If an object is in motion its position changes with time. Rate of change of displacement with time is called velocity while the rate of change of distance is called speed. Displacement is the shortest distance between two places or we can say the distance between final position and initial position is called displacement.
Now, we have the quantity which define how fast is the position changing with time.
Average Velocity: It is defined as the change in displacement or position divided by the time elapsed or the time interval. Let us consider the final position is xf , initial position xi, the displacement Δx (delta x) and time interval Δt.
So that, Average velocity = Change in displacement/change in time
Average speed: It is defined as the total distance travelled divided by the time elapsed or the time interval. Let us consider the distance travelled is d and time interval Δt.
So that, Average velocity = Total distance travelled/ change in time
Let’s we have an example:
Example:- Suppose a car is moving along a straight line as given in the figure. Car moves from A to C in 10 sec and returns from C to B in 5 sec.
(a) What are the average velocity and average speed of the car in going from A to C?
Answer:- Average velocity = Displace/time interval = 50/10 = 5 m/s
Average speed = Total distance/ time interval = 50/10 = 5 m/s
In this case both average velocity and average speed is equal in magnitude.
(b) What are the average velocity and average speed of the car in going A to C and back to B?
Answer:- Average velocity = Displacement/time interval = 30/(10+5) = 30/15 = 2 m/s
Average speed = Total distance/ time interval = (50+20)/(10+5) = 70/15 = 4.66 m/s.