# Need some help with two Dimensional velocity Vectors

• Giu1iano
In summary, the man plans to cross the river at a certain angle to reach his destination. He needs to know the river's velocity to do this. If the river had zero velocity, it would take 20 seconds to cross. It takes longer if the river is flowing & he wants to go directly North.
Giu1iano
Mod note: Moved from a technical section, so missing the homework template.
Hi, I'm struggling to understand this problem. I would normal just wait for class but I'm away traveling for work.

The question is as follows.
A man has a boat that can travel at 15.0 m/s relative to the water. He starts out from the south shore of a river and plans to cross so that he lands directly across the river from his starting point. The river has a velocity of 8.00 m/s East relative to the shore.

a) at what angle should he point his boat?
b) what will be his velocity relative to the shore?
c) if the river is 300 meters wide, how long will it take him to cross?

Answers: a) [N32°W] b) 12.7 m/s [N] c) 23.6s

For a) I used Tan=8/15, Tan=28° or [N28°W] (which is wrong)
b) I used a^2+b^2=c^2 to solve the resultant Lane. 8^2+15^2=289, √289=17 or 17 m/s[N] (which is wrong)
c) I used time=displacement/velocity, t=300m/17m/s, t=17.6s.

I missed the lecture for this class because of work, and my teachers notes confuse me. I'd appreciate any help!

Thank you :)

Last edited by a moderator:
Giu1iano said:
For a) I used Tan=8/15
Welcome to the PF.

The above is not right... If the boat's speed is 15, that will be the hypotenuse. The tan function is for the two opposite sides. What would the correct trig function be?

I'm sorry to say but you're not right either. The speed of the boat is 15 m/s. If the boat starts from the south shore of the river and plans to cross so that he lands directly across from where he started that would put him traveling North. 15 is adjacent and 8 is the opposite.

Last edited:
Giu1iano said:
I'm sorry to say but you're not right either. The speed of the boat is 15 m/s. If the boat starts from the south shore of the river and plans to cross so that he lands directly across from where he started that would put him traveling North. 15 is adjacent and 8 is the opposite.
No, I don't think so. The diagram I drew has a triangle with the short 8m/s side on the bottom (north points up), the hypoteneuse is 15m/s, and the left vertical side is the river crossing speed which is less than 15 but more than 8. The angle at the top of the triangle is the same as the angle of the 15m/s vector to the left of the vertical. The angle at the top of the triangle has 15m/s as the hypotenuse and 8m/s as the base at the bottom (opposite side)... I do get the same answers as you have posted, BTW.

berkeman said:
No, I don't think so. The diagram I drew has a triangle with the short 8m/s side on the bottom (north points up), the hypoteneuse is 15m/s, and the left vertical side is the river crossing speed which is less than 15 but more than 8. The angle at the top of the triangle is the same as the angle of the 15m/s vector to the left of the vertical. The angle at the top of the triangle has 15m/s as the hypotenuse and 8m/s as the base at the bottom (opposite side)... I do get the same answers as you have posted, BTW.

Mmk, let me try

The hypotenuse does correspond to the velocity of 15 m/s. His velocity (not speed) relative to the water must have a component of 8m/s toward the West to counter the velocity of the river.

If the river had zero velocity, it would take 20 seconds to cross. It takes longer if the river is flowing & he wants to go directly North.

Thank for helping me realize my mistake!

## 1. What is a two dimensional velocity vector?

A two dimensional velocity vector is a mathematical representation of a moving object in two-dimensional space. It includes both the speed and direction of the object's movement.

## 2. How do you calculate the magnitude of a two dimensional velocity vector?

The magnitude of a two dimensional velocity vector can be calculated using the Pythagorean theorem, where the length of the hypotenuse (velocity vector) is equal to the square root of the sum of the squares of the two sides (horizontal and vertical components).

## 3. How do you find the direction of a two dimensional velocity vector?

The direction of a two dimensional velocity vector can be found using trigonometry. By dividing the vertical component by the horizontal component, you can determine the tangent of the angle between the vector and the horizontal axis. This angle can then be used to determine the direction of the vector.

## 4. How can two dimensional velocity vectors be used in real world applications?

Two dimensional velocity vectors are used in many real world applications, such as navigation, robotics, and video game design. They can also be used in physics and engineering to analyze the motion of objects in two-dimensional space.

## 5. Can two dimensional velocity vectors change over time?

Yes, two dimensional velocity vectors can change over time as the speed and direction of an object can change. This can occur due to external forces acting on the object, such as gravity or friction, or by the object's own propulsion or steering mechanisms.

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