Introduction Lecture 2

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Transcript Introduction Lecture 2

INTRODUCTION
Today’s objectives:
• Fundamental concepts
• Newton laws
• Units
• Numerical calculation
WHAT IS MECHANICS??
• Study of what happens to a particle/body when FORCES
are applied to it.
• Either the body or the forces could be large or small.
WHERE IS MECHANICS??
Physics
Electromagnetism
Mechanics
Optics
Thermodynamics
Mechanics
Rigid Bodies
(Things that do not change shape)
Statics
EnPh130
Dynamics
EnPh 131 and
Mece 250
Deformable Bodies
(Things that do change shape)
Fluids
Incompressible
Compressible
WHO STARTED IT ALL!
He managed, for the first time, to explain
the relation between different
physical quantities using:
1.
2.
A Mathematical language (calculus)
Experimental laws such as:
F  Ma
Fg  G
Sir Isaac Newton
1643 - 1727
See his biography at:
http://www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Newton.html
m1m2
d2
DYNAMICS
Dynamics is the study of the motion of the objects:
 Kinematics is the study of the geometry of the
motion or how position, velocity and acceleration of
a moving object are related. (First part of this
course)
 Kinetics deals with how motion is caused by the
applied forces. (Second part of this course)
TWO IMPORTANT IDEALIZATION
1. Particle vs. Rigid Body
•
In Rigid body, we should consider
both mass and size
•
Rigid body has both translational and
rotational motion
•
In Particle, mass is considered but
“size” is neglected
•
Particle has no rotational motion
Rule of thumb: a finite body can be considered
as a particle as long as the rotation of the
body is not important in the resulting motion
TWO IMPORTANT IDEALIZATION
2. Concentrated force vs. Distributed force
•
The hydrostatic force of the water
behind a dam is distributed over the
surface
•
The cable force is a concentrated force
or point force since it is applied to a
point on the crate.
•
In dynamics of particle we usually deal
with point force or concentrated forces
QUANTITIES
Basic Quantities:
1. Length,
2. Time
3.
 velocity, acceleration
Mass
Force (not independent from the first three)
m
F=ma
UNIT SYSTEMS
Note:
1.
Unit is a part of a physical quantity. Don’t forget to report the units in
your calculations and final answers
2.
It is worth memorizing some simple unit conversion factors:
1 ft = 0.3048 m, 1 lb = 4.4482 N, 1 slug = 14.5938 kg
NUMERICAL CALCULATIONS (review)
• Must 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
answers, 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)
- equal to 5 and zeros in the following digits, then round to
the closest even digit (2415  2420 or 2425  2420.
SAMPLE CONCEPT QUIZ
1. Give the most appropriate reason for using three significant
figures in reporting results of typical engineering calculations.
A) Historically, slide rulers could not handle more than three
significant figures.
B) Three significant figures gives better than one-percent
accuracy.
C) Telephone systems designed by engineers have area codes
consisting of three figures.
D) Most of the original data used in engineering calculations do
not have accuracy better than one percent