This is less of a strict math problem than me thinking my online teacher is wrong. I will, however, format the question as per PF's requirements, and I think it should be in this forum because it involves my coursework. 1. The problem statement, all variables and given/known data Essentially, when should I use relativistic calculations (considering gamma in equations like momentum, speed, length, etc.)? 2. Relevant equations γ = 1/sqroot(1 - v^2/c^2), and dependent equations 3. The attempt at a solution My teacher says that this should only be used for cases where "it makes a difference", and uses "like, 99% of c" as his example. I was given a problem on a quiz in which a particle was traveling at 16% of c, I used the relativistic momentum (I was finding the De Broglie wavelength), and got the answer wrong (presumably because I used the relativistic momentum). In the example the problem showed, it used the regular definition of momentum. SO- who screwed up? My answer was wrong enough for it to be counted wrong, which pretty much justifies my argument. The value of gamma at 16.6% of c is 1.014, which I think is an appreciable difference (1.4 percent). At 50% of c, which my teacher also seems to be discounting, gamma is 1.1547. 15.5% is definitely an appreciable difference. What do you all think?