Physics
Physics has an important role in our life. Without physics and the work of
physicists, our modern life would not exist. Using physics, people created machines,
instruments and some different divices from the crudest to the most modern aspect.
Techology is developed more rapidly, more modern day by day.
Moreover, all other natural sciences- example chemistry, biology, medicine-
depend upon physics for the foundations of their knowledge. Physics holds this key
position because it is concerned with the most fundamental aspect of matter and
energy and how they interact to make the physical universe.
Physics has some main problems: mechanics, electricity and magnitism, heat,
wave and sound, optics, nuclear physics, atomic particles.
CHAPTER 1: MECHANICS
Mechanics is a branch of physics concerned with the behavior of physical
bodies under the effect of the bodies on their enviroment. The early modern period,
scientists, such as Galileo, Kepler and especially Issac Newton, laid the foundation
for a field of mechanics and now it is known as classical mechanics or Newtonian
mechanics.
Mechanics has two major divisions: classical and quantum mechanics.
Classical mechanics came first while quantum mechanics did not appear until 1900.
Both commonly constitute the most certain knowledge that exists about physical
nature.
Classical mechanics is concerned with the physical laws governing the motions
of bodies. It is used for describing the motion of marcroscopic objects, such as: parts
of machinery, astronomical objects inclue spacecraft, planets, stars, galaxies. It is one
of the oldest and lagest subjects in science, engineering and technology.
Classical mechanics is divided into: statics, dynamics and kinematics. Statics
studies matter at rest or in motion with constant velocity. It deals with the balancing
of forces with approriate resistance to keep matter at rest. It is commonly used for
designing buildings and bridges. Different from statics, dynamics studies matter in
motion, example motion of stars, baseballs, gyroscopes of the water pumped, and
even air plane. Kinematics studies motion without regard to the forces present. It is
simply a mathematical way to describe motion.
Three Newton’s laws:
Classical mechanics is governed by three basic principles, which were first
formulated in the 17th and 18th centuries by Isaac Newton. These principles are
known as Newton’s laws.
The first law describes a fudamental property of matter, and often called the
“Law of Inertia”, as follows: Every object in a state of uniform motion tends to
remain in that state of motion unless an external force is applied to it. The key point
here is that if there is no net force acting on an object (if all the external forces cancel
each other out), the object will maintain a constant velocity, if that velocity is zero,
the object remains at rest and if having an external force to apply, the velocity will
change.
Newton’s second law describes the manner in which a force compel a change
of motion, at a rate of change called acceleration. It can be state as follows:
F=ma
Where
F: the applied force
m:mass of the object
a: the object’s accerleration
Acceleration and force are vectors, in this law the direction of the force vector
is the same as the direction of the acceleration vector.
This law allows quantitative canculations: how do velocity change when forces
are applied. Notice the fundamental difference between Newton’s 2nd law and the
dynamics of Aristotle: according to Aristotle there is only velocity if there is a force,
but according to Newton an object with certain velocity maintains that velocity unless
the force acts on it to cause an acceleration.
Newton’s third law can be stated as follows: For every action in nature there is
an equal and opposite reaction. In other words: if object A exerts a force on object B,
then object B also exerts an equal force on object A. Notice that the forces are exerted
on different objects.
This law explains what happens if we step off a boat onto the bank of a lake: as
we move in a direction, the boat tends to move in the opposite direction.
Mass, force and acceleration
Mass is the amount of matter in a body. The mass of a body remains constant.
In the metric system mass is measured in kilogram (kg). Sometimes we use weight,
or the pull of gravity upon matter. The object’s weight depends on the gravitational
pull acting on it. An object’s weight is much less on the moon than it is on the Earth,
and in outer space a body’s weight may be nearly zero.
When an object’s velocity changes, it accelerates. Acceleration shows the
change in velocity of a body in a unit time. According to Newton’s 2nd law, it is direct
result of the applied force. In the metric system, acceleration’s unit is (m/s)/s.
When we study mechanics, we can know a concept: force. Force is a vector
quantity that has both a specific magnitude ( size or length) and direction. It is
characteristic for a body’s acting to other. It changes the motion of a free body or
cause stress in a fixed body. It can also be described by concepts such as a push or
pull that can cause an object with mass to change its velocity, to accelerate, to
deform.
If two forces applied simultaneously to the same point have the same effect as
a single equivalent force, called resultant force. We can canculate the net force:
F=F1+F2+… .
If two forces acting on an object is the same direction (parallel vectors), the
resultant force is equal to F1+F2, in the direction that both two forces. If two forces
acting on a object is opposite directions, the net force is equal to |F1-F2 |, and
direction of whichever one has greater magnitude. If the angle between the forces is
anythingelse, the net force must be added up using the parallelogram rule.
The same forces can have different effects depending on applied way and
applied body. A force may cause a body to spin or rotate if applying in a certain way.
The tedency of a force to rotate the body is known as torque, it is also a vector
quantity. Its magnitude can be calculated by multiplying applied force to the distance
between the line of force and the axis of rotation.
A kind of force which resists the motion of a body along a path is friction. It
appears only when other forces are applied or if a body is already in motion. It may
be undersirable in some cases, example: air resistance that slows down an airplane,
but in some other cases, it is useful, example: car brakes.
