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I Am In Class 9 Please Help Me With My Doubts A car driver took a total of 2 hours to make a journey of 75 km. He had a coffee break of half an hour and spent a quarter of an hour stationery in a traffic jam. What was his average speed during the journey ? Solution The definition of AVERAGE SPEED is: TOTAL distance traveled divided by the TOTAL time to travel that distance. The TOTAL distance = 75 km The TOTAL time = 2 h therefore by definition the AVG SPEED = 75/2 = 37.5 km/h If the driver doesn t move for 1/2 h + 1/4 h = 3/4 h then she must make the TOTAL distance in 2 - 3/4 = 1 1/4 = 1.25 h to do this, by definition, she must have an AVG SPEED = 75/1.25 = 60 km/h 2) In the hare-tortoise race, the hare ran for 2 min at a speed of 7.5km/h, slept for 56 min and again ran for 2 min at a speed of 7.5km/h. Find the average speed of hare in the race. total distance covered by the hare, s= 2(7.5*2/60) km = 0.5 km total time taken, t = 60min = 1h average speed, v = s/t = 0.5km/h In both d questions we were asked to find average speed but in first question we did not included the time that he wasted or he was not moving and in second question we have included the time in which he was not moving please tell which is correct

I Am In Class 9 


Please Help Me With My Doubts



A car driver took a total of 2 hours to make a journey of 75 km. He had a coffee break of half an hour and spent a quarter of an hour stationery in a traffic jam. What was his average speed during the journey ?


Solution


The definition of AVERAGE SPEED is:

TOTAL distance traveled divided by the TOTAL time to travel that distance.

The TOTAL distance = 75 km

The TOTAL time = 2 h

therefore by definition the

AVG SPEED = 75/2 = 37.5 km/h

If the driver doesn t move for 1/2 h + 1/4 h = 3/4 h

then

she must make the TOTAL distance in 2 - 3/4 = 1 1/4 = 1.25 h

to do this, by definition, she must have an

AVG SPEED = 75/1.25 = 60 km/h

2) In the hare-tortoise race, the hare ran for 2 min at a speed of 7.5km/h, slept for 56 min and again ran for 2 min at a speed of 7.5km/h. Find the average speed of hare in the race.


total distance covered by the hare, s= 2(7.5*2/60) km = 0.5 km


total time taken, t = 60min = 1h


average speed, v = s/t = 0.5km/h


In both d questions we were asked to find average speed but in first question we did not included the time that he wasted or he was not moving and in second question we have included the time in which he was not moving


please tell which is correct




Grade:9

3 Answers

KUMAR PRASSU
43 Points
8 years ago

I thinkthat car''s calculation of avg speed is wrong.The formula of finding the avg speed is

total distance travelled divided by total time time taken.

Finally I want to say that in case of tortoise hare problem the avg speed of hare is also not correct it should be

hardik joshi
37 Points
8 years ago

I THINK THAT ,,CAR''S CALCULATION IS RIGHT,,,

BCZ,,,WHEN CAR IS STOPPED ND DRIVER IS RESTING,,,

WE CAN NOT COUNT THAT TIME IN FINDING AVG. SPEED...

EX..IF UR CAR IS STOP ND,,SOMEONE PUSH UR CAR,HE PUSHED UR CAR UP TO 500m,,,

IN THAT CASE CAN YOU SAY MY CAR''S SPEED IS___!!!!,,,,,

SO,,,UR IS FIRST CALCULATION IS RIGHT,,,,,

I HOPE ,YOU GET IT,,,,,

FITJEE
43 Points
8 years ago

YOUR 2ND ANSWER IS CORRECT BECAUSE THE AV.. SPEED IS ASKED FOR THE WHOLE JOURNEY NOT FOR THE TIME THE CAR WAS IN MOTION...............

IN MY OPINION YOUR 2ND ONE IS CORRECT CALCULATION FOR SUCH QUESTIONS..............

HOPE YOU''LL GET IT

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