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Bài giảng vật lý đại cương chương 1


Nguyen Quoc Thinh

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

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.

Lecture and exercise section grades are combined..
Midterm Exam


The Branches of 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
These steps are called
the scientific method.

Chapter 1

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

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

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.

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),
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,
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

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

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

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

seconds, s in all three systems

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

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

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

v = v 0 + at


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


a has dimensions


t has dimension


Example 2
Newton’s law of universal gravitation is represented


= G

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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