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'Building with Solid Shapes' printed from https://nrich.maths.org/

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Alistair and his sisters Rachel and Abigail, all from Histon and Impington Infants School, sent a joint solution:

Rachel and Abigail discussed the shapes, and said that shape B would fall down - because balancing on the pointy bits would be hard.
Shape F would roll down.
Shape I has only got one "leg" at one edge, and there is no weight on the other side to balance it. (Their mum believes they were thinking of a T shape without the right arm.)

Alistair said:
Shape I has a fulcrum at the top right, and a weight on the top left so the cuboid will turn downwards and fall over.
Shape D has sort of wheels so if it is made on a flat surface it should be OK, but if it is tilted, the cuboid will roll off.

Rachel and Abigail suggested that if the shapes were given a little "shove", E might fall over. (Mum's translation: unstable)

Alistair, Rachel and Abigail don't mention shapes A, C, G or H. I wonder if this is because they think these are stable?

Adriana's son from Australia sent in this lovely picture which shows solutions and ways of fixing the unstable ones.
 
Thank you - that was great!