Center of gravity and equilibrium :
It’s difficult to apply the laws of mechanics to a particular body. The problem
is more simple if we study the behavior of an object’s center of gravity instead of
studying the behavior of entire pbject. The center of gravity is a point at which the
weight of a solid object can be considered to be concentrated . all forces appear to act
upon this center. If the line of exerted force does not pass through the center of
gravity, a torque is created.
A body can be completely at rest if all forces and all torques are balanced. A
complete balance exists. If the sum of all forces and torques acting on a body is equal
zero, we say that the body is in equilibrium.
A body in equilibrium may be in one of three states: stable, unstable, neutral
equilibrium. When a torque apply to a body, after the torque ceases to act, if the body
tends to return to its original position, it is in stable equilibrium. If it continues to turn
to a new position, it is known as unstable equilibrium. The body is in neutral
equilibrium if it comes to rest wherever it may be when the torque is removed.
Work, energy and power
Work: when a force makes a body move, the product of the force times the
distance through which the force acts is called the work done by the force. There are
some example of work which we can observe in everyday life: a horse pulling a plow
through the field, a man pushing a cart, a weightlifter lifting e barbell above his head,
etc. Mathematically, work can be canculated by the following formula:
A=F d cosα
Where
F: the force ( in Newton)
d: the distance through which the force acts (the displacement), (in meters)
α : the angle between the force and the displacement vector.
Energy is the capacity for doing work. If work is done on a body, the energy of
the body increases. Energy is consists of kinetic and potential energy. Energy
associated with motion is kinetic energy. It is equal to one half the product of its mass
times the squre of its velocity represented by a formula:
KE = (1/2)mv2
Where:
KE: kinetics energy (in Joule)
m: mass ( in kg)
v: velocity (in m/s)
Potential energy exists whenever an object which has mass has a position
within a force field. The most everyday example of this is the position of objects in
the earth’s gravitational field. In this case, the potential energy of an object is given
by:
PE = mgh
Where: PE: potential energy ( in Joules)
m: mass ( in kg)
g: gravitational acceleration of the earth ( 9.8 m/s/s)
h: height above earth’s surface ( in m)
Conservation of energy
This principle asserts that in a closed system energy is conserved. This
principle will be tested by the experiment in the case of an object in free fall. When
the object is at rest at height h, all of its energy is PE. As the object falls and
accelerates due to the earth’s gravity, PE is converted into KE. When the object
strikes the ground, h=0, so that PE=0, the all of the energy has to be in the form of
KE and the object reaches the maximum velocity. In this case we are ignoring air
resistance.
Power is the rate of doing work or the rate of using energy. Unit of power is
watt. If we do 100 joules of work in one second ( using 100 joules of energy), the
power is 100 watts.
Some simple machines.
Many principles of mechanics are clearly demonstrted in devides called simple
machines. These machines have been known since antiquity with crude machines or
now with modern machines. They are the lever, the wheel and axle, the inclined
plane, the screw, the rope-and-pulley system. They are designed to amplify the effect
of forces or to do work to move weight or to overcome resistances.
Chapter 2: Heat
Definition and applications
All living things need heat. Heat is a form of energy transferred from one
object to another caused by a different in temperature between these objects. Some
other words:
- Heat is defined as energy in transit from a high-temperature object to a lower
one.
- Heat is a form of energy possessed by a substance by virtue of the vibrational
movement of its molecules or atoms.
- Heat is the transfer of energy between substance of different temperatures.
Heat has an important role in our life. It causes natural changes which occur in
an endless cycle. To explain some phenomena in the nature, we can use concept of
heat. Example the atmosphere in tropical areas is hotter than it in polar areas because
tropical areas receive more heat from sun.
The amount of heat from the sun that falls on the region determines the
temperature range of the region. The temperature of environment effect to plant,
animal and even man. Heat is a very important factor in making our life and our
world.
The nature of heat.
Despite having many definitions of heat, heat has one nature. We can know
heat when we were a child. We could detect it easily through its effect: burning. But
do you know what heat itself actually is? Heat cannot be weighed and cannot be seen
or heard too.
To understand the nature of heat, we may study its acting, we can use the
kinetic theory of matter. According to this theory, all matter made of atoms and
molecules in constant motion. When matter absorbs energy, the random internal
energy and the motion of these atoms and molecules are increased. This increase
makes itself in the form of heat, and when it occurs, the temperature of the matter
rises. This leads a conclusion: when the energy of motion has been transferred to the
random motion of the atoms that make up the matter, the motion of the atoms is
speeded up and heat is produced. That is the nature of heat.
Sources of heat
Heat is very necessary for life, so it is important to know where it comes from
and how it can be used. The most important source of heat for our Earth is the
radiation from the sun. The Earth absorbs a part of heat from the sun. This keeps the
temperature of the Earth’s surface and atmosphere at a level which permits life to
continue.
The second important source of heat is the store of natural fuel on and in the
Earth, such as: coal, oil, gas, wood. They do not provide heat constantly and
automatically as the sun does. They are composed of carbon, hydrogen, and other
elements. In a certain temperature, the combustion occurs, the fuels react chemically
with oxygen. This reaction releases a large quantity of heat.
The definition of specific heat.
The specific heat is the amount of energy that is transferred to or from one unit
of mass or mole pf a substance to change its temperature by one degree. Specific heat
is a property, it depends on the substance under consideration and its state.
The temperature
Temperature is the property that gives physical meaning to the concept of heat.
And object has low temperature if it is cold, and vise versa. When contacting with a