IIT JEE , AIEEE Forum - Physics , Mechanics

tejas.ahobala

a motor boat going downstream overcame a raft at a point A ;

=60 min later it turned back and after some time passed the raft at a distance l=6.0 km from the point a . find the flow velocity assuming the duty of the engine to be constant.

iitkgp_bipin

Analyze the situation from the frame of the river.
In this frame :

velocity of raft = 0
and velocity of boat is constant.

Since it takes 60 minutes in downstream it will take 60 minutes in upstream to reach the raft. Hence total time interval involved = 120 minutes = 2 hour

Now analyze the situation from the frame of ground.

If the speed of the river is u
then that of raft will also be u.

In 2 hour it covers 6 km.

so, u = 6/2 = 3 km/h

cutepooja

it was tough

mukulaish

This is similar to irodov's I question....

U can hav a luk ova there....

paulparthapratim2

come on this can be done in a simole way !!

In 2 hrs the raft covers 6 km

so velocity is 3 km / hr

paulparthapratim2

And this a irodov sum.

paulparthapratim2

in fact the first sum

gpranay123

${v}_{b}\rightarrow velocity of boat{v}_{r}\rightarrow velocity of river$

let motor boat reach a point B then return and pass the raft at point C after a time t

${v}_{b}+{v}_{r}*1=AB\\({v}_{b}-{v}_{r})t=BC\\$

AD+BC=AB-AC D be the point down stream where AD is the distance travelled in 1hr

$AD={v}_{r}*1\\DC=({v}_{r})t\\$

by solving above equations we get t=1hr

for AC (V_r)(1+1)=6 V_r=3kmph

debmalya.roychoudhuri

gpranay has done it in a good way i also did it like it it is easier to understand

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