Monday 17 February 2020

homework and exercises - Why aren't trigonometric functions dimensionless regardless of the argument?


Consider this equation :-



$$y = a\sin kt$$ where $a$ is amplitude, $y$ is displacement, $t$ is time and $k$ is some dimensionless constant.




My instructor said this equation is dimensionally incorrect because the dimension of $[kt] = [\text{T}^1]$ and since $\text{angles}$ are dimensionless, we can conclude that it is dimensionally incorrect.


I don't understand why it is so. Why do we need to check the dimension homogeneity of the term inside the $\sin$ to conclude whether the equation is dimensionally correct or not?


Why isn't the whole sine function is dimensionless $(\sin kt = \text{[T}^0]) $ regardless of the dimension of the argument inside as the range of sine function is $[-1, 1]$.



Answer



One definition of the sinus function (in fact, the one probably used by your calculator) is $$ \sin x = x - \frac{x^3}{3!} + \frac{x^5}{5!} - \cdots $$ This definition is dimensionally inconsistent unless $x$ is dimensionless. The proof that this gives you the familiar trigonometric sine if $x$ is the ratio of two carefully chosen lengths takes you through a surprisingly interesting swath of mathematics.


No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...