How many Gs do you pull in a lift? Or an elevator, as Americans call it? Well, to find out, I
am on the 39th floor of Canary Wharf, one of the tallest buildings in London -- hello
London! -- with a kitchen scale and a kilogram of sugar.
First things first: kilogram is a measure of mass, not of weight. That's important.
If we were in zero-g right now, this would still mass one kilogram, and moving it would
take the same amount of effort. Weight is the force that you experience due to gravity.
Now technically it should be measured in Newtons, but we use kilograms on Earth as shorthand
because it's easier and our gravity doesn't really change much.
So: inside this scale is a little spring that gets compressed by the stuff on top of it.
The heavier the stuff, the more force on the spring to compress it, the higher the reading
on the scale.
The force that compresses that spring is the "normal force". Everything - including this
building - wants to fall through the earth, but the earth pushes back with an equal and
opposite force, the "normal force". That push from the earth gets passed on into the supports
of this building, into the floor, into my legs, into my arms, and onto this scale. We
experience that force as 1g.
Now if you're in free-fall, you don't have that force pushing back on you, so you experience
And if you're in a lift that's accelerating downwards, the force from the acceleration
works against the normal force from the Earth, and you get lighter. So. In this fast lift,
as it accelerates down, this one kilogram of flour appears to be about 920 grams. That's
wobbling about a bit. So that means I'm pulling about 0.9g, I weigh 10% less, I can jump about
10% higher...
But here's the important thing. The lift's now stopped accelerating, we're at speed,
it's back to 1g. It doesn't matter how fast I'm going. I could be plunging at a thousand
miles an hour. I'm not accelerating, I'm experiencing 1g. That's the reason why you're not constantly
pushed back into your seat when you're in a car or a train.
So! We're now slowing down -- we're pulling about 1.1g, there's 1.1kg on the scale. The
force of the lift is being added to the normal force -- which means we're pulling about 0.1gs
in this particular lift.
This building wants to fall through the earth, the building pushes back with --
Ha! Well, that's, er... merry Christmas VT.
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