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How many Gs do you pull in a lift? Or an elevator,
as Americans call it? Well, to find out, I

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am on the 39th floor of Canary Wharf, one
of the tallest buildings in London -- hello

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London! -- with a kitchen scale and a kilogram
of sugar.

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First things first: kilogram is a measure
of mass, not of weight. That's important.

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If we were in zero-g right now, this would
still mass one kilogram, and moving it would

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take the same amount of effort. Weight is
the force that you experience due to gravity.

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Now technically it should be measured in Newtons,
but we use kilograms on Earth as shorthand

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because it's easier and our gravity doesn't
really change much.

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So: inside this scale is a little spring that
gets compressed by the stuff on top of it.

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The heavier the stuff, the more force on the
spring to compress it, the higher the reading

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on the scale.

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The force that compresses that spring is the
"normal force". Everything - including this

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building - wants to fall through the earth,
but the earth pushes back with an equal and

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opposite force, the "normal force". That push
from the earth gets passed on into the supports

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of this building, into the floor, into my
legs, into my arms, and onto this scale. We

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experience that force as 1g.

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Now if you're in free-fall, you don't have
that force pushing back on you, so you experience

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And if you're in a lift that's accelerating
downwards, the force from the acceleration

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works against the normal force from the Earth,
and you get lighter. So. In this fast lift,

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as it accelerates down, this one kilogram
of flour appears to be about 920 grams. That's

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wobbling about a bit. So that means I'm pulling
about 0.9g, I weigh 10% less, I can jump about

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10% higher...

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But here's the important thing. The lift's
now stopped accelerating, we're at speed,

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it's back to 1g. It doesn't matter how fast
I'm going. I could be plunging at a thousand

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miles an hour. I'm not accelerating, I'm experiencing
1g. That's the reason why you're not constantly

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pushed back into your seat when you're in
a car or a train.

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So! We're now slowing down -- we're pulling
about 1.1g, there's 1.1kg on the scale. The

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force of the lift is being added to the normal
force -- which means we're pulling about 0.1gs

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in this particular lift.

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This building wants to fall through the earth,
the building pushes back with --

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Ha! Well, that's, er... merry Christmas VT.

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