Im just a little confused with what to do next:

a river otter wants to cross a river. It can swim at a speed of 2.75m/s. the distance acorss the river is 45.0m and the current velocity is 2.5m/s. 

first it asked me to calculate the time taken, so I calculated the resultant (3.72m/s) and used that to calculate the time (12.1s). 

But now its asking me to calculate how many metres downstream the otter wil be when it gets to the other side, i dont know what to do for this? do i use the time i got previously and use the river current (2.5m/s) velocity???

then its also asking me to find the velocty relative to the bank where it started, didnt i already do that when I had calculate the resultant???

Please help Im so lost.

4 years ago

Answered By Rohtaz S

Ok, here is what went wrong.

The time will be calculated from the velocity  of otter which is assumed to be towards the other bank and 90 degrees to the river flow.Only this velocity can give you the time to cross river, not the velocity of river.

So, time taken to croas the river = (45m)/(2.75m/s) = 16.36 sec

 

Distance travelled down stream = velocity of river x time taken to cross river = 40.90m

 

Relative velocity with bank that you calculated 3.72m/s is correct in this case.

 

Note: This changes if the scenario would be to go straight toward the opposite end and then otter will have to negate the river velocity component with its own velocity.

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