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Negative Numbers Lift

Number · Year 4–Year 6

Send the lift through zero and read the journey as a number sentence.

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About the lift

A building in cross-section, with floors above ground and basements below, and nought at ground level. Tap a floor and the lift travels there, and it travels THROUGH zero rather than jumping, because the counting between the two floors is the whole lesson.

When it lands the board says what happened twice over: in words, "Start at 3. Go down 5. Land at minus 2", and as arithmetic, 3 minus 5 equals minus 2. Down is written as a subtraction and up as an addition, so the same pair of floors read one way is 3 minus 5 and the other way is minus 2 plus 5, which is the pair of facts a class needs.

Note what is being worked out. The floor is chosen by tapping it, so the landing is never a mystery: what a class has to count is the distance from where the lift was to where it is, and that is exactly the subtraction that crosses zero. Press Hide to take the sentence off and the class says it instead of reading it.

Up to eight floors and five basements, which is as many as fit while a floor is still big enough to tap on a whiteboard. For a longer run of negative numbers the number line does it better, and this is here for the context: a lift is a thing children have been in, and going down two from the ground floor is not an abstraction.

How to use it on your whiteboard

  1. 1Tap a floor. The lift travels there and the board writes the journey underneath as a number sentence.
  2. 2Press Hide to take the sentence away, then send the lift and ask the class to say it.
  3. 3Use the Floors up and Basements buttons to change the building. The lift comes inside it if it has to.
  4. 4Press Ground to send the lift back to the ground floor, which is itself a journey worth reading.
  5. 5Tap Fullscreen to fill the board. Share copies a link with the building and the journey on it.

Questions

Why does the lift move slowly?
So the class can count with it. It takes longer for eight floors than for one, and it passes the ground line rather than jumping over it, which is the part of counting through zero that a still picture cannot show.
How do I ask the class to work something out?
Press Hide, then tap a floor. The lift travels and the sentence does not appear, so the class has to say where it started, which way it went and how far. That distance is the subtraction across zero.
How many basements can it have?
Five, and eight floors above ground. Fourteen floors is as many as fit while each one is still big enough to hit with a finger on a whiteboard. For a range like minus twenty to twenty, use the number line.
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