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When the asteroid hit the earth 65 million years ago, did the earth's gravity pull change? By how much?

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The gravity of Earth did not change substantially.

The strength of gravity on Earth is about 9.8, but varies between 9.76 and 9.83 ms-2(due to Earth not being a perfect sphere)

This can be approximated from the mass of the Earth using $$g=GM_e/r_e^2$$

Where $M_e = 5.97219\times 10^{24}kg$ and $r_e= 6371000m$.

The mass of the asteroid was about $10^{16}kg$ That changes the mass from $5.97219\times 10^{24}$ to $5.97219001\times 10^{24}$. It changes the gravity from 9.8 to 9.800000001

In other words. The change in gravity was utterly insignificant. You probably get a greater change in gravity by walking downstairs.

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    $\begingroup$ Hah - so you're leaving the calculation of $\Delta F_G$ when altitude changes by 4 meters to the reader, huh? :-) $\endgroup$ – Carl Witthoft Apr 2 '18 at 17:24
  • $\begingroup$ So, did the earth's atmosphere increase in depth by 4 meters? $\endgroup$ – Patrick Dawe Apr 3 '18 at 8:26
  • $\begingroup$ @PatrickDawe I think Carl was refering to the change in gravity when walking downstairs. The depth of the atmosphere changes all the time, by much more than 4m, due to solar flares heating the outer layers. $\endgroup$ – James K Apr 3 '18 at 12:03
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    $\begingroup$ 4 meters $\Rightarrow$ $\Delta g \simeq 10^{-5}\,\mathrm{m}\,\mathrm{s}^{-2}$. The change from a $10^{16}\,\mathrm{kg}$ asteroid corresponds to a change in height of $5\,\mathrm{mm}$, i.e. the difference between your upper and lower teeth. $\endgroup$ – pela Apr 26 '18 at 4:19
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It is possible the gravity changed locally. Depending on what the asteroid was made of, and the details of what happened in the collision, you might end up with a lump of denser asteroidal material buried in the Earth's crust under the impact crater. That could produce a measurable (although not noticeable to a human) increase in gravity locally.

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