# Speed And Distance

The rate at which any moving body covers a particular distance is called its speed.

Distance = Speed Ã— time

**Unit :**SI unit of speed is metre per second (mps). It is also measured in kilometers per hour (kmp) or miles per hour (mph).

**Conversion of units :**

- 1 hour = 60 minutes = 60 Ã— 60 seconds.
- Average speed

- If a certain distance (d), say from A to B, is covered at â€˜aâ€™ km/hr and the same distance is covered again say from B to A in â€˜bâ€™ km/hr, then the average speed during the whole journey is given by :
- If a body travels a distance â€˜dâ€™ from A to B with speed â€˜aâ€™ in time t
_{1}and travels back from B to A i.e., the same distance with of the usual speed â€˜aâ€™, then the change in time taken to cover the same distance is given by :_{1}; for n > m = Ã— t_{1}; for m > n - If first part of the distance is covered at the rate of v
_{1}in time t_{1}and the second part of the distance is covered at the rate of v_{2}in time t_{2}, then the average speed is

**Relative Speed :**When two bodies are moving in same direction with speeds S

_{1}and S

_{2}respectively, their relative speed is the difference of their speeds.

i.e. Relative Speed = S

_{1}â€“ S_{2}When two bodies are moving in opposite direction with speeds S

_{1}and S_{2}respectively, their relative speed is the sum of their speeds.i.e. Relative Speed = S

_{1}+ S_{2}- If two persons (or vehicles or trains) start at the same time in opposite directions from two points A and B, and after crossing each other they take x and y hours respectively to complete the journey, then
- A man covers a certain distance D. If he moves S
_{1}speed faster, he would have taken t time less and if he moves S_{2}speed slower, he would have taken t time more. The original speed is given by - If a person with two different speeds U & V cover the same distance, then required distance