In these types of problems we consider the relative speed of the two objects. When the two objects are moving in the same direction, the relative speed is the difference of the speed of the two objects. When the two objects are moving in the opposite direction , the relative speed is found as the sum of speed of the two objects.
When two object are moving, then you have to taken into consideration, their relative speed in place of their individual speeds. Relative speed is the speed of one object with respect to other.
Case I : When two objects are moving in the same direction with x km/h and y km/h respectively, then
Relative speed = (x - y) km/h (if x > y)
or (y - x) km/h (if y > x)
Case II : When two objects are moving in the opposite directions with x km/h and y km/h respectively, then
Relative speed = ( x + y) km/h
Illustration . A bus is moving at a speed of 30 km/h ahead of car with speed of 50 km/h. How many kilometres apart are they, if it takes 15 min for the car to catch the bus ?
A. 5 km B. 7.5 km C. 12.5 km D. 15 km
Answer: A
Solution:
As the car and the bus are traveling in the same direction, the relative speed of car as compared to the bus
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