Below zero

Hello, and welcome to Myriora Maths! It's three degrees outside, and it gets five degrees colder. What's the temperature now? Let's find out!

Where do we meet numbers below zero? On an outdoor thermometer in winter. In a lift, where the car park might be floor minus one. And below sea level, deep under the water. Numbers below zero are called negative numbers.

Here's an outdoor thermometer, the kind that hangs outside. Zero degrees is where water freezes. Above zero, it's warmer: one, two, three. Below zero, it's colder: minus one, minus two, minus three. We write them with a little minus sign in front. The thermometer for when you're poorly is different: it only shows warm numbers, about thirty-five to forty-two degrees, so it never needs a minus sign.

Turn the thermometer on its side, and it's a number line. Zero is in the middle. Positive numbers go to the right. Negative numbers go to the left, like a mirror. Minus three is three steps to the left of zero.

So which is bigger: minus two, or minus seven? On the number line, numbers get bigger as you go right. Minus two is further right. So minus two is bigger, even though seven is bigger than two! Think of the weather: minus two degrees is warmer than minus seven.

Let's try one together. Which is colder: minus one degree, or minus four degrees? Think of the thermometer. Say it out loud!

Minus four is further down the thermometer. So minus four degrees is colder. Well done if you said minus four!

Now back to our question. It's three degrees, and it gets five degrees colder. Start at three, and take five steps down. Three steps take us to zero. Two more steps take us to minus two. It's minus two degrees!

Here's a question that catches lots of people. Which is bigger: minus five, or minus one? Some people say minus five, because five is bigger than one. But minus five is further below zero. So minus one is bigger.

Your turn! It's two degrees, and it gets six degrees colder. What's the temperature now? Pick the right card. You've got fifteen seconds, or pause the video. Go!

Time's up! Two steps down take us to zero. Four more steps take us to minus four. It's minus four degrees!

Quick review. Negative numbers are below zero, and have a minus sign. On a number line, the further right, the bigger. To cross zero, count the steps down to zero first, then keep going. See you next time!