Why Physics?

Here is the deal this primer offers. The first proper course on this site expects you to read a sentence like the ball accelerates at 10 m/s per second because gravity pulls it without flinching. Right now some of those words may be strangers. That is fine – it is the point. Six short chapters from now, every word in that sentence will belong to you: not memorised, but understood well enough that you could explain each one to a friend, and say why physicists chose it.

Nothing gets defined in this chapter, and nothing needs solving. Before the vocabulary, you should see the game it was invented for – because physics really is a game, with rules and a way to win, and it is far more fun than its reputation suggests.

Physics is not a list of facts to memorise. It is a loop, played over and over: describe the world in measured numbers, spot the pattern, use the pattern to predict something you have not yet seen – then check. If the world agrees, the pattern earns a little more trust. If it disagrees, the pattern is wrong, however beautiful it was and however famous the person who proposed it. The world always gets the final word.

Every triumph the later courses celebrate is this loop, run with more patience. Astronomers measured Uranus drifting off its expected path, described the drift in numbers, and predicted an unseen planet – point the telescope there. Neptune was waiting. A century later, an equation predicted a particle nobody had asked for; four years on, antimatter turned up in a photograph. The same loop, on grander and grander stages.

The figure below is the loop at its smallest. A ball rolls along a measured track at a steady speed you choose, toward a flag you place. Before the ball moves, physics names its arrival time: one division, printed in the corner. Roll the ball and the clock races the prediction. Then move the flag, change the speed, and try your honest best to make the prediction fail.

Notice what made the game winnable: the prediction was a number. “The ball will get there fairly soon” can never lose – and a prediction that cannot lose cannot be checked, so it teaches you nothing. “The ball will reach the flag in 20 seconds” sticks its neck out. It can fail, and that is exactly why it counts for something when it succeeds.

This is the entrance fee to physics: trading adjectives for measurements. “Fast” becomes “3 metres every second”, “far” becomes “60 metres”, and arguments that could have gone in circles forever get settled by a clock. The fee buys a great deal, but it does have to be paid before anything else – which is why the next chapter is about measuring, before anything on this site is allowed to move.

People bounce off physics believing it demands years of mathematics. Here is the honest inventory for this primer: adding, multiplying, dividing, one kind of rearranging (chapter 2 walks through it at full slowness), and reading a graph (chapter 4 teaches it from scratch). That is the complete list. If you can split a restaurant bill, you have the arithmetic.

The same promise holds, in spirit, across the whole site: when a course needs an idea you do not have, it builds the idea in front of you, on screen, rather than assuming it. You will never turn a page here and find that some classroom you missed was silently required.

The route is short and straight. First, learn to measure: numbers with units attached, and a trick for the very big and very small (chapter 1). Second, learn to read the compact sentences physics writes with those measurements: equations (chapter 2). Then climb the ladder the whole of classical mechanics stands on – where a thing is (chapter 3), how fast that changes (chapter 4), how fast that changes (chapter 5), and what does the changing (chapter 6). One idea per chapter, one toy per idea, no detours.

And at the far end, the sentence from the top of this page – the ball accelerates at 10 m/s per second because gravity pulls it – will read as plainly as a bus timetable. That is the whole promise. Let’s start measuring.