Science Questions

Is gravity getting stronger?

Sun, 16th Jan 2011

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Question

Natalie and Ray Etherton asked:

With all of the dust falling on our planet, and the occasional rock like the one from 65 million years ago, has this increased the gravity of our planet since it was first formed, and if so, by how much?

Answer

Chris -  I know that the consensus of opinion for the amount of mass descending onto Earth from space annually is now about 40,000 tonnes or the weight of a modest aircraft carrier every single year.  The Earth is 4,500 million years old and weíll assume that itís been roughly the same ever since because otherwise, weíre going to get tied up into all kinds of knots.  So what is 4,500 million times 40,000?

Dave -   It comes out roughly about 160 million billion tonnes.

Chris -   Itís quite a lot, isnít it?

Dave -   Itís an awful lot compared to us and itís quite a lot compared to anything we might come across.  However, the Earth itself weighs about 6 million trillion tonnes, so itís only one 10 millionth of the mass of the Earth.

Chris -   So in the grand scheme of things, itís trivial.

Dave -   Yes, if the rate of stuff falling down had been constant all the way through the Earthís life.  However, that obviously isnít the case because the Earth was formed by stuff crashing into it and coagulating in the form of a big lump from lots and lots of meteorite type things.  So, the assumptions weíve made are completely wrong and in fact, the whole of the Earthís mass has come from meteorites.

Chris -   ...and stuff accreting, you're right because if you take it to its logical conclusion, the Earth came from particles falling together, so you could say, well 100% of the gravity of the Earth is because of stuff raining on to the Earth effectively.

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