Dimensional Reasoning Math Example 2

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Example 2

medium
A formula for the period of a pendulum is proposed as T=2πL/gT = 2\pi\sqrt{L/g} where LL is length (m) and gg is gravitational acceleration (m/s²). Verify dimensional consistency.

Solution

  1. 1
    Compute [L/g][L/g]: mm/s2=s2\frac{\text{m}}{\text{m}/\text{s}^2} = \text{s}^2.
  2. 2
    Compute [L/g][\sqrt{L/g}]: s2=s\sqrt{\text{s}^2} = \text{s}.
  3. 3
    Compute [2πL/g][2\pi\sqrt{L/g}]: 2π2\pi is dimensionless, so the result has units of seconds.
  4. 4
    The period TT should be in seconds. The formula is dimensionally consistent.

Answer

[T]=s   (dimensionally consistent)[T] = \text{s}\;\checkmark \text{ (dimensionally consistent)}
Dimensional consistency is a necessary (though not sufficient) condition for a formula to be correct. A formula that fails dimensional analysis is definitely wrong; one that passes may still be wrong by a numerical factor.

About Dimensional Reasoning

Using the units and dimensions of physical quantities to check formulas, guide derivations, and eliminate impossible answers.

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