texas blues said:
What's the dealio with the freight trains?
The most massive planet is Jupiter. It has a mass of 1.90*10^27 kg (that's one followed by 27 zeroes) and at its closest, it is 3.9516 AU = 591,000,000 km away from the Earth.
The closest planet (obviously the moon is closer, and can affect tides etc...but it doesn't seem to trigger earthquakees on a monthly cycle, so we just want to look at other effects on top of the tidal cycle) is Venus, which can be as close as 38,000,000 km away at its closest, with a mass of 4.87*10^24 kg.
So how much gravitational pull can these planets exert on the Earth's surface at their closest? Let's compare the force exerted by Jupiter, by Venus, and by a single freight train.
F=Gm1m2/r^2 where G is the gravitational constant, m1 and m2 are the masses of two objects, and r is the distance between them.
Let's say m1 = the mass of a particular patch of ground...since we will be comparing the pull on the patch of ground by different objects, we can just leave it as m1 and look at the ratios between the forces exerted by various objects. Same goes for G - it will enter into all the force calculations equally, so we can ignore it and just look at relative magnitudes, so long as we use consistent units for m2 and r in all calculations. Let's just call G*m1 "x".
OK, how much does Jupiter pull on a patch of ground, at its closest?
F=G*m1*m2*/r^2 = x*(1.9*10^27)/(591,000,000^2)= 5,439,746,222 * x.
And Venus:
F=G*m1*m2*/r^2 = x*(4.87*10^24)/(38,000,000^2)= 3,372,576,177 * x
Now, consider a freight train. Its center of mass is maybe 10 meters off the ground, or 0.01 km, and it weighs maybe 150,000,000 kg. For it:
F=x*(150000000)/(0.01^2) = 1,500,000,000,000 * x.
So a *single* freight train exerts about 275x as much gravitational pull on the Earth's surface as does the most massive planet at its closest, and about 444x as much pull as the closest planet. And there are a WHOLE LOT more freight trains than there are planets. Even if you argue we should look at pull on some point deep into the thickness of a tectonic plate, given the MUCH smaller distances involved not to mention the far greater number (and faster relative movements) of trains, you can't escape the conclusion that freight trains create much more (and more rapidly variable) pull on the Earth's surface than do the planets.