Another puzzle

[ban-all-sheds]

A man is rowing a boat upstream, along a stretch of river where it's speed is constant, and he passes under a bridge. 1 mile further upstream, he passes under a second bridge, and as he does so his closed tupperware sandwich box falls into the river.

He doesn't notice this immediately - in fact he carries on rowing for 5 minutes before he does. When he realises, he turns round, and begins rowing after his sandwiches, and he catches up with them as they pass under the first bridge.

Assuming that he has not rowed any harder on the return trip, how fast is the river flowing?

 

[big-all]

no way off telling because-----

when we get a time we dont get a distance
and when we get a distance we dont get a time
so the answer is -----
a ham sandwich a bannana yoghurt and an apple!!!

 

[Slogger]

the answer is

the river is not flowing it is still not moving stopped

i was out rowing yesterday no sarnies and the river was going 3 knots coming in

 

[Nige F]

It`s a TIDAL river Like the Ouse ..............one of the bridges is Cliffe, so he ties off and goes for a pint of Sussex beer

 

[ban-all-sheds]

no way off telling because-----

when we get a time we dont get a distance
and when we get a distance we dont get a time

Wrong.

 

[ban-all-sheds]

the answer is

the river is not flowing it is still not moving stopped

Wrong.

 

[ban-all-sheds]

It`s a TIDAL river Like the Ouse ..............one of the bridges is Cliffe, so he ties off and goes for a pint of Sussex beer

Wrong.

 

[ban-all-sheds]

This could be a good one....

 

[empip]

River flowing at approximately 8.8 ft/sec.

 

[WoodYouLike]

Empip, can you check here please?

 

[ban-all-sheds]

Any more answers?

 

[Steve]

Any more answers?

(reply to ban's pre-edit post)

yes, the plane question was answered on page 1. which answer was it?

and ban, if you dont mind, what do you do for a living?

 

[tigger]

He obviously thinks up questions for you lot to ponder over

 

[empip]

.

Ok all done ...

Let:
Mv = Man's slack water rowing velocity Ft/min
Rv = River flow hence box velocity Ft/min

Man rows against current for 5min, distance covered (time x velocity = dist) 5Mv-5Rv
Box floats with current in opposite direction to man for 5min distance covered 5Rv
Result after 5 mins 5Mv - 5Rv + 5Rv = 5Mv the distance of max separation man to box
Man having reversed course has velocity Mv + Rv - Rv therefore Mv is the rate of convergance
Velocity x time = distance .. Hence time = distance / velocity
Distance to close down = 5Mv : Convergance speed = Mv, therefore time 5Mv/Mv = 5 mins, being the time for the man to catch the box whilst rowing at his slack water velocity plus the river current velocity.
Add to this time the 5 mins prior to course reversal following box 'overboard', Therefore it takes 10 mins for box to cover 1 mile hence box speed = River current speed = 1*60/10 = 6 MPH. or 6*52.80/36.00 = 8.8 Ft/sec.

 

[toasty]

It is obvious that the box does not move relatively to the water (i.e. has the same speed as the water).

So if the rower is rowing away from the box for 5 mins (up stream), it will take him another 5 mins to row back to the box again.

Because the rower is rowing with constant speed (constant relatively to the speed of the water!)

You can look at it as if the water in the river doesn't move, the box doesn't move, and the rower rows a certain time away from the box and then back. So in that 10 mins, the box has floated from 1 mile up stream to the bridge.

Conclusion: The water in the river flows at a speed of 1 mile in 10 mins, so 6 mph.

Do I get a prize????

-Dan