I just read the article "New trailer for 'Moonfall' promises a disaster movie on a scale like no other, but how accurate is the physics?" on the upcoming movie "Moonfall," the interview with Peter Travers and the physics involved.
https://www.space.com/moonfall-new-trailer-vfx-interview
I take issue with the excerpt:
"Even if the moon and the Earth were touching with our current assumptions of the moon, and its density, because the moon is 1/100 of the mass of the Earth. And so the gravitational equation doesn't support that even if the moon and the earth were touching, that you wouldn't, you wouldn't really feel much from the moon."
The article goes on to say that to fix the "problem," they had to "inject a ton of mass." The moon's mass is actually about 1/80 that of earth's, but never mind. With the actual masses and distance of the earth/moon system, tidal effects are already significant. The average moon-earth distance is slightly less than 240,000 miles. If the moon and earth were just touching, the distance between centers would be the sum of their radii, i.e. roughly 4,000+1,000, or about 5,000 miles (only slightly less). The ratio of those distances is then about 48. Suppose that before contact, the surface-to-surface earth-moon distance is a bit less than 1,000 miles, such that the center-to-center distance is 6,000 miles, and the ratio of the present distance to the near-contact distance is 40. Tidal effects vary as the CUBE of the center-to-center separations. Thus the near-contact tidal effects would be 40 to the 3rd power, or 64,000 times as much as the present tides (!). I think that the moon's tidal effect would wreak havoc on earth MUCH before getting that close. Moreover, before it got that close, the moon would be ripped apart by the earth's tidal effects.
This is very basic physics, and widely known. Either one of us is terribly wrong, or the article misleads the reader into thinking an intact moon crashes into earth, or that tidal effects are not included in the physical basis of the movie.
Sincerely,
Jack Jewell
https://www.space.com/moonfall-new-trailer-vfx-interview
I take issue with the excerpt:
"Even if the moon and the Earth were touching with our current assumptions of the moon, and its density, because the moon is 1/100 of the mass of the Earth. And so the gravitational equation doesn't support that even if the moon and the earth were touching, that you wouldn't, you wouldn't really feel much from the moon."
The article goes on to say that to fix the "problem," they had to "inject a ton of mass." The moon's mass is actually about 1/80 that of earth's, but never mind. With the actual masses and distance of the earth/moon system, tidal effects are already significant. The average moon-earth distance is slightly less than 240,000 miles. If the moon and earth were just touching, the distance between centers would be the sum of their radii, i.e. roughly 4,000+1,000, or about 5,000 miles (only slightly less). The ratio of those distances is then about 48. Suppose that before contact, the surface-to-surface earth-moon distance is a bit less than 1,000 miles, such that the center-to-center distance is 6,000 miles, and the ratio of the present distance to the near-contact distance is 40. Tidal effects vary as the CUBE of the center-to-center separations. Thus the near-contact tidal effects would be 40 to the 3rd power, or 64,000 times as much as the present tides (!). I think that the moon's tidal effect would wreak havoc on earth MUCH before getting that close. Moreover, before it got that close, the moon would be ripped apart by the earth's tidal effects.
This is very basic physics, and widely known. Either one of us is terribly wrong, or the article misleads the reader into thinking an intact moon crashes into earth, or that tidal effects are not included in the physical basis of the movie.
Sincerely,
Jack Jewell