Introduction
Physics is the science of matter, energy, motion, forces, space, time, and the interactions that connect them. It seeks mathematical rules that explain phenomena ranging from a falling ball and a musical note to electricity, atoms, stars, and the expansion of the universe.
Physics provides foundations for engineering, astronomy, chemistry, electronics, medicine, computing, and many other sciences. Concepts such as force, energy, momentum, waves, electricity, magnetism, heat, light, and gravity appear throughout everyday life.
Measurements in physics usually use the International System of Units (SI). The SI is built around seven base units, including the metre, kilogram, second, ampere, and kelvin.
Learning Objectives
After studying this guide, you should be able to:
Use basic SI units and common physics formulas.
Distinguish distance, displacement, speed, velocity, and acceleration.
Explain Newton's laws of motion and gravity.
Understand work, energy, power, momentum, and pressure.
Explain waves, sound, light, electricity, magnetism, and heat.
Describe the basic ideas of atomic, nuclear, quantum, and particle physics.
Measurement and SI Units
Physics depends on accurate measurement.
The seven SI base quantities and units are:
Quantity | SI Unit | Symbol |
Length | metre | m |
Mass | kilogram | kg |
Time | second | s |
Electric current | ampere | A |
Temperature | kelvin | K |
Amount of substance | mole | mol |
Luminous intensity | candela | cd |
The modern SI defines these units through fixed physical constants.
Many other units are derived units.
Examples include:
Force → newton (N)
Energy → joule (J)
Power → watt (W)
Pressure → pascal (Pa)
Frequency → hertz (Hz)
Electric charge → coulomb (C)
Scalars and Vectors
Physics quantities can be divided into scalars and vectors.
Scalar
A scalar has magnitude but no direction.
Examples:
Mass
Time
Temperature
Speed
Energy
Vector
A vector has both magnitude and direction.
Examples:
Displacement
Velocity
Acceleration
Force
Momentum
Quiz Tip
Speed → scalar
Velocity → vector
Distance and Displacement
Distance measures the total path traveled.
Displacement measures the change in position from starting point to ending point, including direction.
Suppose someone walks 100 m east and then 100 m west.
Total distance:
200 m
Final displacement:
0 m
Speed
Speed measures how quickly distance changes.
Speed = Distance ÷ Time
or:
v = d / t
The SI unit is:
metres per second (m/s)
Example
A runner travels 100 m in 10 s:
Speed = 100 ÷ 10 = 10 m/s
Velocity
Velocity is speed with direction.
Examples:
20 m/s north
5 m/s downward
An object can therefore have constant speed while its velocity changes if its direction changes.
A car traveling around a circular track at constant speed is continuously changing velocity.
Acceleration
Acceleration measures the rate of change of velocity.
Acceleration = Change in Velocity ÷ Time
or:
a = Δv / Δt
The SI unit is:
m/s²
Acceleration can result from a change in:
Speed
Direction
Both
Newton's Laws of Motion
Isaac Newton formulated three famous laws describing relationships between forces and motion.
First Law
An object remains at rest or continues moving at constant velocity unless acted upon by a net external force.
This tendency is called inertia.
Objects with greater mass have greater inertia.
Second Law
Newton's second law relates force, mass, and acceleration:
F = ma
Where:
F = net force
m = mass
a = acceleration
The SI unit of force is the newton (N).
One newton is the force required to accelerate 1 kg at 1 m/s².
Third Law
For every interaction, forces occur in equal and opposite pairs.
If you push on a wall, the wall pushes back on you.
Common Mistake
The two third-law forces act on different objects, so they do not simply cancel each other on one object.
Mass vs Weight
Mass measures the amount of matter and inertia of an object.
Its SI unit is:
kilogram (kg)
Weight is the gravitational force acting on an object.
Near Earth's surface:
Weight = mass × gravitational acceleration
W = mg
where Earth's gravitational acceleration is approximately:
g ≈ 9.8 m/s²
A person has essentially the same mass on Earth and the Moon but weighs less on the Moon because lunar gravity is weaker.
Memory Tip
Mass → kg
Weight → force → N
Gravity
Gravity is the attractive interaction between masses.
Newton's universal law of gravitation can be written:
F = Gm₁m₂ / r²
where:
F = gravitational force
G = gravitational constant
m₁ and m₂ = masses
r = distance between their centers
Gravity explains phenomena such as:
Falling objects
Planetary orbits
Moon orbits
Tides
Free Fall
An object in free fall moves under gravity alone, ignoring effects such as air resistance.
