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Motion In A Plane

Question
CBSEENPH11018625

A body travelling along a straight line traversed one-third of the total distance with a velocity 4 m/s. The remaining part of the distance was covered with a velocity 6 m/s for the other half of time. What is the mean velocity averaged over the whole time of motion? 

Solution

Let s be the total distance travelled by body.
If t1 is the time taken to cover first one-third distance, then 

               straight t subscript 1 space equals space fraction numerator straight s divided by 3 over denominator 4 end fraction space equals space straight s over 12 space
Let t2 be the time for each of the remaining two journeys. 
Then, 

                 fraction numerator 2 straight s over denominator 3 end fraction space equals space 2 t subscript 2 space plus space 6 t subscript 2 space
space space space space space space space space equals space 8 t subscript 2 

                    straight t subscript 2 space equals space fraction numerator 2 straight s over denominator 3 space straight x 8 end fraction space equals space straight s over 12 space 
Therefore, 
Average velocity = fraction numerator Total space displacement over denominator total space time end fraction 
                         = fraction numerator straight s over denominator straight t subscript 1 space plus space 2 straight t subscript 2 end fraction space equals space fraction numerator s over denominator left parenthesis s divided by 12 right parenthesis space plus space 2 left parenthesis s divided by 12 right parenthesis end fraction space equals space 4 space m divided by s