In this lesson:
By the end, you will:
A weather app shows tonight's low: −5°C.
Last summer's hottest day: +30°C.
Both are temperatures. What does the minus sign actually mean?
A thermometer shows °C — twenty degrees above the freezing line.
A thermometer shows °C — thirty degrees above.
You already read these — bigger means warmer.
Above zero is positive. Below zero is negative.
The numbers and are the same distance from zero — five units away — but on opposite sides.
The same is true for and , and for and .
Floors above ground: . Below ground: .
A diver stands on the diving board, 5 ft above the water → ft.
The same diver swims to 10 ft below the water surface → ft.
Water surface is 0. Above is positive; below is negative.
Write the signed number:
Pause and try, then advance.
Imagine three thermometers showing the same physical state (water freezing):
Where is "zero," really?
A common error: thinking zero means "no quantity" or "nothing."
But zero is a chosen reference point — a position from which the positive and negative directions extend.
The choice depends on the context.
Each context picks its own zero.
In Celsius, 0°C is the freezing point of water.
Freezing is °C, but °F. The choice of zero is conventional.
A student says: "0°C means there's no temperature there — nothing's happening."
Is that right?
Pick A or B, then advance.
The answer is B.
At °C:
Zero is somewhere, not absence.
For each context, write what zero represents:
Pause and try.
Answers:
Watch out: don't say "no temperature" or "no money" — zero is somewhere.
✓ Positives and negatives go in opposite directions from zero ✓ Zero is a chosen reference point — what it means depends on context
Watch out:
In Lesson 2, you will:
The conceptual hinge from today carries every move tomorrow.