Actually that's very easy question but I have some difficult to understand the logic behind . So-"The initial velocity of an object (m/s) is Vi=1i+5j+2k. And the final velocity is Vf=3i+5j+7k. What was the change in speed of the object?"X Solution - |Vf|-|Vi| = √(32+52+72)-√(12+52+22) = 3.63 m/s My question - If I would do |Vf-Vi| - why is it wrong ? And what is the meaning of the scalar I get from this equation? Has to be some connection to the "change in speed". I just can't see it clearly. Thanks again, just started to learn physics and I love it.