Lessons · Teaching · measuring, and choosing the instrument
The instrument decides how good the answer can be
Every measurement needs a unit and an instrument suited to the size of what is being measured. A ruler marked in centimetres cannot report millimetres honestly, and repeating a measurement is what shows whether the number is reliable.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a state certification officer. Practice material, not professional advice. What that means.
What it is for
A pupil who writes 4.735 cm from a ruler marked in millimetres has claimed a precision the instrument cannot deliver, and that is a real scientific error rather than a fussy one. It is also how a class learns that a number is a claim.
How to think about it
Choose the instrument by the size of the thing and the precision needed. Read at eye level to avoid reading across an angle. Write the unit every time, because a number without one means nothing. Repeat and compare, because two identical readings are evidence and one is not.
Worked example
Length in millimetres, centimetres, metres or kilometres, by the size of the thingMeasuring a field in millimetres is technically right and practically useless.
Mass in grams and kilograms, using a balance; volume in millilitres, using a measuring cylinderMass and volume are different quantities and need different instruments.
Temperature in degrees, read at eye level so the angle does not shift the readingThe commonest source of a small consistent error.
Report only the precision the instrument can give, and repeat to check4.7 cm from a millimetre ruler is honest; 4.735 cm is not.
Your turn
Name what every measurement must be written with, besides the number.
Every measurement needs a number and a
Solve one, graded on the server
The trap
Confusing mass with weight in the classroom and then being asked the difference. Mass is how much matter there is and does not change with location; weight is the force of gravity on that mass and does.