1.4 Speed, velocity
Displacement is the vector version of the scalar distance.
If I travel from London to New York, clearly I can’t walk straight there! I need to catch an airplane 
take a car in detour 
etc… you get the idea. So even though the displacement from London to New York is 5585km, I’m going to be traveling FAR greater than this, because I’ll be going off track. The total distance I’ve traveled is referred to as distance.
So Miley, if after a Hannah Montana shoot (Disney Channel studios, 3800 Alameda Ave, Burbank CA 91505), all the way to church (First Baptist “The People’s” Church, 828 Murfreesboro Road, Franklin, TN 37064)… and bank. What is the ‘displacement’ traveled? It’s a trick question! A very small number… zero; because the essential distance traveled is 0.
Speed is a combination of a distance within a constrained amount of time. For instance, if I walk 100m in 1 minute, I am walking 100 meters per minute, which is my speed.
Velocity is the vector version of speed, meaning whereas speed = distance / time, velocity = displacement / time. So we’re using the vector displacement, instead of the scalar distance. This thus means speed is a scalar, and velocity is a vector.
Average velocity
Instantaneous velocity
What is the practical effect of this? Say for instance if I travelled from new London to NY, back to London, the distance is 5585+5585=11,170m; whereas displacement is 5585-5585=0m. This means velocity (which uses displacement) will be 0m/s, whereas distance (which uses distance) will be a non-zero number! Get it?
http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/kinema/trip.html
http://lectureonline.cl.msu.edu/~mmp/kap2/cd028a.htm
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