Comment on xkcd #3084: Unstoppable Force and Immovable Object

• Cocodapuf@lemmy.world ⁨1⁩ ⁨day⁩ ago

  Well I don’t know about any objects more massive than black holes. I think a black hole is really the only viable form a body can take once there’s enough matter in one place, like there’s an upper limit for the size of stars and after that anything larger collapses into a black hole.

  An object of infinite mass is a contradiction, a universe can’t exist with a single object of infinite mass, it would consume everything instantly.

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  • ripcord@lemmy.world ⁨20⁩ ⁨hours⁩ ago

    OK, but being very massive is not the same as what was being discussed.

    You can also “lift” a finitely massive black hole with anything else massive.

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    • Cocodapuf@lemmy.world ⁨9⁩ ⁨hours⁩ ago

      OK, but being very massive is not the same as what was being discussed.

      Are you sure? I mean the word “heavy” was what I was going on, but there is a distinction I suppose.

      You can also “lift” a finitely massive black hole with anything else massive.

      Yeah, that’s true… But again, I do have to stress that there is no alternative to “finitely massive” you really can’t have an object of infinite mass in our universe.

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  • Snazz@lemmy.world ⁨20⁩ ⁨hours⁩ ago

    It may be worth it to decide how we define ‘unstoppable force’ and ‘immovable object’.

    An Immovable Object has 0 velocity:

    v = 0

    Acceleration is the time derivative of velocity:

    a = d/dt(v(t))

    a = d/dt(0)

    a = 0

    And we know that

    a = F~net~ / m

    An object with infinite mass would satisfy this equation, but an object with no net force would too. We could add a correction force that will satisfy the constraint of 0 net force.

    |F~net~| = 0

    ∑F~i~ = 0

    F~correction~ + … = 0

    To satisfy Newton’s 3rd law, we would need a reaction force to our correction force somewhere, but let’s not worry about that for now.

    A physics definition of ‘Unstoppable Force’ is:

    |F~unstoppable~| =/= 0

    In this case the gravitational force fits this description, given a few constraints

    F~g~ = Gm∑ M~i~ / x~i~^2^

    As long as the gravitational constant G is not 0, our object has mass, and

    ∑ M~i~ / x~i~^2^ =/= 0, then

    |F~g~| > 0

    But this does feel kinda like cheating because it’s not really what people mean by ‘unstoppable force’. the other way to define it is just immovable object in a different reference frame.

    a = 0, |v| > 0

    I’m gonna stop here because this is annoying to type out on mobile

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