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Is better to say the e.g. kinetic energy is 0 joule or kinetic energy is 0 ? since 0*joule equals 0.Is a mathematical concept behind because in many books i find both ways.

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Fools rush in where angels fear to tread, but I think it's usual to leave out the unit, on the grounds that zero KE is zero of whatever units we use: joule, erg, foot pound weight, foot poundal…

Then there's the argument that units are to be treated as algebraic symbols, so 50 J means 50 multiplied by J and 0 J is zero multiplied by J, which is simply 0.

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    $\begingroup$ Keep in mind that doing that with temperature scales won't work as zero is not generally the same point in these scales. Always safer to state a unit than not. $\endgroup$ – StephenG Jul 11 '18 at 8:23
  • $\begingroup$ Good point, but I'd suggest that temperature is a special case. [A bolder argument would be that absolute temperature and centigrade temperature are different quantities and that we could say "the temperature is 0" as long as we specified what sort of temperature we're talking about. But I acknowledge that the convenient way of specifying is by writing °C or K after the number, as if it were just a unit!] My 'special case' claim would break down if there turned out to be many such cases! $\endgroup$ – Philip Wood Jul 11 '18 at 17:09

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