School in a Book: Physics

Of the hard sciences, physics is definitely my favorite. Biology is the most relatable and chemistry is possibly the most practical, but physics is the most philosophical. What is energy? What is matter? What is reality? How did it all begin? We’ll be debating these questions for a very long time.


Physics: The study of movement and energy, including the study of mechanics, gravity, electricity, sound, light, magnetism, nuclear energy, quantum mechanics and more

Energy: The massless, volumeless, invisible something that allows for movement, chemical change and work. Note that everything in the universe is made of either matter or energy. Note also that energy cannot be either created or destroyed; in order to get energy out of a system, you must first get it from somewhere else and put it into the system. Energy is stored in wood, fuel, batteries, light, food, etc.—anything that releases energy when burned. (Remember, food isn’t turned into energy. It stores energy, then releases it from the food. Energy does not become matter or vice versa.)

Energy conversion: A change in the form of energy from one type to another. For example, during photosynthesis, sun energy becomes stored energy, then kinetic energy used for growth.

Energy chain: The sequence of energy conversions that happen when energy is transformed from one form to another, with each transformation leading to the next

The two fundamental forms of energy: Potential and kinetic

Kinetic energy: Energy that is currently active, such as wind energy and the movement of water

Potential energy: Energy currently in storage, such as seed energy or the energy inside a full balloon

Solar energy: The light and heat energy whose source is the sun

Nuclear energy: The energy found in an atom’s nucleus

Thermal energy: Internal heat energy resulting from something’s temperature

Heat energy: The energy that is transferred from a warmer object to a cooler object as a result of a temperature difference between the two objects. Heat moves from a warmer place to a cooler place, like air pressure moves from high-pressure to lower-pressure places and water flows downhill.

Chemical energy: Energy stored in the bonds between atoms, in molecules. It is released in chemical reactions in the form of heat.

Electrical energy: The energy carried by the movement of electrons in an electric conductor

Mechanical energy: The total potential and kinetic energy something has due to its motion and/or position

Gravitational energy: The potential energy something has due to the its position in a gravitational field. An example is an apple that has not yet fallen off the apple tree.

Force: Any push or pull on an object. This includes the force of gravity, the force of a human hand picking something up, and much more. Note that all objects not in motion still have forces acting on them at all times, but when not moving, these forces are canceling each other out. For example, in order to sit still I must hold my body upright in a way that perfectly balances the force of gravity on it.

The four fundamental forces in the universe: The strong nuclear force, the weak nuclear force, the electromagnetic force, and gravity

The strong nuclear force: The force that holds an atom’s nucleus together

The weak nuclear force: The force responsible for the decay of neutrons and more

The electromagnetic force: The force responsible for electric and magnetic interactions

Gravity: The force everywhere in the universe that pulls every object towards every other object simultaneously. This includes planets, stars, galaxies, electrons and even light. It holds heavenly bodies in orbit around each other; causes planets to attract particles and grow larger; causes the Moon to pull Earth’s water toward it, creating tides; gives things on Earth weight; and more. The greater the mass an object has, the greater gravitational force it exerts. It is the weakest of the four fundamental forces.

Albert Einstein’s General Theory of Relativity: The scientific theory that states that gravity is not a true force, but the simple physical result of the curvature of spacetime which in turn is caused by the uneven distribution of mass across the universe

E=mc^2: Energy equals mass times the speed of light squared. This is the formula that Einstein discovered that shows that a small amount of mass can be converted into a large amount of energy, and vice versa, as long as the speed of light is involved in the process.

Dynamics: The study of how forces affect motion

Velocity: The speed of something in a given direction. Velocity changes when direction changes even when speed stays the same.

Terminal velocity: The maximum velocity something reaches while in freefall. When something falls through gas or liquid it accelerates at a decreasing rate until the force of gravity equals the forces of friction and it reaches this state.

Friction: The resistance of one surface to the motion of another. For example, a patch of ice provides little friction compared to a rubber mat. Friction is everywhere. Without its effects, nothing would stop moving until a force acted upon it. Friction causes movement energy to be converted to heat energy. 

Equilibrium: The physical state that exists when forces or energies or systems are in balance, and there is therefore no change in motion

Inertia: The property of matter by which a stationary object remains stationary or a moving object remains moving until acted upon by another force

Freefall: The physical state that occurs when gravity is the only force affecting an object

Weightlessness: The physical state that occurs when the effects of gravity are balanced out by other forces, such as the centrifugal force of a spacecraft, and therefore seems to have no effect

Centripetal force: The force that causes something to turn in a circular path instead of in a straight line. It is not a fundamental force itself, but the net result of all the forces acting on the object that result in the circular movement.

