Question:

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?

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.

## Comments

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 $90^\circ$.

Cheers,

Mr. Dychko