Question

The speed of a mortar boat w.r.t. still water is 7m/s and the speed of the stream is 3m/s.when the boat began travelling up stream,a float was dropped from it. The boat travelled 4.2 km up stream turned about and caught up with the float. How long is it before the boat reached the float?

Answer 1

So,

Speed of the boat when it travels upstream = 7-3 = 4 m/sec

Speed of the boat when it travels downstream = 7+3 = 10 m/sec

Now,

As soon as the float falls , it will travel downstream at the speed of 3 m/sec

And in the meantime, boat has also traveled upstream a distance of 4.2 km

So,

Time spend on travelling upstream by the boat = 4200/4

=1050 sec

In the meantime the float would had traveled downstream by,

=1050×3

=3150 m

Hence, the total distance between the float and the boat will be

=3150 + 4200

=7350 m

Now, the relative speed between boat and the float

=10-3

=4 m/sec

Hence time taken = 7350/4

=1837.5 sec

Hence, total time spent = 1837.5+1050

=2887.5 sec

This is 2887.5 /60

= 48.125 mins

Hence the float and the boat will meet in 48.125 mins

Answer 2

Answer:

So,Speed of the boat when it travels upstream =7−3=4m/sec

Speed of the boat when it travels downstream =7+3=10m/sec

Now,

As soon as the float falls, it will travel downstream at the speed of 3m/sec

And in the meantime, the boat has also traveled upstream a distance of 4.2 km

Time spend on traveling upstream by boat =  

4

4200

​  

=1050sec

In the meantime, the float would had traveled downstream by =1050×3 = 3150 m

Hence, the total distance between the float and the boat will be  =3150 + 4200 = 7350 m

Now, the relative speed between a boat and the float =10-3=4 m/sec

Hence time taken = 7350/4=1837.5 sec

Hence, total time spent = 1837.5+1050=2887.5 sec  

This is 2887.5 /60= 48.125 mins

Hence the float and the boat will meet in 48.125mins

Explanation: