We have observed moving objects around us. From the observation we know that some objects move faster than other and some slower. For example, a man walks slower than a biker and a train run faster than a bike. In physics, there are two ways to measure the motion known as; Speed and Velocity.
To measure how fast or slow an object is moving, we simply note the time taken by the object to cover a particular distance. Lesser the time is taken; greater speed will be or greater the time is taken lesser the speed is. Simply it is defined as, total distance travelled by an object in a time unit or total distance is covered in 1 second. SI unit of speed is m/s. mathematically speed is calculated as;
Speed = distance travelled (in meter) / time (1 second).
Speed is something we measure, how much an object can travel in a unit time. Now suppose, in a particular time, some distance has been travelled by an object. There are time intervals when speed of the object was increased or decreased.
For example, a boy went to the park 50 meter away from his house. His walking speed was 2 m/s. In park he jogged a circular path of 100-meter radius one round. His jogging speed was 10 m/s. After jogging he sat on the park bench for 5 minutes. At this moment, his speed was 0 m/s. then he walked to home with 2 m/s speed. In this total motion he covers a total distance of (Home to park = 50 m + jogging in circular path = 2*π*100 = 628 m + Sitting on the bench = 0 m + walking back to home = 50 m) = 728 m and total time taken is (Time taken to walk from Home to park = 50/2 = 25 s + time taken in jogging = 628/10 = 62.8 s + time taken while sitting on bench 60*5=300s + time taken while walking back to home is 50/2=25s) = 412.8s.
In this situation it is not possible to calculate the speed of the complete motion because speed of the object is not uniform. Hence, we calculate the average speed of the object. To calculate an average speed of the motion we divide total distance travelled by total time taken. Hence average speed of this motion will be = 728/412.8 = 1.7636 m/s.
Speed is a quantitative unit in which direction of motion is not defined. But if we fix the direction of motion for example train is moving towards north with speed 50 m/s then this quantity becomes Velocity. Hence, Velocity is defined as total distance travelled by an object per unit time in a particular direction. It is calculated exactly as speed.
As we calculate average speed for non-uniform motion, we will also calculate average velocity for a non-uniform motion but with different approach. First way to calculate is when the velocity is changing with non-uniform rate. In this case average velocity is calculated by dividing net displacement of object by total time taken. Second way to calculate is when velocity is changing with uniform rate. In this case average velocity is calculated by dividing the sum of initial and final velocity by 2.
