Physics: an introduction

What physics studies

Physics is the science of matter, energy, space and time, and of how they interact. Where other sciences describe particular systems, physics looks for the deepest rules underneath them: why a rainbow has its colours, why a bridge stays up, how a heart monitor picks up tiny electrical signals. Those rules turn out to be few in number and very general. The same law of gravity that pulls an apple to the ground keeps the Moon in orbit, and the same rules for electric charge explain lightning and the chemistry of your cells. Physics is also quantitative: its ideas are written as equations and tested against measured numbers.

Physics and the other sciences

Because it deals with the most basic rules, physics sits underneath every other science. Chemistry rests on the physics of atoms and the forces between them. Most branches of engineering are applied physics, from bridge design to electronics. Medicine uses physics in X-ray imaging, ultrasound, MRI scanners and radiotherapy. Geologists use it to locate earthquakes and date rocks, and climate scientists use it to model how heat moves through air and oceans. Some fields are hybrids with names that show it: biophysics, geophysics and astrophysics.

How physicists find things out

Science starts with observation: describing the world as it actually is. The process usually called the scientific method runs roughly like this. Notice something and ask a question. Propose a hypothesis, a tentative explanation or prediction that can be tested. Design an experiment that changes one quantity, the independent variable, and measures the effect on another, the dependent variable, while holding everything else fixed (the controlled variables). Analyse the results and draw a conclusion, then repeat. It is a loop rather than a recipe: a failed hypothesis is replaced by a better one, and results only count once other groups can reproduce them.

Worked example: testing a pendulum hypothesis

A student guesses that a heavier pendulum swings more slowly. 1. Hypothesis: the time for one swing (the period) increases with the mass of the bob. 2. Variables: mass is the independent variable, the period is the dependent variable, and the string length (0.50 m) and the starting angle are controlled. 3. Method: time 10 complete swings and divide by 10, which makes the error from starting and stopping the stopwatch ten times smaller per swing. 4. Results: bobs of 50 g, 100 g and 200 g take 14.2 s, 14.1 s and 14.2 s, so each period is about 1.42 s. 5. Conclusion: quadrupling the mass made no measurable difference, so the data do not support the hypothesis. The next hypothesis to test is that the length matters.

Models

A model is a simplified representation of a real system that keeps the features that matter and leaves out the rest. It can be a picture, a diagram, a set of equations or a computer simulation. Treating a car as a single point to work out its journey time is a model; so is picturing a gas as tiny bouncing balls, or an atom as electrons circling a nucleus. Models let us reason about things that are too big, too small or too complicated to handle directly. Every model rests on assumptions, such as no air resistance or a perfectly rigid object, and it is only trustworthy where those assumptions are close to true.

Laws, theories and principles

A scientific law is a concise statement of a pattern that nature follows without exception in its domain, often written as one equation. The conservation of energy is a law, and so is Newton's second law, \(F = ma\). A law says what happens. A theory is a broad, well-tested explanation of why, built from many laws, models and observations: the kinetic theory of gases, the theory of relativity and quantum theory are examples. In science, a theory is the strongest kind of idea, not a guess, and theories never turn into laws, because the two do different jobs. A principle is a statement with a narrower range, such as one that applies only to fluids.

Evidence decides

Every law and theory is provisional. No number of agreeing experiments can prove that a law holds in every possible case, but one careful, reproducible experiment that disagrees forces the idea to be changed or its range to be limited. Good theories also predict things no one has seen yet. Small irregularities in the orbit of Uranus led astronomers to predict a new planet, and in 1846 Neptune was found within about a degree of the calculated position. Einstein's general relativity predicted gravitational waves in 1916; detectors first recorded them in 2015. A prediction that fails, however elegant the idea, means the idea must be revised or dropped.

Classical and modern physics

Classical physics is the physics developed up to about 1900: mechanics, heat, electricity and magnetism, and light. Modern physics began in the early twentieth century with two new theories. Relativity is needed near very strong gravity and for anything moving faster than roughly 1% of light speed, where \(c = 2.998 \times 10^8\) m/s. Quantum mechanics is needed for very small things, such as atoms, electrons and light interacting with them. When both apply at once, physicists use relativistic quantum mechanics. Classical physics is not thrown away: it is an excellent approximation for everyday sizes, speeds and gravity, and it is far simpler, so this course starts there.

Worked example: is classical physics good enough?

Decide which kind of physics each case needs, using 1% of the speed of light as the rough dividing line. 1. The threshold is

2. A jet airliner at 250 m/s: \((250\ \mathrm{m/s}) \div c = 8.34 \times 10^{-7}\), so its speed is that fraction of \(c\), less than a millionth, so classical physics is ample. 3. A space probe at \(2.0 \times 10^4\) m/s: \(6.7 \times 10^{-5}\) of \(c\), or about 0.007%, still classical. 4. An electron in an X-ray tube at \(1.0 \times 10^8\) m/s: 0.33 of \(c\), or 33%, so relativity is needed. 5. Check: the electron is about 33 times faster than the threshold, while even the probe is about 150 times slower than it.

Common mistakes to avoid

First, saying an idea is "just a theory": in science a theory is a thoroughly tested explanation, not a hunch. Second, thinking that a theory becomes a law once it is proved; laws describe patterns and theories explain them, and neither is ever proved beyond all doubt. Third, treating the scientific method as a fixed list of steps done once; it is a cycle. Fourth, using a model where its assumptions fail. Fifth, believing that modern physics made classical physics useless: within its limits it remains highly accurate.

Key takeaways

Physics studies matter, energy, space and time, looks for the most general rules and expresses them as equations tested by measurement. It underpins chemistry, engineering, medicine and the earth sciences. The scientific method is a cycle: observe, question, hypothesise, experiment with one independent variable while controlling the others, analyse and repeat. Models simplify reality under stated assumptions. Laws concisely describe what happens; theories broadly explain why; both stay open to new evidence. Classical physics works for everyday conditions; relativity covers speeds above about 1% of c and strong gravity, and quantum mechanics covers the very small.

Question 1 of 5

Which statement best describes a scientific theory?

Question 2 of 5

A student times a pendulum with bobs of different masses, keeping the string length and starting angle the same. What is the independent variable?

Question 3 of 5

A particle in an accelerator moves at 6.0 × 10⁷ m/s. Which description fits?

Question 4 of 5

A careful experiment, repeated by several independent groups, disagrees with a well-established law. What should happen?

Question 5 of 5

Why does the pendulum student time 10 complete swings and divide by 10, instead of timing a single swing?