This is a learning project, an exercise in the explanation of physics in simple English. See Wikipedia:Physics and Simple English Wikipedia.


Purpose of the pageEdit

This page want to provide everyone a basic knowledge of Physics concepts, phenomena and terminology.
Have you ever asked yourself what is quantum mechanics? String theory? Relativity? Have you ever wanted to understand in deep actual knowledge of nature and universe by scientific community?

If yes, this is the place with the aim to made, who learn all of it, able to basically understand how universe works, and specially understand (why don't almost as a scientist?) the other branches of physics (Particle Physics, String Theory, Quantum Physics, General Relativity etc), etc.

So, it isn't an High School nor a University Level resource, it'll contains only essential concepts, terms and explanations, only what is useful, without long and useless examples, stories or insights, nor any depths at the technical level, in this case there'll be links in the right place (for example, to read in deep what is plasma, there'll be a link to another resource such as wikipedia).

If you want to contributeEdit

If you want to contribute, or discuss something, you can take a look here, in the discussion area.

Scientific MethodEdit

The study of natural phenomena may be done through the Scientific method, established by Galileo Galilei. This involves four main steps:

  1. Characterization, which includes definitions, observations, and creating records of measurements and fact relating to the subject of inquiry.
  2. Hypotheses, that is, possible explanations of prior observations and measurements.
  3. Prediction, that is, making predictions based on prior observation combined with hypotheses.
  4. Experiment, designed to test the predictions through new observations.
Compound Microscope, 1876

The observations of the new experiment add to the body of knowledge in the characterization, and then the steps may repeat. In some cases, hypotheses have been well enough established that they change little or not at all with new observations, they become established theory, but human knowledge may never be complete, and new observations and experiments may show something missing from existing theories.


Characterization is of two types: qualitative and quantitative. The first is based on our senses. The second gathers information that has a numeric value, through measurement.

Measurement consists of comparing information about an object and comparing it with a unit of measure, such as comparing the length of an object to a yardstick. There are many kinds of units.

The International System Of Units define units of wide scientific usage, and their respective names, and symbols.

Measurement can be direct or indirect, and scientific instruments have range, precision or sensitivity, accuracy, and speed, or how rapidly or how often measurements may be taken and recorded.

Vector quantitiesEdit

As seen above, a quantity has a simple unit. This one has a number value and a unit of measure.

Now, let's look a quantity a little complex: a vector. A vector has, other than a number value and a unit of measure, also a direction, and the number value is known as magnitude, because it's also a way.
That is, we are talking that a vector has two features: direction and magnitude. Now, consider magnitude has two sub-features: way and a unit (this one again has a number value and unit of measure).


Matter is made of atoms, particles originally considered indivisible. When atoms are close enough they repel each other; a little further apart, they are attracted to each other (This is why objects we see seem to be encapsulated, compacted).

The size of atoms is extremely small. Approximately, we consider an atom is to an apple as an apple is to the Earth.


What is known as temperature, which we experience as hot or cold, is motion of atoms, either loosely in a plasma or gas, or vibrationally, jiggling in place. If this motion increases, the temperature increases. If the motion decreases, matter is colder. When the motion has decreased to the minimum possible, the temperature is called absolute zero.

States of MatterEdit

The four main states of matter are: solid, liquid, gas and plasma. For any given substance, the state of matter is related to the temperature. In order of increasing temperature, the states will be considered



At sufficiently low temperature, molecules of a substance have enough kinetic energy to move instead of sticking to a particular place but that energy is not sufficient for them to break out of a particular shape. The bonding of the molecules are strong enough to hold them close but not enough to make them immobile. A liquid, for this reason, has a definite volume (as long as constant temperature is maintained) but not a definite shape. As temperature rises, the molecules gain more kinetic energy and they tend to spread apart and this effect causes expansion of volume. A liquid takes the shape of the container and its open surface is horizontal on earth till it is undisturbed.


