Why is displacement used in the formula for velocity instead of distance?

[Taterpeel]

Hello everyone. I want to learn Physics, so I've been trying to teach myself. I still don't know much, and am just now getting to the formula for Velocity. The internet tutorial I am using says it is this..

Average Velocity = Displacement / Elapsed Time

And I've been plugging in the numbers, and it's not hard or anything. I was just wondering.. Why would displacement be in the numerator? Shouldn't it be distance? If I travel somewhere and then travel back to my destination 30 times in an hour, won't my velocity be different than if I do it 5 times? But I will end up with the same Displacement, so why do I use displacement here, and not distance?

 

[russ_watters]

Velocity is a vector quantity. It includes direction. So if you travel around and eventually end up where you started, your average velocity was zero.

Ie, if you travel one direction for an hour at 60 mph and travel back to where you started also at 60 mph, that's 60*1+(-60)*1=0 displacement. Average velocity is (60+(-60))/2=0 But speed is a scalar quantity, so your average speed is (60+60)/2=60.