Is gravity getting stronger?

16 January 2011

Question

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 we'll assume that it's been roughly the same ever since because otherwise, we're 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 - It's quite a lot, isn't it?

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

Chris - So in the grand scheme of things, it's trivial.

Dave - Yes, if the rate of stuff falling down had been constant all the way through the Earth's life. However, that obviously isn't 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 we've made are completely wrong and in fact, the whole of the Earth's 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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