Units

Don't read anything yet. Choose your topic from the dropdown, then drop the unit of each quantity into the equation and watch the unit of the answer build itself. Then try the match-up against the clock, and the conversions — that last tab is where most marks are quietly lost.

Units are the first thing in every section of the spec — 1 (a), 2 (a), 3 (a), and so on all the way to 8 (a). That is not an accident. Edexcel put them first because every single calculation in the section that follows depends on getting them right.

A measurement is a number and a unit

"The current is 3" means nothing. 3 amps? 3 milliamps? A number on its own is not an answer in physics, it is half an answer.

Examiners are strict about this because the unit is what tells you what was measured. Every mark scheme that asks for a calculation expects the unit with it.

Where a unit comes from

Here is the bit almost nobody is told: you do not have to memorise every unit separately. The unit of an answer comes straight out of the equation you used.

Take power. The equation is:

power = energy transferred ÷ time

Energy is in joules and time is in seconds, so power must be in joules per second. That unit is used so often it was given its own name — the watt. So 1 W = 1 J/s, and a 60 W lamp transfers 60 joules every second.

The same trick works everywhere:

Read that last column out loud with the word "per" where the divide sign is: metres per second, joules per second, kilograms per cubic metre. "Per" always means divide, and it always means for every one of the thing underneath.

The units you need

These are the ones that come up. Learn the block for the topic you are revising rather than trying to swallow the lot in one go.

Some units have a name. Some are just written out.

A handful of units were used so often that they were given a name and a letter of their own:

• the newton (N) — instead of writing kg m/s² every time
• the joule (J) — instead of newton metres
• the watt (W) — instead of joules per second
• the coulomb (C), the volt (V) and the ohm (Ω) for electricity
• the pascal (Pa) for pressure, and the becquerel (Bq) for activity

The rest never got a name, so you write them out in full: m/s, m/s², kg m/s, kg/m³, J/(kg °C). Neither kind is harder than the other — they are just written differently.

Three traps worth knowing

Convert first, then calculate

This is the single biggest source of dropped marks in the whole subject, and it has nothing to do with physics — it is arithmetic done in the wrong order.

Every equation on the formula sheet expects seconds, metres and kilograms. If a question gives you minutes, centimetres or grams, convert before you substitute.

Three letters get stuck on the front of units to save writing zeros. They mean exactly the same thing every time, whatever unit follows them:

Watch the two ms: a small m in front of a unit means milli (mA, mm), but m on its own is metres. And a capital M is a million, not a milli.

Worked examples

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.

Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.