Speed

In an athletic meet different participants start running at the same time and cover the same distance. The person who takes the minimum time to cover the distance will be judged as the winner. Suppose Ram and Krishna take 20 minutes and 10 minutes respectively to cover a distance of 1000 m, who is faster? To find out who is faster we have to calculate the distance covered by Ram and Krishna in one minute.

i.e., Krishna covers more distance in unit time or in other words we can conclude that Krishna covered the distance with greater speed.
Speed can be defined as the distance covered by a moving object in unit time
taken. SI unit of speed is m/s or ms^-1. Speed is a scalar quantity.

 

Uniform speed

The figure shows the distance covered by a ball after every 2 seconds.

The ball covers 10 m in every 2 seconds. The speed with which the ball is moving at any point between A and E is 5 m/s. That is, the object is moving with uniform speed. An object is said to be moving with uniform speed if it covers equal distances in equal intervals of time.

Variable speed or Non-uniform speed

The previous example does hold good only if we ignore the friction or resistance offered by the surface. The distance covered varies with time.

For example, a rubber ball dropped from a height, on reaching the ground bounces off to a height less than the initial height and slowly the height decreases. The distance covered by the ball in unit time decreases. That means the speed of the ball varies from point to point. Such a speed is called a variable speed. An object is said to be moving with variable speed or non-uniform speed if it covers equal distances in unequal intervals of time or vice-versa.

Average speed and Instantaneous speed

When we travel in a vehicle the speed of the vehicle changes from time to time depending upon the conditions existing on the road. In such a situation, the speed is calculated by taking the ratio of the total distance traveled by the vehicle to the total time taken for the journey. This is called the average speed. If an object covers a distance s₁ in time t_1, distance s₂ in time t₂ and distance s_n in time t_n then the average speed is given by,
When we say that the car is traveling at an average speed of 60 km/h it does not mean that the car will be moving with the speed of 60 km/h throughout the journey. The actual speed of the car may be less than or greater than the average speed at a particular instant of time.
Hence the speed that the body possesses at a particular instant of time, is called instantaneous speed.