Near Earth's surface, objects in ideal free fall accelerate downward at approximately the same rate regardless of mass.
This means that, in a vacuum, a heavy object does not fall faster merely because it is heavier.
Momentum
Momentum describes an object's quantity of motion.
Momentum = mass × velocity
p = mv
Momentum is a vector.
A massive truck moving slowly can have substantial momentum because of its large mass.
Conservation of Momentum
In an isolated system with no net external force, total momentum remains constant.
This principle is useful for studying:
Collisions
Explosions
Rocket motion
Impulse
An impulse changes momentum.
A simplified relationship is:
Impulse = Force × Time
FΔt = Δp
Increasing the time over which momentum changes can reduce the average force.
This helps explain why:
Airbags protect passengers.
Crash mats reduce injury.
Athletes bend their knees when landing.
Work
In physics, work occurs when a force causes displacement.
For a constant force acting in the direction of movement:
Work = Force × Distance
W = Fd
Work is measured in joules (J).
If you push hard against a wall but the wall does not move, no mechanical work is done on the wall in the physics sense.
Energy
Energy is the capacity to cause change or perform work.
Important forms include:
Kinetic energy
Gravitational potential energy
Elastic energy
Chemical energy
Thermal energy
Electrical energy
Nuclear energy
Kinetic Energy
Kinetic energy is energy associated with motion.
KE = ½mv²
Because velocity is squared, doubling an object's speed increases its kinetic energy by a factor of four if mass remains unchanged.
Gravitational Potential Energy
Near Earth's surface:
GPE = mgh
where:
m = mass
g = gravitational acceleration
h = height
Lifting an object increases its gravitational potential energy.
Conservation of Energy
Energy cannot simply disappear.
It can:
Transfer between objects.
Transform from one form into another.
For example, when a ball falls:
Gravitational potential energy → kinetic energy
When it hits the ground, some energy becomes:
Sound
Thermal energy
Deformation
Power
Power measures how quickly work is done or energy is transferred.
Power = Work ÷ Time
P = W / t
The SI unit is the watt (W).
One watt equals:
1 joule per second
Energy vs Power
Energy → amount transferred
Power → rate of transfer
Density
Density measures mass per unit volume.
Density = Mass ÷ Volume
ρ = m / V
A dense material has more mass packed into a given volume.
Density helps explain why some objects:
Float
Sink
Layer within fluids
Pressure
Pressure is force distributed over an area.
Pressure = Force ÷ Area
P = F / A
The SI unit is the pascal (Pa).
A small force applied over a tiny area can produce high pressure.
This explains why sharp knives and needles work effectively.
Temperature and Thermal Energy
Temperature measures how hot or cold something is and is related to the average microscopic kinetic energy of particles.
Thermal energy depends on the total microscopic energy within a system.
Therefore, temperature and total thermal energy are not identical.
A bathtub of warm water can contain more thermal energy than a small cup of much hotter water.
Heat Transfer
Thermal energy moves through three major processes.
Conduction
Transfer through direct microscopic interactions.
Example:
A metal spoon becomes hot in soup.
Convection
Transfer through movement of fluids such as liquids or gases.
Example:
Warm air rising.
Radiation
Transfer through electromagnetic waves.
Example:
Energy from the Sun reaching Earth.
Radiation can travel through a vacuum.
Waves
A wave transfers energy without permanently transporting matter over the same distance.
Important wave quantities include:
Wavelength
Frequency
Amplitude
Speed
The basic wave equation is:
Wave Speed = Frequency × Wavelength
v = fλ
Frequency and Period
Frequency is the number of cycles per second.
Unit:
hertz (Hz)
Period is the time required for one complete cycle.
They are related by:
T = 1 / f
Transverse and Longitudinal Waves
Transverse Wave
Oscillations occur perpendicular to the direction of wave travel.
Electromagnetic waves are transverse.
Longitudinal Wave
Oscillations occur parallel to the direction of travel.
Sound waves in air are longitudinal.
Sound
Sound is a mechanical wave produced by vibrations.
Sound requires a medium such as:
Air
Water
Solid material
Sound therefore cannot travel through a perfect vacuum.
Pitch and Frequency
Higher frequency generally corresponds to higher pitch.
Loudness and Amplitude
Greater amplitude usually corresponds to greater sound intensity.
Common Mistake
Pitch → frequency
Loudness → amplitude/intensity
Light and Electromagnetic Waves
Light is an electromagnetic wave.
Electromagnetic radiation does not require a material medium and can travel through a vacuum.
