What Is Physics and Physical Quantities
Physics is the science that studies matter, energy, and the forces and interactions between them, and a physical quantity is any property of a body or phenomenon that can be measured and expressed as a number multiplied by a unit. In brief: a physical quantity always has a value and a unit (for example, a length of 2.5 m). Quantities are divided into base quantities, such as length, mass and time, and derived quantities, such as speed and force, which are built from the base ones.
Physics tries to explain how the universe behaves, from the motion of a ball to the structure of atoms and galaxies. To do this, physicists measure things and look for mathematical patterns. This guide explains what physics is and what its branches are, defines physical quantities and their units, compares base and derived quantities in a table, works through an example and corrects common mistakes.
What is physics?
Physics is the most basic of the natural sciences. It looks for general laws that describe how matter and energy behave in space and time. The main branches are:
- Mechanics: motion, forces and energy.
- Thermodynamics: heat, temperature and the behavior of large numbers of particles.
- Waves, sound and optics: oscillations, sound and light.
- Electricity and magnetism: charges, currents, fields and circuits.
- Modern physics: relativity, quantum physics, atomic, nuclear and particle physics.
- Astrophysics: stars, galaxies and the universe as a whole.
Physics is linked with other fields: chemistry uses it to explain bonding, biology uses it to explain the working of eyes and nerves, geology uses it to study earthquakes, and engineering applies it to make machines and buildings.
What are physical quantities?
A physical quantity is something that can be measured. Its value is written as a numerical value and a unit: Q = number × unit. For example, the mass of a book can be 0.80 kg, where 0.80 is the number and kg is the unit. Without a unit, the number has no meaning.
- Base quantities are the quantities chosen as independent starting points. In the International System of Units (SI) there are seven: length (meter, m), mass (kilogram, kg), time (second, s), electric current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol) and luminous intensity (candela, cd).
- Derived quantities are defined by combining base quantities. Speed = length ÷ time, with the unit m/s. Area has the unit m2, density kg/m3, and force kg·m/s2, which is called the newton (N).
- Scalars and vectors: some quantities have only a size (mass, time, temperature), and others also have a direction (velocity, force, displacement).
Symbols for quantities are written in italics (m for mass, t for time), and unit symbols are not italic (kg, s). The same quantity can be given in different units, such as 1.5 km and 1,500 m, and the physical length is the same.
Base and derived quantities compared
| Quantity | Type | Symbol | SI unit |
|---|---|---|---|
| Length | Base | l or L | meter (m) |
| Mass | Base | m | kilogram (kg) |
| Time | Base | t | second (s) |
| Speed | Derived | v | m/s |
| Force | Derived | F | newton (N = kg·m/s2) |
| Energy | Derived | E | joule (J = N·m) |
How do physicists work?
- Observe a phenomenon and ask a question.
- Hypothesize: propose an explanation that can be tested.
- Experiment: measure quantities under controlled conditions, changing one variable at a time.
- Analyze the data, often by graphs and equations.
- Conclude and communicate the results so that others can repeat them.
A law describes a regular pattern, often as an equation (Newton’s laws), and a theory is a well-tested explanation that connects many observations (the theory of relativity). A scientific idea must be testable, and it is changed if the evidence disagrees with it.
Worked example: identifying quantities and units
A runner covers a distance of 100 m in a time of 12.5 s, and has a mass of 65 kg. Identify the quantities and calculate the average speed.
- List the quantities: distance d = 100 m (length, base quantity), time t = 12.5 s (base quantity), mass m = 65 kg (base quantity).
- Choose the formula: average speed = distance ÷ time.
- Substitute: v = 100 m ÷ 12.5 s = 8.0 m/s.
- Check the unit: m ÷ s = m/s, which is the correct unit for speed, and speed is a derived quantity.
The mass was not needed for this calculation. Data that are not needed can be ignored.
How did physics develop?
Ancient Greek thinkers such as Aristotle and Archimedes studied motion and floating bodies. Galileo Galilei introduced systematic experiments and mathematics in the early 1600s, and Isaac Newton published his laws of motion and gravitation in 1687. In the 1800s, the theories of heat, electricity and light were developed, and in the early 1900s, Albert Einstein’s relativity and quantum theory changed the understanding of space, time and matter. The SI system was formally named in 1960, and in 2019 it was redefined in terms of fixed constants of nature.
Where is physics used?
Physics underlies medical imaging (X-rays, ultrasound, MRI), electronics and computers, power generation, communication by radio and optical fibers, transport, and the study of climate. Understanding physical quantities lets people design safe bridges, predict weather and develop new materials.
Which mistakes are common?
Mistake 1: Writing a number without a unit
A result such as “8” is incomplete. It must be 8 m/s (or another unit).
Mistake 2: Confusing mass and weight
Mass (kg) is the amount of matter. Weight is the gravitational force on the object, measured in newtons.
Mistake 3: Mixing units in one calculation
Convert all quantities to consistent units before substituting, for example meters and seconds.
Mistake 4: Assuming that a theory is only a guess
In science, a theory is a tested explanation supported by a lot of evidence.