What is the Angle Between Two Vectors?

  • Thread starter Thread starter coolamasta
  • Start date Start date
  • Tags Tags
    Angle Vectors
Click For Summary

Homework Help Overview

The discussion revolves around finding the angle between two vectors, specifically focusing on the relationship defined by the equation v . w = ||v|| ||w|| cos q. The original poster expresses difficulty in determining the value of lambda in the context of the angle being arccos(4/21).

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the original poster's calculations and the value of lambda, with some questioning the assumptions made in the calculations. There are requests for clarification on the problem statement and the steps taken to arrive at the current understanding.

Discussion Status

Some participants have offered guidance on the calculations and pointed out potential errors in reasoning. The discussion includes attempts to clarify the problem and explore different interpretations of the equations involved. There is an acknowledgment of the challenges faced by the original poster, with encouragement to revisit the problem.

Contextual Notes

Participants note the original poster's struggle with the assignment and the lack of guidance from the lecturer. There are references to previous attempts and the need for clearer communication in the posts.

coolamasta
Messages
7
Reaction score
0

Homework Statement



http://www.flickr.com/photos/coolamasta/4563408911/


Homework Equations



v . w = ||v|| ||w|| cos q

The Attempt at a Solution



i calculate until i got the lambda value is 8/3. then i compare it with 21 and the answer is wrong. i don't know what to do. please help. tq.
 
Last edited by a moderator:
Physics news on Phys.org
coolamasta said:
http://www.flickr.com/photos/coolamasta/4563408911/
A login is needed to see this page.

Please post the question here, without pictures.
 
Last edited by a moderator:
The angle between two vectors v1 = (6,3,-2) and v2 = (-2,lambda,-4) is arccos (4/21). Find the value(s) of lambda. i hope u can understand the questions. thanks in advance.
 
Last edited by a moderator:
http://1.bp.blogspot.com/_2w6xphCWyPE/S9ntDt8niJI/AAAAAAAAAE8/6KISjbB_YCU/s1600/soalan1.jpg

http://4.bp.blogspot.com/_2w6xphCWyPE/S9ntPNm08TI/AAAAAAAAAFE/8w4Fsfu8ISA/s1600/soalan3.jpg

my lecturer gave me this assignments 3 days ago. i spent 2 days already tried to solving it but still can't solve it. i manage to solve 3 out of 5 of his questions but the other two i cant. i went to see him this morning n he's not giving me any tips or solution at all.. i tried to find example questions in his tutorial books but did not find anywhere near this questions. please help me. thanks in advance.
 
Last edited by a moderator:
coolamasta said:
i calculate until i got the lambda value is 8/3. then i compare it with 21 and the answer is wrong. i don't know what to do. please help. tq.
FIrst off, please do learn to use proper English. Sentences start with a capital letter. So does "I".'tq' is not a word. Stop with the silly txtspeak.

OK, had to get that off my chest.

Now as far as your calculated lambda value, show some work. It's a bit hard to figure out where you went wrong when you didn't show us what you did.
 
Sorry for that. This is my work so far.

http://2.bp.blogspot.com/_2w6xphCWyPE/S9n0ILgCiXI/AAAAAAAAAFU/W1NtWNirRBc/s1600/IMG00296-20100430-0459.jpg
 
Last edited by a moderator:
Correction post first:

The problem statement has v1=(6,3,-2). You stated v1=(6,3,2) in post #3. I edited post #3 to reflect the statement.
 
If [itex]\theta = \arccos(4/21)[/itex], what is [itex]\cos \theta[/itex]?
 
Is it there where i went wrong?
 
  • #10
Never mind, I see that you get that.

What you did wrong was to assume that your work meant that

[tex]4=-12 + 3\lambda +8[/tex]

and the same for the denominator.

Look at it this way: 3/6=1/2. certainly neither of 3=1 nor 6=2 is true. Now let's put a variable in there: x/6=1/2. What is x? The solution is found by multiplying both sides by 6, yielding x=6/2=3.

In this problem you have a radical, so at some point it might help to square both sides.

A problem can arise when you do this. Just because you have a solution to the squared expression does not mean you have a solution to the original expression. For example, -1=1 is obviously false, but squaring both sides yields 1=1, obviously true. You will need to double-check your solutions.
 
  • #11
This is the only solution that i know. I've asked everyone around me and they told me they also couldn't find the answer for it. What should I do now?
 
  • #12
What you did was wrong back in elementary school days when you had to learn fractions.

What is the first step often used to solve a problem involving fractions? You want to change the problem to one not involving fractions.
 
  • #13
I think i get it now. I'll try it again. Thank you.
 
  • #14
Good luck! Let us know if you run into problems.
 

Similar threads

But, as I said, you don't actually need the coordinates at all.
  • · Replies 2 ·
Replies
2
Views
2K
Calculating Angle Between E-Field and Current Vectors in Anisotropic Mat.
  • · Replies 3 ·
Replies
3
Views
2K
Vectors & Angles Problem Analysis
  • · Replies 2 ·
Replies
2
Views
2K
Angle between vector and tangent vector
  • · Replies 2 ·
Replies
2
Views
4K
Angle between vector and z-axis
  • · Replies 3 ·
Replies
3
Views
5K
Undergrad Angles between 4-vectors in special relativity?
  • · Replies 5 ·
Replies
5
Views
1K
Graduate Preservation of the angle between two vectors
  • · Replies 9 ·
Replies
9
Views
2K
Is the Angle Between Vectors [2, 1] and [1, -3] Obtuse?
  • · Replies 2 ·
Replies
2
Views
1K
Determining if a subset W is a subspace of vector space V
  • · Replies 8 ·
Replies
8
Views
2K
Finding the angle between two vectors
  • · Replies 2 ·
Replies
2
Views
1K