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Huck Finn walks at a speed of 0.70 m/s across his raft (that is, he walks perpendicular to the raft's motion relative to the shore). The heavy raft is traveling down the Mississippi River at a speed of 1.50 m/s relative to the river bank (Fig. 3-42). What is Huck's velocity (speed and direction) relative to the river bank?

Problem 39.

Source: Giancoli, Douglas C., Physics: Principles with Applications, 7th Edition, 2014.

Quick Answer: 

$1.66 \textrm{ m/s, } 65^\circ\textrm{ downstream from straight across}$

Note: technically a better answer is $25^\circ$ since the textbook asks for the answer "relative to the river bank". Please see the comment below for more details.

Giancoli 7th Edition, Chapter 3, Problem 39


Chapter 3, Problem 39 is solved.

View sample solution


In the back of the book it says 25 degrees. I got the same answer as you when I tried it on my own before looking at your video. Is that a mistake the book made?

Hi p.zeppelin, thanks for the question. I just re-read the question in the textbook, and the wording is to give the angle "relative to the river bank", in which case the book is correct in saying $25^\circ$. We both found answers relative to "straight across". I would say that if we are explicit about our choice of reference then our answer is fine, but I suppose the better answer is $25^\circ$. I'll make a note above in the quick answer. Just to make it clear, and as a way of checking our conclusion, notice that our answer and textbook's answer are complementary (add to 90): $25^\circ + 65^\circ = 90^\circ$, which is expected since the angle between the river bank and straight across is $90^\circ$.

Mr. Dychko