A particle with mass $m$ moving with constant velocity $v$ has a total energy $T$ equal to its kinetic energy. If we consider the reference system (inertial) moving with the particle, the velocity is 0, so the total energy is 0. Where is the energy gone? What about energy conservation law?
I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total energy is   $$\begin{eqnarray}E=E_1+E_2&amp;=&amp;\frac{1\mathrm t\times(50\mathrm{km}/\mathrm h)^2}2+\frac{1\mathrm t\times(50\mathrm{km}/\mathrm h)^2}2\\&amp;=&amp;1\mathrm t\times(50\mathrm{km}/\mathrm h)^2\\&amp;=&amp;2500\frac{\mathrm t\times\mathrm{km}^2}{\mathrm h^2}.\end{eqnarray}$$  Now if we look at it from the point of view of one of the cars, then the total energy is   $$\begin{eqnarray}E=E_1+E_2&amp;=&amp;\frac{1\mathrm t\times(0\mathrm{km}/\mathrm h)^2}2+\frac{1\mathrm t\times(100\mathrm{km}/\mathrm h)^2}2\\&amp;=&amp;\frac{1\mathrm t\times(100\mathrm{km}/\mathrm h)^2}2\\&amp;=&amp;5000\frac{\mathrm t\times\mathrm{km}^2}{\mathrm h^2}.\end{eqnarray}.$$  I know that kinetic energy is supposed to change when I change the frame of reference. But I understand that then there must be some other kind of energy to make up for it so that the energy in the system stays unchanged. But I don't see any other kind of energy here. I only see two total energies of the same system that seem to be different. Could you explain this to me?  Please note that while I don't understand any physics, I do understand college level mathematics, so if necessary please use it. (I doubt anything more than high school maths should be needed here, but I want to say this just in case.)
Re-enabling backspace as "go back" in recent Chrome Since start of this summer, I think that Google removed the option of using backspace for returning to previous page or so.   I was wondering, if there exist any chance, of undo-ing this thing? Is there such thing that would help me, such as downgrading Chrome or using any additional add-ons or something such as this for getting this glorious function back?  It's nearly impossible for me to forget this shortcut (as old habits die hard, lol) and keep pressing it and later on using the mouse to go back. How to remove headers and footers for pages between chapters? Prediction interval for binomial random variable Variables are often adjusted (e.g. standardised) before making a model - when is this a good idea, and when is it a bad one? In what circumstances would you want to, or not want to scale or standardize a variable prior to model fitting?  And what are the advantages / disadvantages of scaling a variable? I want to write a program for this: In a folder I have n number of files; first read one file and perform some operation then store result in a separate file. Then read 2nd file, perform operation again and save result in new 2nd file. Do the same procedure for n number of files. The program reads all files one by one and stores results of each file separately. Please give examples how I can do it. Find My iPhone Email Notification Is the email notification from Apple that your device has been tracked using Find My iPhone a new feature? Can I use PCA (or should I use regression) for testing the effect of multiple variables on one dependent variable? I have 2000 soil property measures and 14 different variables like rainfall, temperature, slope, etc. I want to check the effect of those 14 variables on soil property measures, including which variable affects my soil property the most. Is PCA a suitable solution to this? Or should I try multiple regression instead? Is it possible to install MySQL 5.5 or 5.6 on Ubuntu 16.04? I recently upgraded to Ubuntu 16.04. Unfortunately I need to work MySQL 5.5 or 5.6. I tried many solutions to have MySQL 5.5 or 5.6 run on Ubuntu 16.04 like this  . But none of them is working. Ubuntu detect the unmet dependencies but says "but it is not going to be installed" for 5.5 or says Package 'mysql-server-5.6' has no installation candidate...  Is it not possible to force an earlier version of MySQL on Ubuntu? Why is that? Duty Free in the USA. Why do I have to be given my purchase as I board the plane? I am not one for buying things in airports but recently my wife asked me to get her an alcoholic beverage while flying from the USA to Ireland (JFK-DUB). I made the purchase in JFK and was surprised to be told I had to collect it when departing for my flight as I board the plane. The sales assistant's English wasn't their best so I may have misheard and my Yorkshire twang may have thrown her, but I was somewhat surprised as to why I couldn't carry the bag myself to the plane.  Is there a law that prevents me from carrying my purchase to the plane or is this an added service I was charged, which I was duped into paying?  When boarding the plane there were several others who had to pick up their sealed duty free bags (why sealed?) so it wasn't just me. No explanation was given. After update, get "error: file not found." followed by "grub rescue> _" How can I include value of sharepoint's version column in a word document? I'm trying to work out a way to display the contents of the version column from SharePoint (i.e. the value that changes every time a file is checked in) as a field (or something similar) inside of a Word document.  Ideally, I'd like to know how to configure SharePoint so I could click something like "Insert > Quick Parts > Document Property > Version", and it would include the version in the document.  The goal is to make it easier for someone to correlate a printed version of a document with the version history of SharePoint.  I have been able to add editable text columns to the Document content-type and have them show up as document property quick parts.  I've also been able to add a calculated column which gets the version as a text string... however this calculated column isn't showing up in Word as a document property.  (Perhaps I'm missing a setting on the calculated column) Windows 7 Aero Snap two windows horizontally As many people already know, there are several ways to trigger Windows 7 Aero Snap feature. A user can either press Win+{Left,Right,Up,Down} to snap the window to the left or right or maximize or restore. One can also drag the title bar to all the corner except bottom one.  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