The electromagnetic spectrum includes:
Radio waves
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
All electromagnetic waves travel at the same speed in vacuum:
299,792,458 m/s
The speed of light is one of the fixed constants used in the modern SI definition.
Reflection
Reflection occurs when a wave bounces from a surface.
For light:
Angle of incidence = Angle of reflection
Mirrors use reflection to form images.
Refraction
Refraction is the change in direction of a wave when its speed changes while passing from one medium to another.
Examples include:
A straw appearing bent in water.
Lenses focusing light.
Prisms changing the direction of light.
Electricity
Electricity involves electric charge and its interactions.
There are two basic types of charge:
Positive
Negative
Like charges repel.
Opposite charges attract.
Electric Current
Current is the rate of flow of electric charge.
I = Q / t
where:
I = current
Q = charge
t = time
Current is measured in amperes (A).
Voltage
Voltage, or potential difference, measures energy transferred per unit electric charge.
Unit:
volt (V)
Voltage helps drive current through a circuit.
Resistance and Ohm's Law
Resistance opposes electric current.
Unit:
ohm (Ω)
For an ohmic component under suitable conditions:
V = IR
This is known as Ohm's law.
Electrical Power
Electrical power can be calculated as:
P = VI
where:
P = power
V = voltage
I = current
Electrical appliances with greater power ratings transfer energy more rapidly while operating at the stated conditions.
Series and Parallel Circuits
Series Circuit
Components are connected along one path.
The same current passes through each component in a simple series circuit.
Parallel Circuit
Components occupy separate branches.
The voltage across parallel branches connected to the same two points is the same.
Most household electrical systems use parallel arrangements so devices can operate independently.
Magnetism
Magnets produce magnetic fields.
Every magnet has two poles:
North
South
Like poles repel.
Opposite poles attract.
Electricity and magnetism are closely connected.
An electric current creates a magnetic field, while a changing magnetic field can induce an electric current.
This relationship forms the basis of technologies such as:
Electric motors
Generators
Transformers
Atoms
Matter is composed of atoms.
An atom contains a small nucleus surrounded by electrons.
The nucleus contains:
Protons
Neutrons
Proton
Positive electric charge.
Electron
Negative electric charge.
Neutron
No net electric charge.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Some isotopes are stable.
Others are radioactive.
Radioactivity
Unstable atomic nuclei can release radiation as they transform.
Major forms include:
Alpha radiation
Beta radiation
Gamma radiation
These differ in:
Composition
Charge
Penetrating ability
Interaction with matter
Nuclear Fission and Fusion
Fission
Nuclear fission splits a heavy nucleus into smaller nuclei.
Fission is used in conventional nuclear power reactors.
Fusion
Nuclear fusion combines light nuclei into heavier ones.
Fusion powers stars such as the Sun.
Memory Tip
Fission → split
Fusion → join
Relativity
Albert Einstein's theories of relativity changed scientific ideas about space, time, gravity, mass, and energy.
One famous relationship is:
E = mc²
It expresses the equivalence between mass and energy.
Because the speed of light squared is an enormous number, a small amount of mass corresponds to a very large amount of energy.
Quantum Physics
Quantum physics describes matter and energy at atomic and subatomic scales.
At these scales, familiar everyday ideas are not always sufficient.
Important quantum concepts include:
Quantized energy levels
Wave-particle behavior
Probability
Uncertainty
Quantum physics provides the scientific foundation for technologies including:
Lasers
Semiconductors
Modern electronics
Fundamental Particles
Modern particle physics describes matter in terms of elementary particles.
Two important groups are:
Quarks
Leptons
Electrons are leptons.
Protons and neutrons are made from quarks.
The Standard Model of particle physics describes known elementary particles and three fundamental interactions: electromagnetic, strong, and weak. Gravity is not incorporated into the Standard Model. CERN describes the model as a highly successful framework developed through decades of theoretical and experimental work.
Four Fundamental Interactions
Physics recognizes four fundamental interactions:
Gravitational
Electromagnetic
Strong nuclear
Weak nuclear
Gravity
Acts between masses and dominates large-scale astronomical structures.
Electromagnetism
Acts between electrically charged particles.
Strong Interaction
Binds quarks and helps hold atomic nuclei together.
Weak Interaction
Participates in processes including some forms of radioactive decay.
Common Physics Mistakes
Mass and Weight Are Different
Mass → amount of inertia, measured in kg
Weight → gravitational force, measured in N
Speed and Velocity Are Different
Velocity includes direction.
Distance and Displacement Are Different
Distance measures total path length; displacement compares starting and ending positions.
