Luckily MathsIsFun delivered on its promise and rescued us from a mother-son rift.
I went with their hot air balloon example which has the added bonus of reinforcing the number line concept.
My hot air balloon looks like a flower pot (art is not my thing) but the idea is sound and it works like this:
The orange circles are balloons, worth +1. The grey are weights, worth -1.
- Start with the balloon pointing to zero then remove balloons (subtract a positive) or add weights (add a negative) to make the balloon drop.
- Add balloons to make it rise (add a positive).
- Remove weights to show how subtracting a negative results in the balloon rising (a positive).
It's an effective way of proving to a 9 year old that taking away a negative can make a positive and there was a bonus bit of craft work preparing the number line and stamping out the circles.
Fabulous ideas from : http://www.mathsisfun.com/positive-negative-integers.html
