# Can the size of some dinosaurs be explained by weaker gravity? **URL:** \ **Category:** Factual Questions\ **Created:** [February 4, 2004, 1:07am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409 "2004-02-04T01:07:08Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Capt.Caustic](https://avatars.discourse-cdn.com/v4/letter/c/ecc23a/32.png) [@Capt.Caustic](https://boards.straightdope.com/u/Capt.Caustic)\ **Post date:** [February 4, 2004, 1:07am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/1 "2004-02-04T01:07:08Z")
It’s a simplistic notion, I know. But then I’m kinda simple myself. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 4, 2004, 1:12am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/2 "2004-02-04T01:12:42Z")
The strength of gravity from an object such as a planet depends on (a) the mass of the planet and (b) how far you are from the centre of the planet. For the Earth’s gravity to have been weaker than it is now, the Earth would have had to contain less mass, or have a larger radius. As far as we know, it’s always been the same size and mass. One theory about the size of the giant sauropods (the big long-necked types like brachiosaurs) is that they may have spent part of their lives in swamps or lakes, so the water would help support their body weight. ---
**Author:** ![Capt.Caustic](https://avatars.discourse-cdn.com/v4/letter/c/ecc23a/32.png) [@Capt.Caustic](https://boards.straightdope.com/u/Capt.Caustic)\ **Post date:** [February 4, 2004, 1:21am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/3 "2004-02-04T01:21:18Z")
If the earth spun faster, would the centripetal force make an appreciable impact on the gravitational pull at the surface? ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 4, 2004, 1:26am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/4 "2004-02-04T01:26:28Z")
> [@Capt. Caustic](#): > > If the earth spun faster, would the centripetal force make an appreciable impact on the gravitational pull at the surface? No. An equivalent centripetal force would have acted on the tectonic plates below the creatures, making the relevant calculations the same as they are for us today. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 4, 2004, 1:28am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/5 "2004-02-04T01:28:25Z")
It would have to spin a lot faster, but yeah. (Duh. I’m supposed to be studying classical mechanics, and I forgot about that one.) Not sure how much faster (anyone feel like doing the calculation?), but something would have to have happened to slow it down. (There’s no ‘drag’ force that could account for the Earth’s rotation slowing.) ---
**Author:** ![Capt.Caustic](https://avatars.discourse-cdn.com/v4/letter/c/ecc23a/32.png) [@Capt.Caustic](https://boards.straightdope.com/u/Capt.Caustic)\ **Post date:** [February 4, 2004, 1:31am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/6 "2004-02-04T01:31:15Z")
> [@GorillaMan](#): > > No. An equivalent centripetal force would have acted on the tectonic plates below the creatures, making the relevant calculations the same as they are for us today. I was thinking that the earth’s circumference would increase out from the center, and thereby reduce the force acting on the critters. ---
**Author:** ![Pepsi\_Classic](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Pepsi\_Classic](https://boards.straightdope.com/u/Pepsi_Classic)\ **Post date:** [February 4, 2004, 1:32am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/7 "2004-02-04T01:32:02Z")
> [@](#): > > Not sure how much faster (anyone feel like doing the calculation?), but something would have to have happened to slow it down. (There’s no ‘drag’ force that could account for the Earth’s rotation slowing I thought the rotation of the Earth was slowing. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 4, 2004, 1:33am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/8 "2004-02-04T01:33:01Z")
We did this calculation in first year physics class… how fast would the Earth have to spin in order for a person to fly off the surface? There was an answer, but it was ridiculously fast. ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 4, 2004, 1:34am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/9 "2004-02-04T01:34:04Z")
Pepsi: my point was that I don’t think it could have been going _that_ much faster. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [February 4, 2004, 1:36am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/10 "2004-02-04T01:36:57Z")
The Earth is slowing because of tidal friction with the Moon. As I understand it, in the time of the dinosaurs the day was about 21 hours. It doesn’t matter exactly what the number is, because the corresponding acceleration is not a signficant part of the gravitational pull - [something like 0.45%](http://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&safe=off&q=radius+of+earth+*+4+*+pi%5E2+%2F++%2821+hours%29+%5E+2+in+g&btnG=Google+Search). ---
**Author:** ![Pepsi\_Classic](https://avatars.discourse-cdn.com/v4/letter/p/f19dbf/32.png) [@Pepsi\_Classic](https://boards.straightdope.com/u/Pepsi_Classic)\ **Post date:** [February 4, 2004, 1:37am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/11 "2004-02-04T01:37:06Z")
What is making the Earth rotate slower. ---
