Wikipedia:Reference desk/Archives/Science/2023 September 28

= September 28 =

Weighing less than nothing in Don Rosa's comic (continued)
I asked a question about this before. I have now read Don Rosa's comic The Universal Solvent again, and can verify that there is indeed a scene where the Ducks have travelled close to the centre of the Earth inside a shaft, and they notice that gravity is now working upwards instead of downwards because, as the Ducks cite, "more of the Earth's mass is above them than below them". I could even take photographs of the pages and upload them to Wikipedia but I don't think copyright reasons would allow this.

I remember that the last time I asked, there was a reply that this shouldn't work like this, as the entire other half of the Earth is still below the Ducks from their point of view.

Am I therefore correct that Don Rosa made a mistake in how physics work here? J I P &#124; Talk 09:13, 28 September 2023 (UTC)
 * Either a mistake or literary license. Comic books are not necessarily reliable sources for science. ←Baseball Bugs What's up, Doc? carrots→ 11:05, 28 September 2023 (UTC)
 * For reliable science, you ask random strangers on the Web, as any fool knows. —Tamfang (talk) 17:14, 4 October 2023 (UTC)
 * See shell theorem for the real answer (gravity always points inwards, goes down linearly with radius, and is zero at the center). —Kusma (talk) 11:10, 28 September 2023 (UTC)
 * The shell theorem was also referred to the previous time this question was posed. Earth's mass distribution is not perfectly symmetric, so there will be some discrepancy between the centre of mass centre of gravity, where the intrepid explorers are weightless, and the centre of symmetry of the Earth spheroid, where they experience a force towards the mass centre. --Lambiam 16:34, 28 September 2023 (UTC)
 * For a general asymmetric mass distribution, I don't see why the centre of mass should necessarily be a point of weightlessness. —Kusma (talk) 17:07, 28 September 2023 (UTC)
 * Simplest 1d example: Two point masses a and b at -1/a and 1/b have centre of mass at 0. Gravity there is -1/a^3+1/b^3, which is nonzero unless a=b. —Kusma (talk) 19:22, 28 September 2023 (UTC)
 * You're reading a comic about talking ducks with human-level intelligence, and you're worried about the accuracy of an obscure aspect of gravity? {The poster formerly known as 87.81.230.195} 90.212.130.182 (talk) 16:30, 28 September 2023 (UTC)
 * Short of asking Don Rosa, you can read his thoughts about related questions in the archives of the Disney Comics Mailing List. See threads from April 1995, particularly and . --Amble (talk) 18:49, 28 September 2023 (UTC)