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General Science / Why do office chairs have five wheels?

« on: 29/05/2009 16:34:34 »

Wheeled chairs have a minimum of five wheels for safety reasons.  Because the chair is on wheels it is assumed that people will 'scoot' on them (for short distances) rather than get up and walk, but because the 'scooting' force is applied sideways, to the end of a long arm, the force tends to topple the chair as much as it propels it.  While the wheels are on a smooth surface there are few problems but if someone hits a ridge in the floor, such a rug or a raised floor vent, the chair is much more likely to topple than roll.  This is made even worse by the fact that the wheeled base can rotate freely, so once one wheel has struck a ridge and stopped, the wheeled base is likely to rotate until a second wheel hits the ridge and when this happens the effective wheelbase is no longer twice the arm length but only sin/cos 45 deg of the wheelbase i.e. it is reduced by a factor of 0.7071067.  Increasing the number of wheeled arms on the base mitigates against this.

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General Science / If Humans Were Wiped Out Would The World Survive ? (A nuclear power question)

« on: 22/08/2008 00:36:17 »

Forget about the nuclear plants - after just a few hundred thousand years (an eye blink, in geological terms) you'd not find any signs of damage that might have been caused by unattended melt-downs.  However, nature would still be doing her stuff.  The one thing that everyone who has an interest in geology shares is an appreciation of the time scales involved.

Life on Earth isn't likely to die out until the core cools down and solidifies, at which point we'll probably loose our protective magnetic shield resulting in our atmosphere being stripped away by the solar wind, as appears to have happened on Mars.