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Bài giảng vật lý đại cương chương 1

Phys-211
Instructor:
Telephone:

Nguyen Quoc Thinh
01685036392

E-Mail: nguyenquocthinh@hus.edu.vn
Office: 213 T1 (temporary)
Office Hours: By appointment.
Textbook:
Physics for scientists and engineers, Serway, 4th edition
World wide web: http://user.hus.edu.vn/nguyenquocthinh


Study
Suggested Study Procedure
1. Read the assigned topics/materials before coming to
the class/lab.
2. Attend the class, take good notes, and actively
participate in all the activities in the class.

3. Reread the topics/materials.
4. Doing lots of homework problems is the best way to
do well in the class. As you do each problem, think of
what strategy you are using to solve the problem.


Evaluation
Lecture and exercise section grades are combined..
Assignments
Midterm Exam
Exam

20%
30%
50%


The Branches of Physics


Physics
Physics attempts to use a small number of basic
concepts, equations,
equations and assumptions to
describe the physical world.
These physics principles can then be used to make
predictions about a broad range of phenomena.
Physics discoveries often turn out to have unexpected
practical applications, and advances in technology can
in turn lead to new physics discoveries.


Theories and Experiments
The goal of physics is to develop theories
based on experiments
A theory is a “guess” expressed
mathematically, about how a system works
The theory makes predictions about how a
system should work
Experiments check the theories’ predictions

Every theory is a work in progress


Chapter 1

The Scientific Method
There is no single
procedure that scientists
follow in their work.
However, there are
certain steps common to
all good scientific
investigations.
These steps are called
the scientific method.


Chapter 1

Models
Physics uses models that describe phenomena.
A model is a pattern, plan, representation, or
description designed to show the structure or
workings of an object, system, or concept.
A set of particles or interacting components
considered to be a distinct physical entity for the
purpose of study is called a system.


Chapter 1

Hypotheses
Models help scientists develop hypotheses.
A hypothesis is an explanation that is based on
prior scientific research or observations and that
can be tested.
The process of simplifying and modeling a
situation can help you determine the relevant
variables and identify a hypothesis for testing.


Chapter 1

Hypotheses, continued
Galileo modeled the behavior of falling
objects in order to develop a hypothesis
about how objects fall.

If heavier objects fell
faster than slower
ones,would two bricks of
different masses tied
together fall slower (b) or
faster (c) than the heavy
brick alone (a)? Because
of this contradiction,
Galileo hypothesized
instead that all objects fall
at the same rate, as in
(d).


Chapter 1

Controlled Experiments
A hypothesis must be tested in a
controlled experiment.
A controlled experiment tests only one
factor at a time by using a comparison
of a control group with an
experimental group.


Units
To communicate the result of a
measurement for a quantity, a unit
must be defined
Defining units allows everyone to relate
to the same fundamental amount


Chapter 1

Numbers as Measurements
In SI, the standard measurement system for
science, there are seven base units.
Each base unit describes a single dimension,
such as length, mass, or time.
The units of length, mass, and time are the
meter (m), kilogram (kg), and second (s),
respectively.
Derived units are formed by combining the
seven base units with multiplication or division. For
example, speeds are typically expressed in units of
meters per second (m/s).


Systems of Measurement
Standardized systems
agreed upon by some authority, usually a
governmental body

SI -- Système International
agreed to in 1960 by an international committee
main system used in this course
also called mks for the first letters in the units of
the fundamental quantities


Systems of Measurements,
cont
cgs – Gaussian system
named for the first letters of the units it
uses for fundamental quantities
(centimetre, gram, second)

US Customary
everyday units
often uses weight, in pounds, instead of
mass as a fundamental quantity


Length
Units
SI – meter, m
cgs – centimeter, cm
US Customary – foot, ft

Defined in terms of a meter – the
distance traveled by light in a vacuum
during a given time


Mass
Units
SI – kilogram, kg
cgs – gram, g
USC – slug, slug

Defined in terms of kilogram, based on
a specific cylinder kept at the
International Bureau of Weights and
Measures


Mass
The SI unit for mass is the
kilogram.
A kilogram is defined as the
mass of a special platinumiridium alloy cylinder kept at the
International Bureau of Weights
and Measures in France.


Time
Units
seconds, s in all three systems

9,192,631,700 times the period of
oscillation of radiation from the cesium
atom.


Fundamental Quantities and
Their Dimension
Length [L]
Mass [M]
Time [T]
other physical quantities can be
constructed from these three


Chapter 1

Dimensions and Units
Measurements of physical quantities must be
expressed in units that match the dimensions of
that quantity.
In addition to having the correct dimension,
measurements used in calculations should also
have the same units. For example, when
determining area by
multiplying length and width,
be sure the measurements
are expressed in the same
units.


Dimensional Analysis
Technique to check the correctness of
an equation
Dimensions (length, mass, time,
combinations) can be treated as
algebraic quantities
add, subtract, multiply, divide

Both sides of equation must have the
same dimensions


Dimensional Analysis, cont.
Cannot give numerical factors: this is
its limitation
Dimensions of some common quantities


Example 1
The following equation was given by a student during an
examination:

v = v 0 + at

2

Do a dimensional analysis and explain why the equation
can’t be correct.
ν has dimensions

L
T

a has dimensions

L
2
T

t has dimension

T


Example 2
Newton’s law of universal gravitation is represented
by

F

Mm
= G
2
r

where F is the gravitational force, M and m are
masses, and r is a length. Force has the SI units kg
· m/s2. What are the SI units of the proportionality
constant G?


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