Energy and Power Are Different
Energy measures an amount.
Power measures how quickly energy is transferred.
Heat and Temperature Are Different
Temperature is not the total thermal energy stored in an object.
Sound Cannot Travel Through Empty Space
Sound requires a material medium.
Current Is Not the Same as Voltage
Current measures charge flow.
Voltage measures energy transferred per unit charge.
Force Is Not Required to Maintain Constant Velocity
A net force is required to change velocity, not to maintain ideal constant velocity.
Memory Tips
For motion:
Speed → how fast
Velocity → how fast + direction
Acceleration → change in velocity
For Newton:
1st → inertia
2nd → F = ma
3rd → force pairs
For mechanics:
Momentum → mv
Kinetic energy → ½mv²
Potential energy → mgh
Power → energy ÷ time
For waves:
v = fλ
Frequency → cycles per second
Wavelength → distance between matching points
For electricity:
Current → charge flow
Voltage → energy per charge
Resistance → opposition to current
V = IR
Summary
Physics studies the fundamental behavior of matter, energy, forces, motion, space, and time. Classical mechanics explains motion using concepts such as velocity, acceleration, force, momentum, and energy. Newton's laws provide a foundation for understanding many everyday movements, while gravity explains falling objects and astronomical orbits. Waves transfer energy and include both mechanical waves such as sound and electromagnetic waves such as light. Electricity and magnetism are closely connected, forming the basis of modern power systems, motors, generators, electronics, and communication technology.
Thermal physics examines temperature, heat, and energy transfer. Atomic and nuclear physics study atoms, radioactivity, fission, and fusion, while relativity and quantum physics describe phenomena beyond the range of ordinary classical experience. For quiz preparation, remember the major equations together with what each quantity means. Understanding why an equation works is more useful than memorizing symbols alone.
FAQ
What is physics?
Physics is the science of matter, energy, motion, forces, space, time, and their interactions.
What is the SI unit of force?
The newton (N).
What is Newton's second law?
F = ma, meaning net force equals mass multiplied by acceleration.
What is the difference between mass and weight?
Mass measures inertia and is expressed in kilograms. Weight is gravitational force and is measured in newtons.
What is kinetic energy?
Kinetic energy is energy associated with motion and is calculated using KE = ½mv².
What is the wave equation?
v = fλ, where wave speed equals frequency multiplied by wavelength.
Can sound travel through space?
Not through a perfect vacuum. Sound is a mechanical wave and requires matter through which to travel.
What is Ohm's law?
V = IR, relating voltage, current, and resistance for ohmic materials under appropriate conditions.
What is the difference between fission and fusion?
Fission splits heavy nuclei. Fusion combines light nuclei.
What are the four fundamental interactions?
Gravity, electromagnetism, the strong interaction, and the weak interaction.
Key Takeaways
Physics explains matter, energy, forces, motion, waves, electricity, and the fundamental structure of nature.
Newton's laws, conservation of energy, and conservation of momentum are central principles of classical mechanics.
Waves are described using frequency, wavelength, amplitude, and speed, while sound and light are different types of waves.
Electricity and magnetism are closely linked and form the basis of many modern technologies.
Modern physics includes relativity, quantum mechanics, nuclear physics, and particle physics.
References
Bureau International des Poids et Mesures — The International System of Units The international authority for SI measurement, including base units, derived units, physical constants, and unit definitions.
BIPM — The International System of UnitsBureau International des Poids et Mesures — SI Base Units Official definitions of the metre, kilogram, second, ampere, kelvin, mole, and candela.
BIPM — SI Base UnitsNational Institute of Standards and Technology — SI Units U.S. government reference explaining the SI system, defining constants, base units, derived units, and scientific measurement.
NIST — SI UnitsNational Institute of Standards and Technology — Guide to the SI Detailed authoritative guide to correct scientific quantities, symbols, units, and measurement conventions.
NIST — Guide to the SICERN — The Standard Model Authoritative introduction to quarks, leptons, fundamental particles, and the interactions described by the Standard Model of particle physics.
CERN — The Standard ModelCERN — Physics Educational resources covering particle physics, antimatter, dark matter, fundamental forces, and research at particle accelerators.
CERN — PhysicsNASA Science — Universe Scientific educational resources covering gravity, stars, cosmology, electromagnetic radiation, black holes, and the physical universe.
NASA Science — UniverseNASA Science — The Electromagnetic Spectrum Educational material explaining electromagnetic radiation, wavelength, frequency, and different regions of the spectrum.
NASA Science — Electromagnetic Spectrum