gravity jump

[map08]

Hi there,
I once heard in physics class that if you jump up, not only are you travelling away from the earth, but the earth also moves ever so slightly away from you.
Can this be right? I can understand that the earth is attracted slightly by a human body's gravity, but can't see how it would be repulsed.
Thanks!

[mibn2cweus]

For every action there is an opposite and equal reaction however slight for the Earth�s part considering that the Earth "weighs in� at 5.972e24 kg.
Nevertheless, the mutual attraction of gravity between the Earth and the suspended jumper would bring both back to about the same place when, inevitably, the jumper returns to the Earth's surface.

[ugly_bob]

map08 - it's all about conservation of momentum.

If you jumped away at 20m/s and weighed 60kg, gives a momentum of 1200 kgm/s
thus the earth would need a compensating momentum in the other direction. 1200kgm/s / 6x10^24kg = 2*10^-22m/s

So the earth would move away from you at 200 millionths of a millionth of a millionth of a millionth of a metre per second

[WaldoMcFroog]

Does this mean that if two identical twins, one at each pole, both jumped the same height at the same time, the Earth would compensate by temporarily bulging by a minute amount in the middle?

[ugly_bob]

possibly. That's getting into more complicated situations.

If you say that the earth was completely rigid, one identical twin is leaving the system in either directino, so the earth would not move at all.

[mibn2cweus]

Just wondering which twin would be older when they returned to the Earth?

[mibn2cweus]

. . . never mind, I figured it out, the twin at the South Pole would be older of course. The North Pole spins counter-clockwise, duh!

[China Doll]

<Walks off clutching head in search of strong pain killer >