Why Can an Equation Be Homogeneous but Incorrect?

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Discussion Overview

The discussion revolves around the concept of dimensional homogeneity in equations and the possibility of an equation being dimensionally correct yet yielding incorrect results. Participants explore examples and seek clarification on this phenomenon, touching on theoretical and practical implications.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents an example involving two rods, suggesting that while the equation for total length (L = L1 - L2) is dimensionally correct, it produces an incorrect answer.
  • Another example is provided where the formula L = L1 L2 / (L1 + L2) is also dimensionally correct but yields an incorrect result.
  • Participants inquire about well-known formulas that might exemplify this trend of being dimensionally correct yet incorrect in application.
  • A participant discusses the equation for force (F = ma) and proposes a scenario that leads to a contradiction regarding net force, indicating a potential misunderstanding of the equation's application.
  • Humorous suggestions are made about using exam copies of first-year students as examples, implying that many such formulas may not be dimensionally correct.

Areas of Agreement / Disagreement

Participants express curiosity and share examples, but there is no consensus on specific well-known formulas that fit the trend discussed. The conversation remains exploratory without a definitive resolution.

Contextual Notes

Some examples provided may depend on specific interpretations of physical scenarios, and the discussion does not resolve the underlying assumptions regarding the equations presented.

d1v1n1ty nb
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im posed with the question why an equation may be homogeneous with respect to its units but still be incorrect? i can't think of way to explain this can anyone help me out? Thanx
 
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Consider a rod with length L1 = 15 cm and another rod with length L2 = 10cm.

Now put the two rods one after the other.
Consider the following formula: the total length of the two rods L = L1 - L2 = 5 cm.

This formula is dimensionally correct, and the answer is wrong.

Another formula:

the total length equals L = L1 L2 / (L1 + L2) = 150 cm^2 / 25 cm = 6 cm

Again, the formula is dimensionally correct, but gives the wrong anwer.

...
 
Thanx, thts a good way to describe it. Is there any well known common formulae that fits this trend?
 
Force is ma = kgm/s2

So imagine we have an object with mass m and length L just sitting there for some period of time t. Then the net force F=m*L/t2 doesn't actually turn out to be 0.
 
d1v1n1ty nb said:
Thanx, thts a good way to describe it. Is there any well known common formulae that fits this trend?
A well-known formula that doesn't work? No... :confused:
 
d1v1n1ty nb said:
Thanx, thts a good way to describe it. Is there any well known common formulae that fits this trend?

Pick a pile of exam copies of first year students :biggrin:
 
Except that it is probable that none of those will be dimensionally correct either!
 
HallsofIvy said:
Except that it is probable that none of those will be dimensionally correct either!

:smile: :smile:
Right !
 

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