Question 858305: A train 600 metre long passes a car running at 72 km/hr in same direction in which the train going in 10 seconds. Calculate the speed of the train.
A. 108km/hr
B.252 km/hr
C.162km/hr
D.225 km/hr
Answer by KMST(5328)   (Show Source):
You can put this solution on YOUR website! According to my calculations the answer is "none of the above."
 = speed of the train with respect to the ground, in m/s.
  is the car's speed.
ONE WAY TO CALCULATE IT:
From the point of view of a passenger in the car,
the car is not moving, and the train is going past the car at
 m/s.
One moment the passenger sees the front of the train level with the car,
and 10 seconds later the passenger seens the back of the train level with the car. Since at that point, the front of the train is  ahead of the car, the apparent speed of the train (from the car passenger's point of view is
 m/s.
So  --> -->
So the speed of the train with respect to the ground is 80 m/s.
 
ANOTHER WAY TO SEE IT:
One moment, the train is level with the car, at a certain spot on the road.
After  seconds, at 20m/s, the car is 200 m ahead of that initial spot.
At the same time, going at a faster rate, the front of the train is many meters ahead of that initial spot.
The back of the train is now level with the car.
Since the front of the train is  meters ahead of that,
the front of the train is 200m + 600 m = 800m ahead of that initial spot where car and train first met.
The train has covered 800m in 10 seconds, so the speed of the train with respect to the ground is 80 m/s.
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