# Stuck on a vector problem: Boating across a river

• opus
In summary: Thank you for your help!In summary, to row straight across a 63 meter-wide river with a rowing speed of 1.3 m/s relative to the water, and a river flow of 0.57 m/s, you must aim your boat at a 26 degree angle upstream, and it will take you approximately 54 seconds to cross the river.
opus
Gold Member

## Homework Statement

You wish to row straight across a 63 meter-wide river. You can row at a steady 1.3 m/s relative to the water and the river flows at 0.57 m/s.
In what direction should you head, and how long would it take you to cross the river?

## The Attempt at a Solution

I've attached an image to show my drawing of the situation.
What I have done is make 3 vectors.
##\vec v## represents the boat's vector and it's unit notation is ##\vec v = (0\hat i + 1.3\hat j)##
##\vec u## represents the current of the river with ##\vec u =(0.57 \hat i + 0\hat j)##
##\vec T## represents the sum vector and I get ##\vec T =(0.57 \hat i + 1.3\hat j)## ##=(1.42~m/s,66.3°)##

Now I'm not sure where to go after this. I know the current will be pushing the boat in a manner that causes it to not be perpendicular to the shore and I need to find and angle to aim the boat so that it can get straight across.
The problem mentions the term "relative" so I kind of think of reference frames, but I'm not sure how to go about using such a thing here.

#### Attachments

• Screen Shot 2019-01-22 at 1.15.40 AM.png
8.1 KB · Views: 355
With that ##\vec v## you are getting a resultant velocity that has a component in the river direction and therefore you are not crossing straight over the river (you will end up downstream) as specified by the problem.

opus
Orodruin said:
With that ##\vec v## you are getting a resultant velocity that has a component in the river direction and therefore you are not crossing straight over the river (you will end up downstream) as specified by the problem.
So then I need to find a way to have ##\vec T## to have a horizontal component of 0?

I suppose "relative" means the speed through the water compared to over ground. Each of your vectors has two properties: length and direction, ending up in six properties overall. If you have four of them, all of them are defined unambigously, you know

1) length of current vector (speed of river)
2) direction of current vector (direction of river)
3) length of your movement vector through the water (your relative speed)
4) direction of your movement vector over ground (direction to cross the river perpendicularly)

Now you have to find the the missing properties, which can be done graphically very easily. But of course you can calculate it numerically too.

EDIT: In your case, since you want to cross the river perpendicularly to its direction, the problem becomes even easier, since you have a right triangle

opus
stockzahn said:
Now you have to find the the missing properties, which can be done graphically very easily. But of course you can calculate it numerically too.

Aren't the two missing quantities you mention what I have found for ##\vec T##? It's direction and magnitude? This would give me 6 total properties as you've mentioned.

opus said:
Aren't the two missing quantities you mention what I have found for ##\vec T##? It's direction and magnitude? This would give me 6 total properties as you've mentioned.

But your vector ##\vec T## points in the wrong direction, if I didn't misunderstand your notation the angle should be ##90^o##, not ##66.3^o##.

opus
Looks like I misunderstood how to set this up. I had my original vector pointed straight across because it's the shortest distance. But I need the sum vector to be straight across after I've started swimming against the current. Let me rework some of this.

opus said:
But I need the sum vector to be straight across after I've started swimming against the current.

opus
Yeah my thought was to make it straight, and then the current would give me a sum that pushed it to the right, and I'd need to do something to get it straight again lol. Not very efficient I suppose.

Ok I think I've got it worked out. I had to find what was needed to get the sum vector to be perpendicular and got a 26 degree upstream angle and will cross in 54.3 s!

opus said:
Ok I think I've got it worked out. I had to find what was needed to get the sum vector to be perpendicular and got a 26 degree upstream angle and will cross in 54.3 s!

That looks pretty good, I've only obtained a small difference in the crossing time - I calculated 53.9 s. If in your opinion your answer is accurate enough, well done. Otherwise we could compare results, if you want to.

opus
Looks like you are correct. I made the mistake of carrying a rounded solution through my work. In one go on the calculator, I've got 53.92.

## 1. What is a vector problem?

A vector problem is a type of mathematical problem that involves the use of vectors, which are quantities that have both magnitude and direction. In these types of problems, you will typically be given information about the magnitude and direction of one or more vectors and will be asked to find the magnitude and/or direction of a resulting vector.

## 2. How do you solve a vector problem?

To solve a vector problem, you will need to use mathematical operations such as addition, subtraction, and multiplication to combine the given vectors and find the resulting vector. This may involve breaking the vectors down into their components and using trigonometric functions to find the magnitude and direction of the resulting vector.

## 3. What is the significance of boating across a river in a vector problem?

Boating across a river is a common scenario used in vector problems to illustrate the use of vectors in real-world situations. In these types of problems, the river represents a vector with a known magnitude and direction, and the boat represents another vector that can be broken down into its components to find the resulting vector.

## 4. What are some common challenges when solving a vector problem?

One common challenge when solving a vector problem is visualizing the vectors and their components in three-dimensional space. It can also be challenging to accurately break down the vectors into their components and use the correct mathematical operations to find the resulting vector. Additionally, keeping track of units and converting between different units of measurement can also be a challenge.

## 5. How can I improve my skills in solving vector problems?

To improve your skills in solving vector problems, it is important to have a strong understanding of vector concepts such as magnitude, direction, and components. Practice visualizing and manipulating vectors in three-dimensional space, and familiarize yourself with the mathematical operations and trigonometric functions used to solve these types of problems. It can also be helpful to work through a variety of practice problems and seek guidance from a teacher or tutor if needed.

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