Cohesion: The physical state that exists when molecules of a certain substance are more attracted to each other than to the substance they’re touching. An example is surface tension.

Adhesion: The physical state that exists when molecules are more attracted to the substance they’re touching than to each other. An example is glue on paper.

Diffusion: The spreading out of molecules to fill a space more evenly. An example is the spreading out of perfume to fill the air in a room.

Surface tension: The physical state that exists when a liquid’s surface resists gravity somewhat, remaining cohesive. This happens because the molecules in water at top are more attracted to the molecules in the water below than to the molecules in the air.

Fluid dynamics: The scientific principles that explain the flow of liquids and gases

Turbulence: The uneven movement caused when an object moves through air or water

Drag/air resistance: The friction that occurs on an object moving through air. With no friction at all, objects falling toward the earth would fall at the same rate.

Air compression: The condition created when air particles are pushed closer together (as in a small space such as a tire or a balloon). When this happens, the particles try to escape and expand by pushing on the inside walls, causing visible inflation. Compressed air is an especially highly pressurized type of air.

Vacuum: An area of decreased air pressure that causes areas of higher air pressure to be drawn in towards it. When we suck or otherwise remove air from a container, we create a vacuum in that container. That vacuum, in turn, sucks air into it. Note that it isn’t the motion of pulling out air that causes a vacuum cleaner to suck, but the natural physical reaction of higher-pressure air to rush to fill (and thus balance out) lower-pressure air that causes this behavior. Also note that outer space has no air, so it is considered a vacuum.

Newton’s First Law of Motion: “A body at rest will remain at rest, and a body in motion will remain in motion unless it is acted upon by an external force.”

Newton’s Second Law of Motion: “The force acting on an object is equal to the mass of that object times its acceleration.”

Newton’s Third Law of Motion: “For every action, there is an equal and opposite reaction.” An example of this occurs when a balloon full of air is let go: the air goes one way and the balloon goes another.

Electricity: The effect caused by the presence and movement of charged particles (specifically, the electrons in the charged particles)

Electromagnetism: The force of electricity and magnetism, which occurs when electrically charged particles interact with an electromagnetic field

Electromagnetic spectrum: All types of electromagnetic radiation, whether or not they are visible to the human eye, including (in order): gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves, radio waves (such as those used for radar, FM, TV, shortwave and AM) and more

Electromagnetic waves: Waves made of continually changing electric and magnetic fields that can move through solids, liquids, gases and even a vacuum

Electric field: An area that surrounds an electric charge or an electromagnetic wave that exerts force on other charges

Electric current: A flow of electric charge through a material or along a conductor

Conductor: An object or material that allows an electrical current to flow in one or more directions

Insulator: An object or material that does not allow an electrical current to flow freely or easily through it

Static electricity: Electricity created due to an imbalance of electrical charges that causes some charges to seek a path away from their present location

Magnet: A material or object that produces a magnetic field

Magnetic field: All the (invisible) space around a magnet that can attract or repel certain materials such as iron, cobalt, or nickel. The field is strongest at the poles. The earth is a large magnet, and has a magnetic field with two poles. It is strong enough that magnets will reorient to be parallel to the field, which is why compasses work.

The five properties of magnets: They only act on certain materials (iron, cobalt and nickel); they have two ends, or poles (north-seeking and south-seeking); opposite poles attract and like poles repel; they have magnetic fields; and their magnetic fields pass through other materials

Magnetic north/south: The magnetic poles of the earth (though these poles don’t correspond exactly to the geographical North Pole and South Pole)

Ferromagnetism: The magnetic quality of certain materials (such as iron, cobalt and nickel) that allows these materials to permanently attract or repel. There are also many other materials that have a magnetic quality, but more weakly and not permanently.

Light: A form of energy made up of electromagnetic waves

Visible light spectrum: The (relatively small) part of the light spectrum that is visible to the human eye

Speed of light: The speed that light travels in a vacuum (over 186,000 miles per second). It is also the highest possible speed at which all other massless particles can travel including gravitational waves and electromagnetic energy. Note that particles with any amount of mass can never reach this speed.

Luminous: Having the ability to give off light (as opposed to the mere reflecting of light)

Luminosity: A measurement denoting the total amount of light energy emitted, whether the object is luminous itself or merely reflecting light (such as the moon)

Light intensity: A measurement denoting the amount of light energy that falls on a particular area

Transparent: See-through

Translucent: Almost entirely see-through

Opaque: Not see-through

Umbra: The darkest part of a shadow

Penumbra: The faded part of a shadow

Color: The various visual effects that occur when different types of matter absorb some wavelengths of light and reflect others. Wavelengths that are reflected are perceived by the human eye, while wavelengths that are absorbed are not perceived, which creates color variation.

