Speed

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Ram and Krishna run in different races covering different distances. Ram takes 20 minutes to cover 1000 m and Krishna takes 10 minutes to cover 700 m. Who is faster?

To find out who is faster we shall calculate the distance covered by them in one minute.

Krishna covered more distance in unit time. We conclude that Krishna is faster.

Speed can be defined as the distance covered by a moving object in unit time

where S is the distance covered and t is the time taken.

SI unit of speed is m/s or m s^-1. Speed is a scalar quantity.

Uniform Speed

The figure shows the distance covered by a ball at every 2 second interval.

Uniform Speed

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. 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.

Here we ignore the friction or resistance offered by surfaces.

Variable Speed or Non-uniform Speed

The distance covered varies with time.

Variable Speed

Example: A rubber ball dropped from a certain height (h₁) on reaching the ground bounces up to a height less than the initial one (h₂). It continues to bounce but the height to which it rises keeps decreasing (h₃, h₄). The distance covered by the ball in unit time decreases. 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 travelled 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 travels at an average speed of 60 km/h it does not mean that the car would 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.

 

The speed of a moving body at any particular instant of time, is called instantaneous speed.