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A simple question about gravity

Discussion in 'off topic' started by ks.234, Jan 14, 2020 at 2:41 PM.

  1. ks.234

    ks.234 pfm Member

    In the cartoon image of Einstein’s explanation of gravity, a rubber sheet is pulled tight and a bowling ball is placed in the middle. The distortions in the rubber sheet representing the distortions in space and time.

    If an object like a tennis ball is then thrown onto the rubber sheet, the tennis ball will gravitate towards the bowling ball.


    If this image is correct, will the orbit of the tennis ball get less each turn and eventually rest on the bowling ball?
    gavreid likes this.
  2. stevec67

    stevec67 pfm Member

    Yes it will. The spiral of approach will tighten more and more quickly as it approaches the middle. This is because the depression caused by the bowling ball gets "steeper" as you approach it, like gravity. I'm not sure that the gradient of increase is the same though, and I don't have the maths to test that. It would be complex.

    Obviously the ball can escape the "gravity" field just like any orbiting object. If there were no friction the ball would orbit without being drawn in.
    ks.234 likes this.
  3. Tony Lockhart

    Tony Lockhart pfm Member

    If I threw the tennis ball it’d miss.
    Rob998 and ks.234 like this.
  4. gavreid

    gavreid pfm Member

    You can get quite complex behaviour but here's a good demonstration

    darrenyeats, roman, vuk and 2 others like this.
  5. ks.234

    ks.234 pfm Member

    Does that mean that the moon is drawing closer to the earth, and will one day hits us?
  6. richgilb

    richgilb Admonishtrator

  7. Vinny

    Vinny pfm Member

    The gravitational attraction between any two masses (of any kind, anywhere), is Gmm/r^2. The two m's being the two masses, r the distance between the two centres of mass, and G the universal gravitational constant.

    Hence, as the masses approach one another, the attraction increases in inverse proportion to the square of the separation.

    The reason that g (the earth's gravitational "constant") is regarded as equal, for all intents and purposes, for all masses, is that all masses of concern are minute compared to the mass of the earth. g does vary slightly across the earth due to significant variation in r though.
    roman likes this.
  8. Sunflower Sutra

    Sunflower Sutra Active Member

    Don’t worry! pretty sure the sun will go supernova before that happens.

    *correction* turn into a red giant and evaporate all the seas on earth
    ks.234 likes this.
  9. Joe P

    Joe P certified Buffologist / mod

    One of The Onion's best articles — Evangelical Scientists Refute Gravity With New 'Intelligent Falling' Theory


    MikeMA and ks.234 like this.
  10. ks.234

    ks.234 pfm Member

    That’s brilliant, many thanks
  11. ks.234

    ks.234 pfm Member

  12. stevec67

    stevec67 pfm Member

    No, because there is no friction in space. No air, remember. This is independent of the tidal effect, which is a new one on me.

    However a dry moon going around a dry planet will orbit more or less for ever.
  13. Mike Reed

    Mike Reed pfm Member

    My tennis balls get sliced, smashed and served; delicious. Amazing, the scientific brains on here, and not just hifi technical either !
  14. herb

    herb music live

    I saw an explanation on TV recently as to why we cannot dump our waste into our sun, we would need more speed otherwise it would miss. It would decay into orbit. I think I get it.
  15. ks.234

    ks.234 pfm Member

    Also, if the moon was chipped off the earth, its impetus would be outward, not inward? So the outward force would cancel out the inward gravitational force?
  16. vuk

    vuk \o/ choose anarchy

    the cycling shorts joke was ace.
    gavreid likes this.
  17. ks.234

    ks.234 pfm Member

    Yes, I thought I saw a brown mark...
  18. ks.234

    ks.234 pfm Member

    ...anyway...if I was was to suppose a stupidly simple question like, ‘what does gravity look like?’, would something like....

    ...be a suitably simple cartoonish answer?
    Sue Pertwee-Tyr likes this.
  19. Vinny

    Vinny pfm Member

    Not actually true.
    "Space" is not absolute vacuum, and even if it was, that would not counteract gravitational forces. What keeps the moon in approximately constant orbit around earth, and other planetary objects in stable orbits, is here (and many other places) -


    Even better - go down the page -

    darrenyeats and ks.234 like this.
  20. ks.234

    ks.234 pfm Member

    Excellent, give me pictures over numbers anytime.

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