This question appeared on Bangladesh physics Olympiad 2014, Sylhet, Bangladesh for students of class 9 and 10.

I really tried to solve this, but I failed. Please help.

Suppose that, while lying on the beach near the equator watching the sun set over a calm ocean, you start a stopwatch just as the top of the sun disappears. You then stand, and fitting your eyes by a height $H=1.70\,\mathrm m$, and stop the watch when the top of the sun again disappears. If the elapsed time is $T=574\,\mathrm s$, what is the radius of the Earth?

You can check out the question: http://bdpho.org/pdf/sylhet_cat%20B_2014.pdf The question is on number 7.

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    $\begingroup$ What did you try? Why didn't it work? $\endgroup$ – Allure Jan 3 at 10:00
  • $\begingroup$ I tried to calculate it by using the information 'The earth rotates 1° every 4 minutes' but it doesn't i couldn't get it to work. Then i thought finding out the speed of earth and relate with 2πr. But i got stuck. I did more attempts but i can't remember those at this moment. $\endgroup$ – GDD Jan 3 at 10:06
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    $\begingroup$ Please try to find a descriptive title. The title should indicate the topic of the question in such a way as to distinguish it from other questions, without going into unnecessary detail. Do not use a clickbait title. $\endgroup$ – Loong Jan 3 at 11:38
  • $\begingroup$ I am sorry I just couldn't find a title $\endgroup$ – GDD Jan 3 at 11:39
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    $\begingroup$ Welcome to Physics! Please note that this site is not a place to obtain solutions to worked problems. Please see this Meta post on asking homework-like questions and this Meta post for "check my work problems". $\endgroup$ – Kyle Kanos Jan 3 at 20:35