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What you will learn in Measurement & Units

1. Seven Quantities, Chosen Not Discovered
Everything measurable is a product of powers of seven base quantities. Seven is a decision, and since 2019 every one of them is defined by a constant of nature rather than by an object.
2. What a Quantity Is, in Seven Numbers
A dimensional formula says completely what kind of thing a quantity is — and, as one search shows, says nothing at all about which thing.
3. Checking, Converting, Deriving
One rule — every term carries the same dimensions — rejects most wrong equations in seconds, derives Kepler's third law in one step, and lets one famous wrong equation straight through.
4. A Number and a Range
Every measurement carries an uncertainty, every calculation carries it through, and one input almost always decides the answer. Which one is rarely the one you expect.
5. The Rule and What It Approximates
Significant-figure rules are not a separate subject. They are error propagation done in your head, and knowing that is the difference between remembering a convention and understanding one.
6. Estimating What Nobody Told You
Most useful physics questions have no given data. The answer comes from two or three numbers you already have, and being within a factor of ten counts as being right.

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1. Seven Quantities, Chosen Not Discovered
Everything measurable is a product of powers of seven base quantities. Seven is a decision, and since 2019 every one of them is defined by a constant of nature rather than by an object.
2. What a Quantity Is, in Seven Numbers
A dimensional formula says completely what kind of thing a quantity is — and, as one search shows, says nothing at all about which thing.
3. Checking, Converting, Deriving
One rule — every term carries the same dimensions — rejects most wrong equations in seconds, derives Kepler's third law in one step, and lets one famous wrong equation straight through.
4. A Number and a Range
Every measurement carries an uncertainty, every calculation carries it through, and one input almost always decides the answer. Which one is rarely the one you expect.
5. The Rule and What It Approximates
Significant-figure rules are not a separate subject. They are error propagation done in your head, and knowing that is the difference between remembering a convention and understanding one.
6. Estimating What Nobody Told You
Most useful physics questions have no given data. The answer comes from two or three numbers you already have, and being within a factor of ten counts as being right.