The speed of an object is defined as the distance travelled by the object per unit time.
Speed of an object is measured in km/h, m/s, m/min, cm/s, etc.
Conversion of units:
Distance 
Time 
1 kilometer = 1000 meters 
1 hour = 60 minutes 
1 meter = 100 centimeters 
1 minute = 60 seconds 
1 centimeter = 10 millimeters 

From the above table, can you deduce
i) 1 km/hr = 5/18 m/sec
ii) 1 m/sec = 18/5 m/sec
Note: 1 km/hr is also denoted as 1 kmph.
Moving Trains
The time taken by a train of length x meters in passing a stationary object(such as a single post or a lamp pole or a person standing) moving with a speed of v m/sec
i.e. t = (x/v) sec.
The time taken by a train of length x meters in passing a stationary object of length y meters (such as a platform) moving with a speed of v m/sec
i.e. t = (x + y)/v sec.
Ex: 
Two express trains of lengths 250 m and 300 m long are running in opposite directions on parallel tracks with speeds of 100 km/hr and 80 km/hr respectively. Find the time taken by the slower train to pass the driver of the faster one? 
Sol: 
Relative speed = (100 + 80) km/hr = 180(5/18) = 50 m/sec 
We have to find the time taken by the slower train to pass the driver of the faster train and not the complete train. 
So, distance covered = length of the slower train = 300 m 
Hence, required time = Distance/speed = 300/50 = 6 sec 

Ex: 
A boat travels 20 km in still water in 16 minutes. Find the speed of the boat in:
(i) km/min (ii) m/s 
Sol: 
i) 
Distance travelled by the boat = 20 km.
Time taken to cover the distance = 16 minutes.

ii) 
We know that 1 km = 1000 meters and 1 minute = 60 seconds.
Therefore, distance travelled in meters = 20 × 1000 meters = 20000 m.
Also, time taken in seconds = 16 × 60 seconds = 960 seconds.

