Why Was My Thread Closed on the Physics Forum?

[imbumb]

Homework Statement

A boy is situated at point A in a river, at distance a from the riverbank. He can swim at speed u or run by at speed v > u on the shore; water flows in the river at velocity w > u. The boy wants to reach the point C upstream on the riverbank with minimal time. At what distance x from point B aligned with point A should he get out of the water?

Relevant Equations

n1 sin a=n2 sin b

what i tried to do is solve it using the point of reference of w so i get that AB is tilted to the left at an angle a and that CB would be v+w but the solution says it sould be v+w/u and that i should use the formula above to find a and hence find the time but i dont understand why

 

[berkeman]

Is there a diagram that goes with this problem? Use the "Attach files" link below the Edit window to upload the diagram.

 

[imbumb]

i hope this helps

 

[haruspex]

I think I understand the question but can make no sense of your sketched method. Please post all your working. Type it in; images of handwriting are rarely legible.

 

[Mister T]

i get that AB is tilted to the left at an angle a

Without a diagram for reference, I can't tell which way is left.

the solution says it sould be v+w/u

That can't be right. $v$ has dimensions of velocity, and $\frac{w}{u}$ has no dimensions. So you can't add $v$ to $\frac{w}{u}$ because they don't have the same dimensions. For example, $v$ could have the dimensions of meters per second.

Perhaps you meant $\frac{v+w}{u}$? But that can't be right because it's dimensionless, and you're looking for an answer that has dimensions of length, for example, meters. Try using LaTeX to write your equations. It will make them much easier for us to read.

 

[kuruman]

I don't think that the equation you have as relevant is applicable here. That's the way to go if there is no current. I assume that the boy's swimming speed u is relative to the water and I note that the current speed w is greater than u. What strategy should the boy employ if he is to get to point C even though the current flows faster than he can swim?

 

[imbumb]

Without a diagram for reference, I can't tell which way is left.That can't be right. $v$ has dimensions of velocity, and $\frac{w}{u}$ has no dimensions. So you can't add $v$ to $\frac{w}{u}$ because they don't have the same dimensions. For example, $v$ could have the dimensions of meters per second.

Perhaps you meant $\frac{v+w}{u}$? Try using LaTeX to write your equations. It will make them much easier for us to read.

yes i meant $$\frac{v+w}{u}$$

 

[kuruman]

What is $$\frac{v+w}{u}$$supposed to be?

Also, it seems to me that the distance between the banks as well as the distance along the banks between B and C are needed. Are they given?

 

[imbumb]

What is $$\frac{v+w}{u}$$supposed to be?

Also, it seems to me that the distance between the banks is needed. Is that distance given?

why would it be needed? also $$\frac{v+w}{u}$$ is supposed to be the distance between C and B if we take everything from the perspective of w

 

[imbumb]

I think I understand the question but can make no sense of your sketched method. Please post all your working. Type it in; images of handwriting are rarely legible.

you can just click the picture

 

[haruspex]

What is $$\frac{v+w}{u}$$supposed to be?

Also, it seems to me that the distance between the banks as well as the distance along the banks between B and C are needed. Are they given?

I believe the boy starts in the river, distance a from the target bank,

 

[haruspex]

you can just click the picture

There is text missing at each edge.

 

[kuruman]

I believe the boy starts in the river, distance a from the target bank,

OK. I see now that he starts in the river. I initially thought that he was on dry land across the river at distance $a$ from the nearest bank. That would have been a more interesting problem.

 

[imbumb]

There is text missing at each edge.

you dont need the text, i copy pasted it here

 

[Lnewqban]

you dont need the text, i copy pasted it here

Please, cooperate with the experts currently trying to help you understand the problem.

 

[Mister T]

yes i meant $$\frac{v+w}{u}$$

But, as I said, that can't be right. The problem asks for the distance $x$, and the quantity $\frac{v+w}{u}$ is not a distance. Likewise, the answer you came up with, $v+w$, can't be right either because it's a speed, not a distance.

You need to show us how you came up with your answer. Show us all the steps and describe what you're doing. You see examples of what I'm asking for by looking at the worked examples in your textbook. Note that when they work out the solution they show all the steps and explain what they're doing. That's what we're asking you to do. That way we can help you by guiding you towards the right steps to take.

Oh, and one other thing. Don't use the quantity $a$ to refer to an angle. It's already given in the figure as the distance from point A to point B. The distance $a$ will likely have to appear in your answer.

Edit: Now that I look more closely at Post #3 it looks like it is a worked example in your textbook. But of course we can't read the text because you've cropped the text on the left and right sides of the page. Show us the whole thing. I suspect that you are having trouble following the solution given in the textbook. We can help you with that.

 

[imbumb]

[Insult removed by the Mentors]

 

[Lnewqban]

[Insult removed by the Mentors]

No, that was not what I meant.
They respond to your post on voluntary basis.
Therefore, they do not need to find excuses.

Our old and tired eyes only need to clearly see the wording of the problem and any posted picture.
As per forum rules, we will not solve the problem for you, we will guide you to its full understanding.

 

[erobz]

In spite of the OP's abysmal attitude, does this have an analytic solution? I'm starting to form the standard optimization of total time w.r.t $x$... it looks like it's going to be messy tackling it that way.

 

[Mister T]

you mean experts that find excuses not to solve the problem?

We're not here to solve the problem. We're here to help you solve the problem. We won't do your homework for you, but we will help you do it yourself. That's the only way you'll achieve success in the course. You have to learn how to solve problems on tests. This is not an excuse. It's the way of the world. Students have to pass tests to get their diploma.

i copy pasted the text and sent the photo but they werent satisfied

If you're referring to the photo in Post #3 we have already told you that the text you posted can't be read because the text is missing at both edges.

As I said before, from what I could make out, that's a worked example in your textbook and what you really want is help understanding the author's solution. But we can't know that for sure, or help you, unless you post the entire text of the problem, including the author's solution, if it is indeed a worked example in the textbook.

People can't help other people unless they know what's needed, and that the person really wants to be helped. It seems neither of these criteria are being met.

 

[haruspex]

you can just click the picture

In post #1 you wrote

what i tried to do is solve it using the point of reference of w so i get that AB is tilted to the left at an angle a and that CB would be v+w

I asked to see your actual attempt, as required by forum rules. I still do not see that.

 

[berkeman]

Thread closed for Moderation...

 

[berkeman]

After some cleanup, this thread will remain closed.

@imbumb -- Your thread is substandard:

** Your thread start was incomplete with no diagram posted and no clear working of the problem

** You have been dragging your feet at every request by us to try to clarify your problem statement and show your work

** Your insults are not warranted, and are not allowed at PF.

If you still need help on this problem, start a new thread and please do a much better job showing the problem statement and showing your detailed work on the solution.

Also, I'm guessing that English is not your primary language, or you would realize how ironic your choice of username is...