Stability and centre of mass — Engineering, 11–13 years
Engineers help objects stay upright by thinking about where their mass acts and where their support lies.
What makes something stable?
An object is stable when its weight acts above its supporting base. Imagine a straight line dropping from its centre of mass: if that line stays inside the base, the object can stand. A wide base and a low centre of mass usually make tipping harder.
Why engineers need this idea
A tall object can fall even when nothing pushes it, simply because its weight line has moved beyond its support. Engineers met this problem in towers, vehicles, furniture and machines. Thinking about stability tells them where to put mass, widen a base or add an anchor.
A balancing ruler
A 30 cm ruler balances on your finger at 15 cm because its mass is spread evenly on both sides. Put a 100 g block at the 25 cm mark, and the centre of mass moves towards that block, so the ruler tips that way. Move your finger to about 20 cm and it balances again.
The reasonable mistake
It is tempting to think that a heavier object must be more stable. Weight does press down, but stability also depends on the height and position of the centre of mass and on the size of the base. A heavy object on a narrow, tall stand can tip more easily than a lighter, wide one.
Where it is used
Buses, cranes and bicycles all need controlled stability. A crane carries heavy loads low or uses counterweights so its centre of mass stays over its support. Designers also use this idea for office chairs, camera tripods and playground equipment, where tipping would be dangerous.
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