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 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 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... :) 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 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? Jul 1 comment How to calculate time dilation in approaching speed of light Ah, I see. So I would calculate how long it would take to travel for example 100 000 ly in 0.9c m/s, ignoring relativity, and use it as t_0 (i.e. what it looks like to the "twin" left on Earth)? Jul 1 comment How to calculate time dilation in approaching speed of light @Argus Yes, I meant the pilot would experience the traveling going much quicker when he gets back. Jul 1 comment How to calculate speed difference between objects close to the speed of light? Thanks Bill Wheaton, good to know. Jul 1 comment How to calculate speed difference between objects close to the speed of light? Thank you Alfred Centauri, your answer helped a lot! Jul 1 comment How to calculate speed difference between objects close to the speed of light? @Argus Ah, yes, I meant the total speed of the rockets closing in, not the difference of speed. Thanks for pointing that out.