Find an Equivalent Law for Dimensionless Quantities

In summary, the conversation discusses finding an equivalent law for a physical system described by $f(E,P,A)=0$ in terms of dimensionless quantities. The participants also discuss the correct fundamental units for mass, length, and time and whether Buckingham's theorem can still be applied.
  • #1
evinda
Gold Member
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Hello! (Wave)

A physical system is described by a law of the form $f(E,P,A)=0$ where $E,P,A$ represent, respectively, enery, pressure and area of surface. Find an equivalent law that relates suitable dimensionless quantities.

I have tried the following:

The fundamental units are:

Mass: $M$, Length: $L$, Time: $T$.

Thus:
$$[E]=ML^{2}T^{-2} \\ [P]=ML^{-1}T^{-2} \\ [A]=L^2$$

In this case, the number of fundamental units is equal to the number of the quantities with dimensions, so we cannot apply Buckingham $\pi$ theorem , right?

But how else can we find an equivalent law that relates suitable dimensionless quantities? (Thinking)

Or have I done something wrong at the choice of the fundamental units? :confused:
 
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  • #2
Hey! (Blush)

Those units are correct and they are dependent on each other.
I think that means that we can still apply Buckingham's theorem.
Did you try? (Wondering)
 

1. What are dimensionless quantities?

Dimensionless quantities are physical quantities that do not have units of measurement. They are ratios of two quantities with the same units, resulting in a value that is independent of the units used.

2. Why is it important to find an equivalent law for dimensionless quantities?

Finding an equivalent law for dimensionless quantities allows us to simplify and generalize physical laws, making them applicable to a wider range of situations. It also helps in identifying relationships between different physical quantities and simplifying calculations.

3. How do you find an equivalent law for dimensionless quantities?

To find an equivalent law for dimensionless quantities, we first identify the physical quantities involved in the original law and express them in terms of their fundamental units. Then, we eliminate the units by dividing the quantities with the same units. The resulting equation will be a dimensionless quantity that represents the relationship between the original physical quantities.

4. Can dimensionless quantities be used to compare physical systems?

Yes, dimensionless quantities can be used to compare physical systems as they eliminate the influence of different units and allow for a direct comparison of the underlying relationships between the systems.

5. What are some common dimensionless quantities used in science?

Some common dimensionless quantities used in science include the Reynolds number, the Mach number, and the Froude number in fluid dynamics, the Peclet number in heat transfer, and the Grashof number in convection. Other well-known dimensionless quantities are the coefficient of friction, the damping ratio, and the elastic modulus in mechanics.

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