 # Lakhmir Singh Solutions Class 9 Physics Solutions for Exercise in Chapter 1 - Motion

A motorcyclist drives from place A to B with a uniform speed of 30 km/h and returns from place B to A with a uniform speed of 20 km/h. Find his average speed.

Speed from A to B = 30 km/h

Let D be the distance from A to B

Time taken, T1 to travel from A to B = distance travelled/speed

T1 = D/30

Speed from B to A = 20 km/h

Time taken, T2 from B to a = distance travelled/speed

T2 = D/20

Total time taken, T = T1 + T2 = D/12

Total distance from A to B and B to A = 2D

Therefore, average speed = total distance travelled/total time taken = 24 km/h

Video transcript
"hey kids welcome to lido q a video i am vineet your lido tutor and i am excited to bring you yet another question on your screen a motorcyclist drives from place a to b okay so a to b at a speed of 30 kilometer per hour and from b to a again at a speed of 20 kilometer per hour right find the average speed to find the average speed we need the distance total distance and the total time now we don't know the total distance or the time or the distance one way even so what we are going to do is we are going to find the total distance in time in terms of one variable so let distance from a to b be d then speed is equal to d by t 1 and t 1 be the time taker this implies 30 equals to d by t 1 implies t 1 is equal to d by 30 now we are getting t1 in terms of t then we will get t2 in terms of t and add them in terms of d distance is already in terms of t then we can find the average speed let's see how from b to a b t 2 therefore t 2 will be distance now since the distance f from a to b is d the same distance is covered from b2 by speed that is 20 right therefore total distance is equal to d plus d that is equal to 2t right total time taken is equal to t1 plus t2 that is equal to d by 30 plus d by 20 that will be equal to into 3 sorry into 2 into 3 because 60 is the lcm so 2d plus 3 d by 16 this is equal to 5 t by 60 which is equal to d by 12 right now we are going to find the average speed right so let's draw a box for the average speed because right therefore average speed is equal to total distance that is by time that is t by 12 this is equal to 2 d into 12 by t d d cancels out 24 kilometers per hour right so 24 kilometer per hour is the speed so i am going to make it a little bit clearer so this is equal to 24 kilometer per hour therefore average speed is equal to 24 kilometer per hour that is our answer right guys isn't it easy okay if you have a doubt please leave a comment below do like the video and subscribe to our channel i'll see you in our next video until then bye kids keep learning"
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