# The Naked Scientists Forum

### Author Topic: How does gravity work?  (Read 3954 times)

#### thedoc

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##### How does gravity work?
« on: 16/01/2014 14:30:01 »

I was doing a simple physics experiment with my 7 year old home school son when the question came up about Gravity. What is gravity?

As I read along the explanation of gravity and the forces that hold up a cardboard underneath a glass full of water I began to doubt the explanation given in this particular book.

As I ponder and questioned their theory that gravity from earth pulls and that the pressure as the result of air pushing from below underneath the cardboard is what sustained and holds the cardboard against the water. Meaning that the weight of air is higher than the weight of the water inside the glass that is upside down on the cardboard.

I deduced that it is not so much the weight of air pushing on all directions that hold the board up against the glass of water, but rather, the sum total of gravity pulling from other celestial bodies.

What do you think?
« Last Edit: 16/01/2014 14:30:01 by _system »

#### syhprum

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##### Re: How does gravity work?
« Reply #1 on: 16/01/2014 19:12:44 »
Let us do a calculation, assume the glass is cylindrical 10 cm high and 10 cm diameter.
the pressure of the water on the cardboard at the base will be equivalent to a head of water 0.01 m high pushing the cardboard down whereas the atmospheric pressure pushing the cardboard up will be equivalent to a column of water 10 m high.
There is no need to invoke the theories of Herr Dr Mach

#### CliffordK

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##### Re: How does gravity work?
« Reply #2 on: 16/01/2014 21:28:03 »

I deduced that it is not so much the weight of air pushing on all directions that hold the board up against the glass of water, but rather, the sum total of gravity pulling from other celestial bodies.

Nice experiment.  More or less the same principle as chicken waterers and humming bird feeders.

You can sum all the gravity vectors of the Earth, Sun, Moon, and all the stars in the Milky Way.  Wikipedia lists the acceleration due to gravity of a few of the celestial bodies.  Obviously as the Earth spins, the direction of the acceleration due to the other celestial bodies would vary slightly.  But, when you sum all the different forces, the greatest force we experience down on Earth is that of Earth, 9.8 m/s2, varied slightly by the different bodies by a few millimeters or picometers per second squared.

Anyway, you know that gravity alone isn't keeping the water in your jar because if you did the experiment without the piece of cardboard, all the water would run out.

Your piece of cardboard creates a seal on top of the glass, and then when inverted, the vacuum created in the jar keeps the water in.  The vacuum, of course, being the absence of air pressure.

Our air pressure is about 14.7 psi, created by the column of air above our heads reaching from the surface of the Earth to beyond 100km in space (which it rarefies, but continues somewhat further).  I.E.  it is that 14.7 psi of air that is keeping the lid on your jar.

Your experiment would fail if you used a pipe filled with water that was 34 feet tall, or a tube filled with mercury 760 mm tall.

#### alancalverd

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##### Re: How does gravity work?
« Reply #3 on: 16/01/2014 22:24:18 »
In order for the water to come out of the glass, some air has to go in. The card prevents this from happening.

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##### Re: How does gravity work?
« Reply #3 on: 16/01/2014 22:24:18 »