to supply water to the palace at Versailles© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1 - 15

Marriotte recognized that there must be linearly varying compression as well as tension acting across the beam’s section He had error in calculating resultant moment, and he used Galileo’s formula 1713, A. Parent found correct treatment but was ignored because– – – – Was not pubished by French Academy Many misprints and poorly edited He was not a clear writer He criticized many other’s work

© 2004 The McGraw-Hill Companies, Inc. All rights reserved.

1 - 16

1.2 Continued Basic concepts used in mechanics; space, time, mass and force Space; position of the point P; coordinates of P with reference to the origin In Newtonian mechanics; space, time, and mass are absolute concepts, independent each other (force is not independent) Note; relativistic mechanics time is not independent© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1 - 17

1.2 Continued Force is a vector; point of application, magnitude and direction Study the conditions of rest or motion of particles and rigid bodies in terms of the four basic concepts Particles; a very small amount of matter which may be assumed to occupy a single point in space Rigid bodies; a combination of a large number of particles occupying fixed positions with respect to each other© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1 - 18

Six Fundamental Principles All principles are based on experimental evidence, not from mathematical derivations 1. Parallelogram law for addition of Forces– The two forces acting on a particle may be replaced by a single force; resultant

Parallelogram Law© 2004 The McGraw-Hill Companies, Inc. All rights reserved.1 - 19

本文来源：https://www.bwwdw.com/article/xqyh.html