Answer :
### 16. What are the limitations of dimensional analysis?
Dimensional analysis is a valuable tool for checking the consistency and plausibility of physical equations and for converting units. However, it also has several limitations:
1. Cannot Determine Numerical Factors:
Dimensional analysis can ensure that an equation is dimensionally consistent but it cannot provide numerical constants or coefficients (e.g., the factor [tex]\(2\pi\)[/tex] in the formula for the period of a simple pendulum).
2. Cannot Discover Non-dimensional Numbers:
It cannot predict dimensionless constants such as the Reynolds number, which play crucial roles in fluid dynamics and other fields.
3. Limited in Complex Systems:
In systems involving multiple variables, especially where interactions are not simple multiplicative or power-law relationships, dimensional analysis may not capture the full complexity of the relationships between variables.
4. Not Reflecting Physical Mechanisms:
While dimensional consistency is necessary, it is not sufficient to ensure physical accuracy. An equation might be dimensionally consistent yet physically incorrect because it lacks a proper theoretical basis or experimental verification.
5. Cannot Predict Structures:
It cannot provide information on the internal interactions of a system or predict structural formulas in chemistry.
6. Ambiguity in Choosing Dimensions:
Sometimes there may be ambiguity in the choice of fundamental dimensions or base units.
Dimensional analysis should therefore be used as a complementary tool along with theoretical derivation and empirical validation, rather than a definitive proof of correctness.
### 7. Check the Correctness of the Formula [tex]\( t = 2 \pi \sqrt{l / g} \)[/tex] Using Dimensional Analysis
We aim to check the dimensional consistency of the given formula using dimensional analysis. Let's assign dimensions to each of the variables:
- [tex]\( t \)[/tex]: Time period of a simple pendulum [tex]\([T]\)[/tex]
- [tex]\( l \)[/tex]: Length of the pendulum [tex]\([L]\)[/tex]
- [tex]\( g \)[/tex]: Acceleration due to gravity [tex]\([L][T]^{-2}\)[/tex]
Now, we'll substitute these dimensions into the formula step by step to check for dimensional consistency.
1. Write the Formula with Dimensions:
The formula given is:
[tex]\[ t = 2 \pi \sqrt{\frac{l}{g}} \][/tex]
2. Substitute Dimensions:
Substitute the dimensional forms of [tex]\( l \)[/tex] and [tex]\( g \)[/tex] into the equation:
[tex]\[ [T] = 2\pi \sqrt{\frac{[L]}{[L][T]^{-2}}} \][/tex]
3. Simplify Inside the Square Root:
Simplify the dimensions within the square root:
[tex]\[ [T] = \sqrt{\frac{[L]}{[L] \cdot [T]^{-2}}} = \sqrt{\frac{[L]}{[L]/[T]^2}} = \sqrt{[L] \cdot \frac{[T]^2}{[L]}} \][/tex]
4. Further Simplify:
Continue to simplify:
[tex]\[ [T] = \sqrt{\frac{[L] \cdot [T]^2}{[L]}} = \sqrt{[T]^2} \][/tex]
5. Final Dimensional Check:
Finally:
[tex]\[ [T] = [T] \][/tex]
Since both sides of the equation have the same dimensions [tex]\([T]\)[/tex], the formula is dimensionally consistent.
By checking the formula through dimensional analysis, we conclude that the formula [tex]\( t = 2\pi\sqrt{\frac{l}{g}} \)[/tex] is dimensionally correct.
Dimensional analysis is a valuable tool for checking the consistency and plausibility of physical equations and for converting units. However, it also has several limitations:
1. Cannot Determine Numerical Factors:
Dimensional analysis can ensure that an equation is dimensionally consistent but it cannot provide numerical constants or coefficients (e.g., the factor [tex]\(2\pi\)[/tex] in the formula for the period of a simple pendulum).
2. Cannot Discover Non-dimensional Numbers:
It cannot predict dimensionless constants such as the Reynolds number, which play crucial roles in fluid dynamics and other fields.
3. Limited in Complex Systems:
In systems involving multiple variables, especially where interactions are not simple multiplicative or power-law relationships, dimensional analysis may not capture the full complexity of the relationships between variables.
4. Not Reflecting Physical Mechanisms:
While dimensional consistency is necessary, it is not sufficient to ensure physical accuracy. An equation might be dimensionally consistent yet physically incorrect because it lacks a proper theoretical basis or experimental verification.
5. Cannot Predict Structures:
It cannot provide information on the internal interactions of a system or predict structural formulas in chemistry.
6. Ambiguity in Choosing Dimensions:
Sometimes there may be ambiguity in the choice of fundamental dimensions or base units.
Dimensional analysis should therefore be used as a complementary tool along with theoretical derivation and empirical validation, rather than a definitive proof of correctness.
### 7. Check the Correctness of the Formula [tex]\( t = 2 \pi \sqrt{l / g} \)[/tex] Using Dimensional Analysis
We aim to check the dimensional consistency of the given formula using dimensional analysis. Let's assign dimensions to each of the variables:
- [tex]\( t \)[/tex]: Time period of a simple pendulum [tex]\([T]\)[/tex]
- [tex]\( l \)[/tex]: Length of the pendulum [tex]\([L]\)[/tex]
- [tex]\( g \)[/tex]: Acceleration due to gravity [tex]\([L][T]^{-2}\)[/tex]
Now, we'll substitute these dimensions into the formula step by step to check for dimensional consistency.
1. Write the Formula with Dimensions:
The formula given is:
[tex]\[ t = 2 \pi \sqrt{\frac{l}{g}} \][/tex]
2. Substitute Dimensions:
Substitute the dimensional forms of [tex]\( l \)[/tex] and [tex]\( g \)[/tex] into the equation:
[tex]\[ [T] = 2\pi \sqrt{\frac{[L]}{[L][T]^{-2}}} \][/tex]
3. Simplify Inside the Square Root:
Simplify the dimensions within the square root:
[tex]\[ [T] = \sqrt{\frac{[L]}{[L] \cdot [T]^{-2}}} = \sqrt{\frac{[L]}{[L]/[T]^2}} = \sqrt{[L] \cdot \frac{[T]^2}{[L]}} \][/tex]
4. Further Simplify:
Continue to simplify:
[tex]\[ [T] = \sqrt{\frac{[L] \cdot [T]^2}{[L]}} = \sqrt{[T]^2} \][/tex]
5. Final Dimensional Check:
Finally:
[tex]\[ [T] = [T] \][/tex]
Since both sides of the equation have the same dimensions [tex]\([T]\)[/tex], the formula is dimensionally consistent.
By checking the formula through dimensional analysis, we conclude that the formula [tex]\( t = 2\pi\sqrt{\frac{l}{g}} \)[/tex] is dimensionally correct.