Fluorescence: The ability to glow when exposed to light, which occurs when the material absorbs high-frequency wavelengths, like UV light (which is invisible to the human eye), then emits lower-frequency visible light as a response. UV light works best to create the glow effect because some energy is lost in the energy conversion process, and UV light has a high-energy frequency.

Phosphorescence: The ability to store fluorescent energy, then emit it up to several hours after the absorption

Convex lens/converging lens: A lens that is shaped like an upside-down bowl, whose center is thicker than its edges, which causes light to bend towards the center, making objects appear larger than they actually are. This kind of lens corrects farsightedness.

Concave lens/diverging lens: A lens that is shaped like a bowl, whose center is thinner than its edges, which causes light to bend away from the center, making objects appear smaller than they actually are. This kind of lens corrects nearsightedness.

Mirror: A piece of glass or other transparent material with a silver-colored backing behind it that causes all light to reflect back to the viewer

Reflection: The bouncing back of light rays from a surface

Refraction: A change in the path of a light wave as it passes from one medium to another. For example, a straw placed in a glass of water appears bent due to the changing path of light when traveling through air versus through water.

Diffraction: The bending and spreading out of light waves, as when passing through small openings or encountering an obstacle

Dispersion: The splitting of visible light into its component colors, which is what makes it possible for the eye to see individual colors, as in a rainbow

Prism: A transparent object, such as a diamond or a piece of cut glass, that bends light that hits it, thereby splitting it and causing a rainbow to appear

Interference: A change in the paths, amplitudes and/or frequencies of two waves as they interact with each other

Constructive interference: Interference that results in an increase in a wave’s size, which happens when the peaks and troughs of waves that meet line up with each other

Destructive interference: Interference that results in a decrease in a wave’s size, which happens when the peaks and troughs of waves that meet are placed opposite of each other

Three types of heat transfer: Convection, conduction and radiation

Convection: The transfer of heat through the movement of gases or liquids, such as ocean currents or warm air currents

Conduction: The transfer of heat through solids using direct contact, such as a pan on a burner

Radiation: The transfer of heat through the air or through space, such as the sun heating the atmosphere or a radiator heater heating a home’s air, whether or not that air is moving

Sound: The vibration that occurs in a hearing ear after sound waves transfer energy through the particles in the air until making contact with that ear. Sound waves transfer movement energy while light waves travel as electromagnetic radiation. Sound will only travel through gas, liquid or solid, but not through a vacuum–no particles are there to transfer the energy. Some believe that sound waves are only sound if they find a hearing ear, while others do not.

Sound wave: Fast back and forth movements (vibrations) that produce sound

Tone: A prolonged sound note that vibrates at a steady frequency

Frequency: The speed of a sound’s vibration, with faster vibrations creating higher frequencies and slower vibration creating lower frequencies

Pitch: The perceived highness or lowness of a sound note depending on its frequency. It is made by tightening or loosening vocal cords, guitar strings, etc., thereby slowing down or speeding up the sound vibrations.

Sound intensity: The loudness of a sound, with louder sounds having more energy and lower sounds having less energy

Amplitude: The length of a sound wave from top to bottom. The longer the sound wave, the louder the sound. 

How sound is made from voices: Air passes through the larynx while, at the same time, the vocal cords tense, creating vibration. The human ear can only pick up 20 to 20,000 vibrations-per-second frequencies.

Infrasound: Sounds at frequencies below the ability of humans to hear it

Ultrasound: Sounds at frequencies above the ability of humans to hear it

Decibel: A unit of measurement of loudness

Supersonic: Exceeding the speed of sound

Subsonic: Not exceeding or equal to the speed of sound

Sonic boom: The shock wave produced when an object moves through the air faster than the speed of sound

Sound barrier: The apparent physical boundary stopping the forward progress of an object traveling through the air at or above the speed of sound

Echo: The reflection of sound waves off a surface, resulting in a delayed repetition of the original sound

Sonar: A technique for locating objects underwater by bouncing high-frequency sound waves off of them

Theoretical physics: A branch of physics that relies on mathematical models to explain natural phenomenon, rather than applied experiments

String theory: The scientific theory proposing that the fundamental particles that make up the universe are not particles or matter, but instead string-shaped vibrations

Quantum theory: The scientific theory that attempts to explain wave-particle duality, quantization of energy, entanglement and other phenomenon found at the subatomic level

The theory of everything: Any theory that attempts to explain how all of the different theories, laws and forces can work together in the same universe, even though at times they seem to contradict each other. Note that since general relativity is used for large-scale problems and quantum theory is used for small-scale problems, their incompatibility is usually avoided in practical matters of science.


Babies come. But babies don't go. Get Fights You’ll Have After Having a Baby: A Self-Help Story on Amazon now.