A Car on a Fragile Bridge

A car is crossing a 20km long bridge. The bridge can support at most 1500kg of weight over it. If somehow, the weight on the bridge becomes more than that, it will break.
Now, the weight of the car is exactly 1500kg. At the midway, a bird comes and sits on the roof of the car. This bird weighs exactly 200 gram. 

 
Can you tell if the bridge breaks at this point or not? 

Read what will happen next? 

Source 

 

Bridge Under Load

What was the situation?

At first look, the first impression would be that the bridge will break certainly. But if you wait for a while before concluding anything you will get the right answer.

The bridge will not break in the case! It's 20 km long bridge & now it's in the middle of bridge after traveling 10 km. By now, it must have used half of the fuel that was in the tank initially at the start. This amount of fuel must be weighing more than 200 gm. Hence even a bird sits on the car there are hardly any chances the total weight on the bridge goes beyond 1500 kg! Hence, no chance of breaking of it.

 

Bird's Journey From Train To Train

Suppose distance between SECUNDERABAD & CST is 1000 KM. Railway tracks are in straight lines with no curves in between.

A train with uniform speed of 100 kmph departs from CST at 9:00 AM. Same day on same track other train from SECUNDERABAD departs at 10:00 AM with uniform speed of 80 kmph.

A bird sitting on engine of train that departed from CST starts traveling toward the SECUNDERABAD & after touching the engine of that train travels back to CST train. A bird repeats this until trains collide.

How much distance the bird traveled & how many times did it travel to & fro? 

Click for the answer! 

Distance Traveled By The Bird

Here is the question! 

Let's divide answer in 2 parts.

1.Finding distance traveled by the bird.

The SECUNDERABAD train is one hour late till then CST train covers 100 km.

Hence effective distance is 900 km

Total speed of approach = 100 + 80 kmph

Total distance covered = 900 km

Time passed before trains collide = 900/180 = 5 Hours

Speed of bird = 120 kmph

Distance covered in 5 hours by bird = 120 * 5 = 600 km

Distance covered in first hour by bird = 120 km

Total distance = 600 + 120 = 720 km



2. Finding how many times bird traveled back & forth.


1st travel -

Total speed of approach = 120 + 80 = 200 kmph

Total distance to be covered = 900 km

Time required to meet each other = 900/200 = 4.5 Hours.
 
2nd travel -

Distance traveled by CST train in 4.5 hours = 100 x 4.5 = 450km

Distance traveled by SECUNDERABAD train in 4.5 hours = 80 x 4.5 = 360 km

Distance between 2 trains = 900 - (450+360) = 90 km

Time left for collision is 5 - 4.5 = 0.5 hrs.


So bird ends up with 1 complete round.