At higher temperatures, the average motion of the atoms (or molecules, more-tightly bound combinations of atoms) becomes faster, and strong enough to break up attracted clumps, so the atoms or molecules are free, not bound to each other.


Enclosed in a container, gas atoms are constantly striking the walls, which receive an average push, called pressure.

If we halve or double the amount of gas in the container (the number of atoms or molecules of gas), the pressure will halve or double. If we halve or double the temperature (measured in degrees above absolute zero), the same.



Features of MatterEdit


Mass is a measure of the amount of matter in an object. It is related to weight, because the weight of an object on the Earth is the force (of gravity) by which the object is attracted to the Earth, which depends on its mass. However, mass also shows as inertia, how hard it would be to push or throw an object, which is independent of gravity.


Density is the thickness of a solid, liquid or gas measured by its mass per unit of volume.


Motion is the change of position of an object with respect to time and a reference frame.

The positions through which the object passes during motion form a line, called a trajectory.

Motion has two main features: velocity, the distance traveled by the object per unit of time, together with the direction of travel (magnitude and direction define a vector), and acceleration, the rate of change of the velocity with respect to time. Acceleration is also a vector. Speed is the absolute value of the velocity without regard to direction.

Uniform linear motionEdit

Uniform accelerated linear motionEdit

Uniform circular motionEdit


A force is a natural phenomenon that changes the motion of an object.
It is often said that a force can trigger other many phenomena, such as thrust, drag, torque, deformation, etc. These effects, can be get up into really one: motion changing. Yes! Because, for example, deformation of an object is indeed a change of motion, or position, of the particles composing it.

Force is a vector quantity, so has both direction and magnitude.

Unit for forces is Newton
    9,81 N = 1 KGf

Newton mechanical lawsEdit

First law
if an object is motionless, or its motion is the type of uniform-linear, there are two possible reasons: it isn't subject to forces or the result of forces acting upon it are null.
Second law
Third law

Electromagnetic forceEdit

Atomic TheoryEdit

We have seen above that matter is made of atoms, but now we go a little in deep.

Substances (or matter) can be of two types: chemical compounds (molecules) or chemical elements.

  • A molecule is a combination of two or more atoms attacked each other
  • An element is a single atom, nowadays are known more than a hundred present in nature

Nomenclature and SymbolsEdit

Each element is unique and identified by a Chemical symbol, formed by one or two letters, the first need to be capitalised.
For example:

Iron = Fe
Hydrogen = H
Carbon = C
Titanium = Ti

All known elements and their respective symbols are in the periodic table.

A chemical compound (Molecule) is expressed with a chemical formula. This one contains chemical elements and numbers. Each element has a number subscripted at the right side indicating number of atoms of its type in the molecule. For example:

H2O = two atoms of hydrogen and one of oxygen, this is water
CaCl2 = a salt, one atom of calcium and two of chlorine.
C6H12O6 = glucose
O2 = a molecule of two atoms of oxygen

Atomic structureEdit

A little in deep. What is an atom? How it's made?

An atom is made of a nucleus composed of protons and neutrons, around which orbit some electrons.
Protons and Neutrons are bound to each other by nuclear force. Electrons are bound to nucleus by electromagnetic force. Protons have positive electric charge, Electrons have negative, and Neutrons have no electric charge.

  • Each atom has a fixed number of protons, called Atomic number. This number is unique for each type of element and therefore identify itself in the periodic table.
  • Each atom has with the atomic number also the Mass number, which represents the number of neutrons contained in the nucleus. This, instead of the atomic number, isn't fixed, that is each atom can have a different number of neutrons. Atoms of the same element, so with the same atomic number, but with a different number of neutrons are called Isotopes.
  • Electrons of each atom are disposed in more levels following a scheme, a.k.a. Electron Configuration of the Atom. Electrons staying in the outest level are Valence Electrons; these are weakly bounded relatively to the nucleus, so get in relation with other atoms establishing the chemical behavior of their owner.

Chemical reactionsEdit