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 Oct 28 awarded Notable Question Mar 4 awarded Popular Question Aug 20 awarded Popular Question Jan 17 accepted Why is the spring constant $W_p''(0)$? Jan 16 asked Why is the spring constant $W_p''(0)$? Sep 15 accepted Force to use in harmonic oscillation through the inside of a planet Sep 15 comment Force to use in harmonic oscillation through the inside of a planet @ChrisWhite I know. I want to understand the concepts. If I was happy with the solution I would have written down what Emilio wrote and been happy with it. I'm not happy until I understand what is happening. The mathematical solution is just a step closer to understanding the concept. Sep 15 comment Force to use in harmonic oscillation through the inside of a planet I understand mathematically how you get $M_r = \frac{r^3}{R^3}M_0$, but I can't grasp why this is done. Why is the density of the sphere multiplied by the volume of the planet, and not the volume of the sphere itself? Sep 15 awarded Commentator Sep 15 comment Force to use in harmonic oscillation through the inside of a planet The first one really was a duplicate. Finding that original question helped me start to visualise the solution I later needed help finishing (in this question). Thank you though. Sep 15 comment Force to use in harmonic oscillation through the inside of a planet Wow, this solution gives me exactly the right answer. Thank you. I'm still not sure I grasp what exactly the solution does, but I'm getting there. It's just about mentally visualising it now... :) Sep 14 asked Force to use in harmonic oscillation through the inside of a planet Jul 21 awarded Scholar Jul 21 accepted How to calculate speed difference between objects close to the speed of light? Jul 21 accepted How to calculate time dilation in approaching speed of light Jul 2 awarded Supporter Jul 2 comment How to calculate time dilation in approaching speed of light Thanks to usumdelphini for editing my question - I'm still new to SE and didn't know I could use LaTeX. :) Jul 1 awarded Student Jul 1 comment How to calculate time dilation in approaching speed of light After an hour of working I realised I did everything right from the beginning, but the online test was unclear as to how the write the answer (ie I wrote 48432 years when I should have written 48000)... I calculated it with t = d/v = 100 000 / 0.9 ; t_0 = t * sqrt(1 - (v^2/c^2)) = t * sqrt(1 - ((0.9c)^2/(c^2)) Thanks a bunch for the help, dmckee! Jul 1 comment How to calculate time dilation in approaching speed of light I calculated t_0 to 111 109 years (sounds plausible, traveling 0.9c 100 000 ly). I calculated t = t_0/(sqrt(1 - (0.9c)^2/c^2) but wound up with 254 901 years, which seems really wrong. Shouldn't t < t_0?