**Author:** ![GorillaMan](https://avatars.discourse-cdn.com/v4/letter/g/50afbb/32.png) [@GorillaMan](https://boards.straightdope.com/u/GorillaMan)\ **Post date:** [February 4, 2004, 1:38am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/12 "2004-02-04T01:38:13Z")
> [@Indefatigable](#): > > We did this calculation in first year physics class… how fast would the Earth have to spin in order for a person to fly off the surface? There was an answer, but it was ridiculously fast. ‘First Year’ suggests that you were regarding the earth as a solid sphere, not a small solid sphere surrounded by molten rock surrounded by floating solid rock ---
**Author:** ![Indefatigable](https://avatars.discourse-cdn.com/v4/letter/i/9d8465/32.png) [@Indefatigable](https://boards.straightdope.com/u/Indefatigable)\ **Post date:** [February 4, 2004, 1:39am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/13 "2004-02-04T01:39:09Z")
Achernar: thanks, that’s what I was after. All the physics has fallen out of my head tonight. And I hate not being able to delete the messages where I said something stupid. :rolleyes: ---
**Author:** ![Capt.Caustic](https://avatars.discourse-cdn.com/v4/letter/c/ecc23a/32.png) [@Capt.Caustic](https://boards.straightdope.com/u/Capt.Caustic)\ **Post date:** [February 4, 2004, 1:41am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/14 "2004-02-04T01:41:38Z")
I guess this notion isn’t new, as a quick google led me to a page that pushes this idea. [http://www.kitombo.com/e/gon/0423.html](http://www.kitombo.com/e/gon/0423.html) Has some calculations that I didn’t try to verify. ---
**Author:** ![DrFidelius](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/drfidelius/32/3447_2.png) [@DrFidelius](https://boards.straightdope.com/u/DrFidelius)\ **Post date:** [February 4, 2004, 2:05am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/15 "2004-02-04T02:05:20Z")
The short answer to the OP is “No.” The longer answer involves population dynamics, energy resource utilization, the differences between reptilian-style dentition and mammalian-type teeth, the square-cube law as applied to bare skinned endotherms, Ted Holden, flying feral chickens, physics equations, and some snickering. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [February 4, 2004, 2:38am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/16 "2004-02-04T02:38:38Z")
> [@Indefatigable](#): > > One theory about the size of the giant sauropods (the big long-necked types like brachiosaurs) is that they may have spent part of their lives in swamps or lakes, so the water would help support their body weight. A theory that has been discredited for some time, I might add. The skeletal structure of large sauropods is such that they are light but strong. Simple engineering principles are sufficient to explain how they were able to function, without the need for swamps, weaker gravity, alien anti-gravity rays, or anything else. ---
**Author:** ![pravnik](https://avatars.discourse-cdn.com/v4/letter/p/8edcca/32.png) [@pravnik](https://boards.straightdope.com/u/pravnik)\ **Post date:** [February 4, 2004, 2:51am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/17 "2004-02-04T02:51:26Z")
> [@Indefatigable](#): > > For the Earth’s gravity to have been weaker than it is now, the Earth would have had to contain less mass, or have a larger radius. As far as we know, it’s always been the same size and mass. Oh yeah? Every time we go to the moon, we _bring rocks back!_ ---
**Author:** ![moriah](https://avatars.discourse-cdn.com/v4/letter/m/b2d939/32.png) [@moriah](https://boards.straightdope.com/u/moriah)\ **Post date:** [February 4, 2004, 6:48am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/18 "2004-02-04T06:48:40Z")
Well, the Earth did have less mass before the addition of the mass of the meteor that killed off all the dinosaurs. Peace. ---
**Author:** ![iidkyimys](https://avatars.discourse-cdn.com/v4/letter/i/d6d6ee/32.png) [@iidkyimys](https://boards.straightdope.com/u/iidkyimys)\ **Post date:** [February 4, 2004, 7:49am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/19 "2004-02-04T07:49:28Z")
I’ve heard that the size of dinosaurs could be contributed to the fact the Earth used to have much more oxygen. More oxygen = larger life forms. It can be attributed to a lot more plant life. It’s just a theory, but it does make some sense. ---
**Author:** ![Latino\_Yoshi](https://avatars.discourse-cdn.com/v4/letter/l/5e9695/32.png) [@Latino\_Yoshi](https://boards.straightdope.com/u/Latino_Yoshi)\ **Post date:** [February 4, 2004, 8:07am UTC](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409/20 "2004-02-04T08:07:07Z")
> [@](#): > > Originally Posted by **Indefatigable** > For the Earth’s gravity to have been weaker than it is now, the Earth would have had to contain less mass, or have a larger radius If the earth had a larger radius, it is highly unlikely that it would have had the same mass. It is more likely that it would have had a larger mass with the larger radius, thus leading to a more powerful force of gravity. [Next page](https://boards.straightdope.com/t/can-the-size-of-some-dinosaurs-be-explained-by-weaker-gravity/227409.md?page=2)