EGR 280 Mechanics 1 – Introduction, forces, particle statics

In the beginning, everything was mechanical.

The first people were only concerned with: Finding or growing food Finding or building shelter Getting water from where it was found to where it was needed As people became better at these things, they lived in bigger and bigger groups… and warfare was invented to get the food, shelter and water away from neighboring groups. To gain an technological edge on their neighbors, some groups began to study the world around them, and invented mathematics to describe and understand it. The first engineers were concerned with building better water delivery systems… and with building better fortified castles and predicting the flight of projectiles.

Electrical and computer engineering are concerned with man-made devices, concepts and systems. In contrast, mechanical engineering is concerned with everything we can touch: –Mechanisms –Heat –Structures –Fluids –Friction –Gasses –Materials –Etc… Mechanical engineers are still forced to deal with materials – minerals, plants, water, the air itself – just as they are found in and around the earth.

Before inexpensive electric motors were developed, power came from moving water or steam engines. Much of the very complex mechanical control systems that were developed largely through trial and error have now been replaced with much less expensive computer-controlled mechanical systems – think just of the development of clocks and other timepieces. Modern mechanical engineering relies heavily on the other branches of engineering – electrical, electronic, computer – to provide power and the intelligent and flexible control systems that make modern products much less expensive and easier to design and manufacture. Every product that interacts with its surrounding environment must have a mechanical interface. For this reason, understanding the concepts of mechanics is necessary for the understanding of engineering itself.

MECHANICS –Statics Particle Rigid Body –Dynamics Particle Rigid Body –Vibrations –Mechanics of Deformable Bodies –Fluid Mechanics

US Customary Units (FPS) –Length measured in feet (ft) –Time measured in seconds (s) –Force measured in pounds (lb, kip=1000 lb) –Acceleration of gravity = g = 32.2 ft/s 2 –Mass is a derived unit, m = W ∕ g SI Units –Length measured in meters (m) –Time measured in seconds (s) –Mass measured in kilograms (kg) –Acceleration of gravity = g = 9.81 m/s 2 –Force is a derived unit, 1 Newton = 1 N = 1 kg-m/s 2 Significant figures: Because most mechanical measurements can easily be made to only 1 part in 1000, perform all calculations to four significant figures, but report only three in your final answers.

In statics, we work with forces. Forces are the way bodies interact with their surroundings A force is a vector quantity Vector: both magnitude and direction, e.g.: force, acceleration, momentum Scalar: magnitude only. e.g.: area, length, mass, energy, speed, temperature

Newton’s First Law of Motion: If the resultant force acting on any particle is zero, the particle will either remain at rest or continue to move with a constant velocity. Static Equilibrium: When the resultant of all of the forces applied to a particle is zero, the particle is in static equilibrium R = ∑F = 0 is necessary and sufficient for static equilibrium. Free-Body Diagrams: In order to account for all of the forces acting on a particle, the use of free-body diagrams (FBD) is mandatory.

To construct a free-body diagram: 1.Sketch, isolated from the rest of the problem geometry, a significant particle. 2.Show on the sketch, with values for known quantities and symbols for unknown quantities, all of the forces that are applied to the particle. 3.Show on the sketch any other information that may be helpful to solve the problem (coordinate systems, dimensions, etc.)

Example: Determine the tension in cables AB and AD for equilibrium of the 250-kg engine shown Graphics and problem statements © 2004 R.C. Hibbeler. Published by Pearson Education, Inc., Upper Saddle River, NJ.