Gravity ?

[topdude]

Ok, i was thinking about gravity, as you do !!

If you were standing at the centre of the Earth do you feel enormous gravity because you are at the centre of a large mass ?

Or are you weightless because there is an equal amount of mass around you pulling in all directions ?

[acey]

You're weightless. Newton proved that inside a solid sphere (like the Earth) the gravitational field falls linearly with radius, reaching zero at the centre. The mass around you is evenly distributed so its gravity cancels out.

[tinvek]

you're also very hot

[johnb]

I was on a boat once and you watch the water going down the plug hole in 1 direction, as you cross the equator it goes straight down and then it goes down the plug hole in the opposite direction - fasinating to see for real

John B

[Macavity]

There was that physics brain teaser re. calculating the time taken for a weight to oscallate along a tunnel drilled right through the earth... About 20 minutes, if I remember correctly.

Another good "exercise for the reader" might be to calculate the gravity of a rocky exo-planet versus radius. Produce a GRAPH, maybe? Now, were I less lazy etc.

[AWR]

Another good "exercise for the reader" might be to calculate the gravity of a rocky exo-planet versus radius. Produce a GRAPH, maybe? Now, were I less lazy etc.

Linear surely?

gravitational field strength g = GM/r^2,

Mass of planet M = kr^3 (k = density x 4/3 x pi)

g = Gkr

Andrew

[AWR]

There was that physics brain teaser re. calculating the time taken for a weight to oscallate along a tunnel drilled right through the earth... About 20 minutes, if I remember correctl

Ok, so I'm bored.

The force F on a mass m is given by:

F = -mgr / re

where g is the gravitational field strength at the earth's surface and re is the radius of the earth

Because F is proportional to -r, then SHM results

Using F=ma

a = -gr / re

This gives the angular velocity w = Sqrt( g / re)

and the time period T = 2 pi Sqrt( re / g)

Now re = 6400km and g = 9.8N/Kg

Giving T = 84.5 minutes

Andrew (sad loser!)

[acey]

I was on a boat once and you watch the water going down the plug hole in 1 direction, as you cross the equator it goes straight down and then it goes down the plug hole in the opposite direction - fasinating to see for real

That's a trick they do for tourists - the Coriolis force has no effect on how water goes down a plughole (at least under normal conditions). It's possible that a large tank of water kept completely still for a long period (24 hours) will show the effect after about 15 minutes draining:

Coriolis effect - Wikipedia, the free encyclopedia

but under ordinary conditions, other factors are far more important. People do continue to argue about it endlessly though, e.g. here:

Why does water going down a plughole spin in one direction in the Northern hemisphere and in the other direction in the Southern? Which way (if any) does it spin on the equator? Also are there any planets which don''t spin? | Notes and Queries | guar