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Question

A car moving on a straight road covers first 1000 s of its journey with speed of 18 km/h and next 2000 s of its journey with speed of 36 km/h. Average speed of the car during whole journey is

A
18 km/h
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B
36 km/h
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C
27 km/h
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D
30 km/h
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Solution

The correct option is D 30 km/h
Given data:
V1=18 km/h, t1=1000 s
t1=1000/3600 h
V2=36 km/h, t2=2000 s
t2=2000/3600 h


Assume the distance covered in two steps be d1 and d2.
Average speed: V=total distanetotal time
V=d1+d2t1+t2
V=V1.t1+V2.t2t1+t2

V=18×10003600+36×2000360010003600+20003600

V=5+205/6=6×5=30 km/h

Alternate Method:

If you divide the total time travel into equal intervals, such that you know the speed in each interval, you can use the formula for average speed.
V=V1+V2+V3+....+Vnn

V=V1+V2+V33

V=18+36+363=903=30 km/h

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