Average speed Average Velocity and Acceleration

Average speed,Average Velocity  & Acceleration
DISTANCE AND DISPLACEMENT
Distance and displacement are two quantities that may seem to mean the same thing yet have distinctly different definitions and meanings.
Distance is a scalar quantity that refers to “how much ground an object has covered” during its motion.
Displacement is a vector quantity that refers to “how far out of place an object is”; it is the object’s overall change in position.
Units of Distance & Displacement:meter is the S.I. units of distance & displacement. 
    In the figure:1, The distance is indicated by dotted lines from A to B. That is entire path travelled by the body to reach the destination ‘B’. That total path travelled by the body is called Distance. And if a straight line drawn from point A to Point B, then that is the possible way to reach point B in shortest path. That shortest path is called as Displacement.

 Some more examples:    

Differences between Distance & Displacement

Distance & Displacement in different cases:
Case 1:
Motion in straight path:
    Displacement is the shortest distance between two points, while distance is the length of the path travelled. Therefore, if the body takes the shortest path between two points, distance and displacement will be equal. Imagine it in this way: Between two points you can draw any line-curved, with turns, etc, but you can draw only one straight line connecting the points which will be of the shortest distance. If an object travels across the curved line, the distance is equal to the length of the curved line, while displacement is equal to the length of the straight line. In case the object travels in this straight line, distance will be equal to the length of this line, which is also equal to displacement.
    When an object is moving in straight path, then distance & displacement can be measured in same way because entire path travelled by the object & shortest path travelled by the object is same, so distance is equals to displacement.
Special case:    
    Here, displacement can be negative in some cases. To understand this, we need to take a fixed reference point for the motion of a body.
Case 2:
Motion in curved path:
    When an object is moving in curved path, then distance & displacement can never be measured in same way because entire path travelled by the object & shortest path travelled by the object can never be same, so distance is not equal to displacement.
Case 3:
Motion in circular path:
    If a body is moving in circular path, then the body will have distance & displacement also. But the displacement will be zero if the body comes back to the same point again in its path. In this giant wheel, there are 20 cabins  situated at different places & each cabin is fixes at 1m apart from each other.

Then if the giant wheel started moving then, Cabin-1 starts moving from its position  (Reference point), if it moves to next position, thendistance & displacement will not be same, because it is not moving in straight path. If the Cabin-1  again comes back to same reference point, then displacement can be considered as zero.