Position Vectors in Physics: r=(x,y,z) to r=xi+yj+zk

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Discussion Overview

The discussion centers on the representation of position vectors in physics, specifically the transition from the coordinate form r=(x,y,z) to the vector form r=xi+yj+zk. Participants explore the implications of this notation in various contexts, including Newtonian and Lagrangian mechanics.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants argue that the vector form r=xi+yj+zk is necessary for clarity in higher-level physics, such as Lagrangian mechanics, while others suggest that it is not exclusive to that framework and can be applied in Newtonian mechanics as well.
  • One participant emphasizes that x+y+z is merely a scalar quantity, whereas (x,y,z) represents an ordered triplet, and the basis vectors i,j,k define directions in space.
  • Another viewpoint suggests that the introduction of basis vectors allows for equations of motion that are independent of the specific coordinate system used.
  • A participant challenges this by stating that the vector form is not independent of the basis, as it relies on the standard Cartesian basis represented by i,j,k.
  • It is noted that Newton's equations tend to be more complex in non-Cartesian coordinates, implying that the choice of basis can significantly affect the formulation of physical laws.

Areas of Agreement / Disagreement

Participants express differing opinions on the necessity and implications of using basis vectors in position representation. There is no consensus on whether the introduction of i,j,k fundamentally changes the independence of equations of motion from the basis used.

Contextual Notes

Some claims rely on specific interpretations of coordinate systems and the role of basis vectors, which may not be universally accepted. The discussion also reflects varying levels of familiarity with historical and contemporary physics texts.

Bim
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For describing position of a particle we need vector coordinates r=(x,y,z). But when you go to higher physics like lagrangian formalism also basis vectors are introduced. The position vector then becomes r=xi+yj+zk, where i,j,k are basis vectors. Why do we have do do that? Why is not enough just to use the r=x+y+z?
 
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One doesn't need to lagrangian mech. to enounter, xi+yj+zk, Newtonian would do. x+y+z is just a number, which is different from (x,y,z), an ordered triplet. The i,j,k specify the directions.
 
It's merely a notational preference.
 
I did not mean it so simply. It has to do with equations of motion, that they are independent of basis. So any elaboration in this regard would be very much appreciated. Why do the equations of motion become independent of basis when we introduce the i,j,k vectors?
 
erg... xi+yj+zk is not independent of basis, the standard basis is being used here. i,j,k are e1,e2,e3.
xi+yi+zk=x(1,0,0)+y(0,1,0)+z(0,0,1)=(x,y,z)

or maybe I am wrong, though this is how this things were shown in some books i have at home(30 years old or so, from uni of california)
 
It's because i,j,k are Cartesian rectangular co-ordiantes: the preferred basis for Newtonian mechanics. Newton's eqns. look ugly in anything other than Cartesian coordinates. So when doing Newton's physics in a Cartesian basis, the vectors themselves don't feature in the equations, so it appears as if they're not really there.

When doing physics in other co-ordinate systems, the basis vectors also appear in the equations. If you tried doing non-Lagrangian (i.e. simpler Newtonian mechanics) in a non-Cartesian basis the basis vectors would still feature. e.g. try finding the equations of motion in planar polar co-ordinates using just Newton and differentiating vectors: you'll (obviously) get the same result that Lagrangian mechanics would give you.

note: I've made the simple association with basis vectors and co-ordinate systems using the notion of the co-ordinate basis.
 
Thanks Masudr!
 

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