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When dinosaurs roamed the Earth was the Earth rotating faster creating less gravity to allow for their size? How fast could have the Earth spin and be slowed down by an asteroid to the current rotational speed not completely wiping all life out? How much would 100 kg weigh then?

With the asteroid the opposite direction of the Earth's rotation unlike the moon's hard interior that the asteroid would just punch through the crust into the mantle allowing the Earth not to instantly be affected by the change in rotation. Also it was theorized that the moon was a large piece of the Earth liquid mantle removed by an asteroid.

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    $\begingroup$ An interesting question, but we really do encourage folks to take a stab at the answer before posting. $\endgroup$ Jan 19 '16 at 13:30
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    $\begingroup$ Apparently we really still are far from a consensus as to why the dinosaurs were so big. I've studied this a great deal since my dinosaur-crazy daughter was born and her enthusiasm's rubbed off on me. There are several possibilities. The most plausible I have heard is, given the low grade food sauropods had to eat (cycads and pine needles) in the Jurassic, you need to become an industrial scale digestion plant on legs to access economies of scale. The bigger you are, the more efficient this processing, and the upper limit is basically that you get so big that you take so long .... $\endgroup$ Dec 18 '16 at 5:07
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    $\begingroup$ ...(about 200 years) to grow up, live and reproduce, and you thus become too slow to adapt to changing ecologies. Once you have big sauropods and herbivores, it becomes clear why the meat eaters get big. Note that, for modern animals, zoologists have theories they are fairly satisfied with: see here for example. $\endgroup$ Dec 18 '16 at 5:07
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    $\begingroup$ The asteroid didn't slow down earth's rotation---it's mostly because of the moon. $\endgroup$ Dec 19 '16 at 16:22
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    $\begingroup$ The asteroid that killed the dinosaurs was the size of a town or city. The planetoid that formed the moon was comparable in size to mars or the entire earth. $\endgroup$ Dec 20 '16 at 19:01
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According to this problem set from Goddard Spaceflight Center in the upper Triassic the day was $23.5$ hours. The rotation speed was then $\frac {24}{23.5} \approx 1.021$ times higher. The equatorial speed of the earth is now $465.1$ m/sec. The centripetal acceleration at the equator is now $\frac {465.1^2}{6378100}\approx 0.034$ m/sec, which reduces $g$ by $0.3\%$. In the upper Triassic, the acceleration would only be $4\%$ of that higher, so about $0.035$ m/sec$^2$. It is not enough to matter.

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    $\begingroup$ The moon was closer too. So twice a day, you would lose some more weight... Also, according to this the earth radius might have been 102% ± 2.8% of today's value. Which would further increase the drop in $g$. $\endgroup$
    – Floris
    Jan 19 '16 at 3:19
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    $\begingroup$ @Floris: I hadn't seen that,but the Wikipedia page you link to says scientific consensus is against. Even so, increasing the radius to $102\%$ will decrease $g$ by $4\%$. That will swamp the decrease due to rotation speed. We still have less than $5\%$ decrease overall. Not a big effect if you are doing mechanical design of animals. $\endgroup$ Jan 19 '16 at 3:24
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    $\begingroup$ @Floris The moon was once even closer than that: aleastory.co.uk/The-Distance-of-the-Moon.pdf $\endgroup$ Jan 19 '16 at 13:38
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    $\begingroup$ @JohnForkosh - the expression "how much would 100 kg weigh" is not a blunder: it's a physically accurate expression. 100 kg is a mass (quantity), and on earth it has a weight (force of gravity). Jen uses the term correctly - she asks about the change in the force of gravity on a 100 kg mass. See for example this answer and this one for a more detailed discussion of the usage of "mass" and "weight". $\endgroup$
    – Floris
    Jan 19 '16 at 13:51
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    $\begingroup$ @JohnForkosh it is abundantly clear from the context that the answer to the question "how much does it weigh" should be given in Newton - today, 100 kg weighs 981 N (at certain points on earth). Back then it might have been 978 N or so, according to Ross's answer. I stand by my defense of Jen on this one. $\endgroup$
    – Floris
    Jan 20 '16 at 12:56

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