Tidal Forces

Ever wondered what happens when a cloud of rocks skims low over the Earth and back out into space? Yes? Well, here’s your golden opportunity to find out.

Here’s what happens:


The rock cloud starts out as a cube of 125 rocks (bottom right), and as it approaches the Earth it first bunches together a bit (probably because of the gravitational attraction between the rocks).

Then, as it gets close to the Earth, it begins to elongate in the direction of the Earth, and gets squeezed laterally at the same time. This is probably because the front end is nearer the Earth, and accelerates slightly faster than the back end. And also the left hand side is pulled rightward, and the right hand side leftward.

And once it passes over the Earth, the back end gets pulled back more strongly than the front end, so the cloud carries on lengthening (if it doesn’t look like it does, it’s probably because the cloud is moving downwards into the page after it passes over the Earth). And the rocks that were on the left hand side go over to the right hand side, and the right hand side rocks go out the left hand side, changing places. At least, I think so.

Anyway, that’s what happens if it’s a cloud of rocks. But if it’s a pile of rocks in contact with each other, the pile will be able to elongate, but it won’t be able to contract laterally very much because the left hand side rocks will be impeded by the right hand side rocks. They won’t swap sides like the cloud. So a rock pile will elongate into a long thin rock pile, a bit like a sausage. It might even break up into a long train of separate rocks.

This is what happens when small asteroids come too close to large planets, and are said to be torn apart by ‘tidal forces’. This because the same forces act to produce the Earth’s ocean tides, so that the Earth’s oceans get pulled towards the Moon on the side nearest to it, but less strongly on the opposite side, much like with the rock cloud.

Anyway, next time someone asks you what happens when a pile of rocks skims low over the Earth and back out into space – like they so often do -, you’ll be able to say: “Easy. It’s like this…”

P.S. Coincidentally, NASA published an article about rock clouds yesterday.

About Frank Davis

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14 Responses to Tidal Forces

  1. c777 says:

    Like this Lyrid fragment that entered the atmosphere over Argentina?
    I suppose they will say the Lyrid meteor shower at the same time was a mere coincidence as well.
    However the Lyrid shower are cometry fragments so they would be going really fast .105,000 kph or so increasing the energy of the object.
    The object was estimated to be about 300mm Ø.


  2. Chuckles says:

    Neutron Star, a short story by Larry Niven.

  3. margo says:

    Fascinating, Frank!

  4. Sheila says:

    Another tidal force ?
    You may want to have a say here,
    Bob Blackman advocating a smoking ban in private vehicles.

  5. harleyrider1978 says:

    Bob Blackman MP: Laws to protect children from smoke in private vehicles reflect true Conservative values


    GO NAIL THIS NAZI RINO as we call them in America! I already nailed his hide.

  6. harleyrider1978 says:

    Get in on this fight DAVE ATHERTON’S COME OUT SWINGING!!!!!

  7. Rose says:

    Russian ‘smokers’ rights’ movement holds founding convention

    “Russia’s Federation of Independent Trade Unions backed the new movement,
    claiming that the anti-smoking laws infringed on the rights of smoking workers. Trade union leaders also expressed fears that the elimination of designated smoking areas at Russian businesses would lead to workplace stress.”

    How very different from our own Trades Union Congress

    10 “The TUC supports the proposed penalties for those who smoke in a public place or workplace, regardless of whether there is a notice in place. In addition we would support a penalty of the same amount for removing or defacing a warning notice.”


    In summary the TUC:
    Warmly welcomes the ban,

    Opposes exemptions for pubs that do not sell prepared food or private members clubs,
    Asks the government to provide additional resources to local authorities for enforcement”

    If the TUC ever showed any consideration for their soon-to-be-denormalised smoking workers, I have yet to find it.

    Also, considering possible adverse consequences was clearly not a strong point.

    “However if there were a blanket ban, it is unlikely that there would be any negative effect on the licensed trade industry, where many union members work.”

    • harleyrider1978 says:

      Rose Ive a feeling the Bulgarians are also joining in on a euro level fight with the Russians. I dont have a hardline link yet on that assumption,but it will likely show up very soon.

  8. Zaphod says:

    “the Earth’s oceans get pulled towards the Moon on the side nearest to it, but less strongly on the opposite side.”

    Explaining tide to the uninitiated is always tricky. That is only half the story.

    The near side bulge is due to the stronger Moon gravity, but the opposite side has a bulge too, caused by greater centrifugal/(petal) force. Hence two tides per day, not one.
    This explanation is also inadequate. (Shrug.) :-)

  9. nisakiman says:

    Off topic, but have we seen this one before?

    Passive Smoking Linked to Hearing Damage


    A sort of “Smoke gets in your ears” – a modern variation on an old theme.

    I lose track of all the maladies they doggedly keep attributing to smoking. Have they done ingrown toenails yet? We should perhaps run a competition to see who can come up with the most bizarre claim from the Tobacco Control Industry.

  10. Bucko says:

    Why do they start out as a cube?

  11. Merv says:

    Hi Frank – wondered if the tidal forces you’re talking about explains yet another close shave coming our way (4 million miles – not much in astronomic terms)!


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