Speed of a body is the distance covered by the body in unit time. Therefore Speed= which implies: 1. In order to find speed, if :
2. In order to find the distance, if:
3. In order to find time, if:
Unit of Speed
Now, the various units of distance are metre, centimetre, Kilometre etc. and the various units of time are hour, minutes and seconds. So, say we take the unit of distance to be centimetre (cm.) and the unit of time to be second (s.). Example: A snail covers a distance of 8 cm in 4 seconds. So the speed of the snail is So here, The two most widely used units of speed are as follows: metre/sec or m/s and Kilometre/hour or Km/h. Example: The speed of sound in air is 330 m/s. Example: The car is travelling at a speed of 60 Km/h. Speed Unit ConversionTo convert speed from Kilometre per hour (Km/h) into metre per second (m/s), we multiply by . To convert m/s into Km/h, we multiply by . Reason: Example: Convert:
Solution: 1. 2. 3. ( ) 4. Example: A boy covers a distance of 1.2 km in 40minutes. Find his speed in:
Solution: 1. In order to get speed in Kilometre per hour , the distance covered must be in Kilometre and the time taken must be in hour Given: Distance =1.2 Km and Time =40 min =
2. In order to get speed in metre per second, the distance covered must be in metre and the time taken must be in second. Given: and, $latex\ therefore $ Uniform and Variable speedIf a body covers equal distances in equal intervals of time ;it’s speed is said to be uniform otherwise it’s speed is variable. For example:
Example: A man runs 200 metre in 25 second. Find:
Solution: 1. 2. Distance run in 5 sec 3. Time taken to cover [ Example: A train covers first 120 Km in 2 hours, next 160 Km in 3 hours and last 140 Km again in 2 hours. Find the average speed of the train. Solution: Average speed of an object= Since, total distance covered =120 Km+160 Km+140 Km =420 Km And, total time taken= 2 hrs + 3 hrs+ 2 hrs= 7 hours Therefore average speed = Example: A man covers first 60 Km of his journey at 30 Km/h and remaining 50 Km at 20 Km/h. Find:
Solution : 1. Time taken to cover first ( ) and, time taken to cover remaining total time taken 2. Since , total distance covered= 60 Km + 50 Km =110 Km and total time taken average speed Note: When a train passes:
Example: A 160 m long train is travelling at a speed of 72 Km/h, find the time taken by the train to pass:
Solution: 1. Distance to be covered= length of the train=160m Speed
2. Distance covered=length of the train +length of the platform=160 m + 200 m =360 m
Exercise
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