Displacement and Velocity

Displacement is the vector difference between the ending and starting positions of an object. It may be very different from the distance the object has travelled along the way.
Velocity is the rate at which displacement changes with time. It is a vector, too.
The average velocity over some interval is the total displacement during that interval, divided by the time.
The instantaneous velocity at some moment in time is the velocity of the object right now!
Instantaneous velocity is the derivative of position with respect to time.
Speed is a scalar, which describes the total distance travelled by some object during some time interval. Speed is sometimes equal to the magnitude of a velocity vector, but not always.
Use some common sense when you have finished a problem: look at the answer and ask yourself, "Is that reasonable?"

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