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Thread: physics brain teaser: fun with spaceships

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    physics brain teaser: fun with spaceships

    Okay, so gravity and acceleration are the same?

    Well then, riddle me this Batman.

    If you're a person on a planet, gravity pulls you "down", because the source of the geodesics is the center of the planet ("center of mass").

    But when you're in an accelerating spaceship, "down" is the direction opposite the acceleration, so if you're accelerating forward, down is behind you in the direction of the engine.

    So now, imagine the spaceship standing vertically on the surface of the earth prior to takeoff. The gravitational force at the tip of the rocket is slightly less than the gravitational force at its base, because the tip is slightly farther away from the earth.

    Here's the question: in an accelerating spaceship, is the "force of gravity" the same throughout the ship, or is it slightly smaller at the tip?
    Last edited by nonsqtr; 11-29-2021 at 07:44 AM.

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    Dip a tennis ball in water. Spin it. What happens to the water? Why wouldn’t water and people fly off the earth the same as water flys off the tennis ball?
    The urge to save humanity is almost always a false front for the urge to rule. H.L. MENCKEN

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    Quote Originally Posted by Big Dummy View Post
    Dip a tennis ball in water. Spin it. What happens to the water? Why wouldn’t water and people fly off the earth the same as water flys off the tennis ball?
    They would, if the Earth had the same density as the tennis ball, and could be spun up to a speed that was the same, relative to its mass, as the speed to which the tennis ball is spun up to.
    So let us stop talkin' falsely now
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    Quote Originally Posted by Oceander View Post
    They would, if the Earth had the same density as the tennis ball, and could be spun up to a speed that was the same, relative to its mass, as the speed to which the tennis ball is spun up to.
    Wet a bowling ball and spin it. What doesn’t fly off is pushed backwards. Why isn’t oceans and people rolling backwards away from the spin direction?
    The urge to save humanity is almost always a false front for the urge to rule. H.L. MENCKEN

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    Quote Originally Posted by Big Dummy View Post
    Wet a bowling ball and spin it. What doesn’t fly off is pushed backwards. Why isn’t oceans and people rolling backwards away from the spin direction?
    Differences in scale; the same physics laws produce apparently different results because of differences in scale, to start with.

    Secondly, there is, in fact, a similar effect - it's called inertia, btw - but it's so small compared to other effects that, at the scale of the Earth, it isn't noticeable. Also, the atmosphere rotates with the Earth, but the air surrounding the bowling ball does not rotate with the ball.

    Puhleeze.
    So let us stop talkin' falsely now
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    Quote Originally Posted by Oceander View Post
    Differences in scale; the same physics laws produce apparently different results because of differences in scale, to start with.

    Secondly, there is, in fact, a similar effect - it's called inertia, btw - but it's so small compared to other effects that, at the scale of the Earth, it isn't noticeable. Also, the atmosphere rotates with the Earth, but the air surrounding the bowling ball does not rotate with the ball.

    Puhleeze.
    So air friction with the surface causes the atmosphere to spin? Then wouldn’t the upper atmosphere be still air and the surface have a constant wind?
    The urge to save humanity is almost always a false front for the urge to rule. H.L. MENCKEN

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    Quote Originally Posted by Big Dummy View Post
    So air friction with the surface causes the atmosphere to spin? Then wouldn’t the upper atmosphere be still air and the surface have a constant wind?
    No.
    So let us stop talkin' falsely now
    The hour's getting late -- Jimi Hendrix

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    My head hurts!

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    All right, let's try a different approach.

    If you have a rocket ship that can accelerate as strongly as the gravitational pull of the earth, you can travel anywhere in the galaxy within 10 years.

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    Quote Originally Posted by nonsqtr View Post
    All right, let's try a different approach.

    If you have a rocket ship that can accelerate as strongly as the gravitational pull of the earth, you can travel anywhere in the galaxy within 10 years.
    Assuming the rocket ship maintains constant acceleration throughout, correct?
    So let us stop talkin' falsely now
    The hour's getting late -- Jimi Hendrix

    #RESISTBidet

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