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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? JIP | Talk 09:13, 28 September 2023 (UTC)[reply]

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)[reply]

For reliable science, you ask random strangers on the Web, as any fool knows. —Tamfang (talk) 17:14, 4 October 2023 (UTC)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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)[reply]

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 [1] and [2]. --Amble (talk) 18:49, 28 September 2023 (UTC)[reply]