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Finding angle through cross and dot product

  • Thread starter cxiangzhi
  • Start date
  • #1
5
0
By using both cross and dot products, the angle between 2 vectors can be found. But there is 1 question that I tried for countless times that the result of cross product and dot product are not the same.

Here is the vectors that I am talking about

A= 2i+3j=k
B= -4i+2j-k

The result by using dot product = 100.08°
The result by using cross product = 79.92°
 

Answers and Replies

  • #2
6,054
390
What is 2i+3j=k?

Show your steps.
 
  • #3
5
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My mistake. Its A= 2i+3j+k

AxB=-5i-2j+16k

abs(AxB) = 16.88

abs(AxB)=abs(A) x abs(B)sinθ

∴ abs(AxB)/(abs(A) x abs(B))= sinθ
∴ θ = 79.92

A.B=-3

A.B/(abs(A) x abs(B))= cos(theta)

∴θ = 100.08

The result of dot and cross product should be the same but its not in this case. Please help.
 
Last edited:
  • #4
Are you using matrices to solve the cross product?
 
  • #5
5
0
Yeap.. I used the determinant method to solve the cross product..
 
  • #6
6,054
390
Observe that sin 79.92 = sin 100.08. So the cross product per se cannot distinguish between such angles.
 
  • #7
5
0
Im sorry but I still have a question here.. What is per se?
 
  • #9
5
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Thanks a lot guys.. Appreciate you help.. :)
 

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