Squaring out coordinates (with trig)

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Discussion Overview

The discussion centers around the mathematical manipulation of vector coordinates, specifically the process of squaring the components of a vector in the context of a worked example involving trigonometric functions. Participants seek clarification on how to properly square coordinates and interpret the notation used in the example.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant expresses confusion about the transition from vector coordinates to a squared form, questioning how coordinates can be squared and why the resulting expression contains specific terms.
  • Another participant clarifies that r is a vector with two components and explains that r² refers to the dot product of the vector with itself, leading to the sum of the squares of its components.
  • A third participant critiques the notation used, suggesting that |r|² would be a clearer representation than r.r.
  • A later reply humorously comments on the informal conventions used in physics notation, implying a tendency to simplify expressions at the cost of clarity.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the notation and clarity of the mathematical expressions. There is a mix of agreement on the interpretation of vector squaring, but differing opinions on the appropriateness of the notation used.

Contextual Notes

Some participants note the importance of using trigonometric identities, such as sin² + cos² = 1, and caution against misapplying the expansion of squared sums, indicating potential pitfalls in the calculations.

Who May Find This Useful

This discussion may be useful for students or individuals working with vector mathematics, particularly in physics contexts, who are grappling with vector notation and manipulation involving trigonometric functions.

UniPhysics90
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I'm following a worked example and really don't get a certain step in it: the step is going between

r=(-(s-2a)sin($) , scos($))

and

(r^2)=(s^2)+(4(a^2)-4as)sin($)

First of all how do you 'square' coordinates? I would have though it stays as coordinates?

Even when i try different ways of multiplying it out i can't get this answer, i always end up with extra terms or wrong coefficients.

any help would be great.

thanks
 
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Note that r is actually a vector, with two components (r = (r1, r2).
So when they write r², they actually mean
r² = r . r = r1² + r2².

You will have to use that sin² + cos² = 1. Just watch out when you calculate (a + b)², it is not а² + b².
 
r2= r.r is not very good notation (although some books use it).
What they mean, and should use, is |r|2.
 
Yes, welcome in the world of lazy physicists ;)
Why write two bars if you can have a convention to leave them out :P
 

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