Dot Product: Understanding and Solving with Vectors | Homework Help

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SUMMARY

The discussion centers on the calculation of the dot product of two vectors, b = x hat + y hat and c = x hat + z hat. The user initially applies two methods: the cosine formula, r * s = rscos(theta), and the component summation method. The component method yields a result of 1, while the cosine method incorrectly suggests a result of 0 due to a miscalculation of the angle, which is actually 60 degrees, not 90 degrees. Both methods are valid, but accurate angle measurement is crucial for the cosine formula to yield correct results.

PREREQUISITES
  • Understanding of vector notation and components in three-dimensional space.
  • Familiarity with the dot product formula: r * s = rscos(theta).
  • Ability to calculate angles between vectors using trigonometric principles.
  • Knowledge of vector addition and multiplication rules.
NEXT STEPS
  • Study the geometric interpretation of the dot product in vector analysis.
  • Learn how to accurately calculate angles between vectors using the law of cosines.
  • Explore vector projections and their applications in physics.
  • Investigate common pitfalls in vector calculations and how to avoid them.
USEFUL FOR

Students studying physics or mathematics, particularly those focusing on vector analysis and dot product calculations. This discussion is beneficial for anyone seeking to clarify the relationship between vector components and their geometric interpretations.

Rijad Hadzic
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Homework Statement


Im given vectors:

b = x hat + y hat
c = x hat + z hat

Homework Equations


http://imgur.com/a/iHTOT

The Attempt at a Solution


so I have 2 eq's... one says:
r * s = rscos(theta)

the other is a summation saying multiply x component 1 with x component 2, add y component 1 with y component 2, and so on.

Ok so I start with method 2: 1*1 x hat + 1* 0 y hat + 0*1 z hat

so you end with 1 (my books answer).

But when I use "r * s = rscos(theta)" I find the angle between them is 90 degrees so I get 0.

Whv does 1 formula work in this case but the other doesnt
 
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Rijad Hadzic said:

Homework Statement


Im given vectors:

b = x hat + y hat
c = x hat + z hat

Homework Equations


http://imgur.com/a/iHTOT

The Attempt at a Solution


so I have 2 eq's... one says:
r * s = rscos(theta)

the other is a summation saying multiply x component 1 with x component 2, add y component 1 with y component 2, and so on.

Ok so I start with method 2: 1*1 x hat + 1* 0 y hat + 0*1 z hat
No, this isn't what it is saying. What you get from a dot product is a number -- no vectors involved.

There are two forms for the dot product of vectors in ##\mathbb{R}^3##: one that involves the cosine of the angle between the vectors, and the other that involves the sum of the products of the components.

Assuming these are your vectors. ##\vec r = <1, 1, 0>## and ##\vec s = <1, 0, 1>##, then ##\vec r \cdot \vec s = 1 \cdot 1 + 1 \cdot 0 + 0 \cdot 1 = 1##

Rijad Hadzic said:
so you end with 1 (my books answer).

But when I use "r * s = rscos(theta)" I find the angle between them is 90 degrees so I get 0.

Whv does 1 formula work in this case but the other doesnt
Both formulas work, but the angle between the two vectors is not 90° - they are actually 60° apart.
 
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