At what velocity should relativity be taken into account

[moonkey]

Homework Statement

I have exams coming up and I'm just looking for an idea of when relativity should be taken into account. For example I'm just going over a question involving photoelectric effect and potential fields and de Broglie and all that jazz and the final velocity of the electron turns out to be around 6.5x106ms-1. Should I take relativity into account for this velocity or is it still safely within the margins of non-relativistic velocity? I'm thinking that it is safe to assume that it is non-relativistic but I would like some advice just in case such a question arises that involves a greater velocity.

Homework Equations

The Attempt at a Solution

I have exams coming up and I'm just looking for an idea of

 

[phinds]

OOPS. Forget I said anything (it turned out to be not relevant, so I deleted my comment)

 

[Quinzio]

At a first glance, I'd say it depends on the precision that you want.
Since c is 300.000.000 m/s your speed is a 2% of c, which gives you an error of 0.03% if you ignore c (if I am not wrong).

If you use just only 3 figures (e.g. 0.134) in your data, then ignore it.

 

[moonkey]

That makes perfectly good sense.

Thanks Quinzio