Easier solution to first order non-linear differential equation?

99 Views Asked by At

Im am dealing with this differential equation:

$$m\frac{dv}{dt}=mg-kv^2$$

where $m,g,k$ are constants.

I am able to solve this by treating this as a separable differential equation, but that method is long and tedious and there is lots of room to make mistakes. I am wondering if there is an easier method to solve this i.e laplace transform, etc?

Thanks in advance

1

There are 1 best solutions below

1
On

One sort of solution to this to make it moderately neater is to write $v(t)=Cf(\lambda t)$, and choose $C,\lambda$ craftily to simplify the algebra:

$$m\frac{dv}{dt}=mg-kv^2\implies mC\lambda\cdot f'=mg-kC^2f\implies f'=\frac{g}{C\lambda}-\frac{kC}{m\lambda}f^2$$

So, we might reasonably take $\frac{g}{C\lambda}=1,\frac{kC}{m\lambda}=1\implies C=\sqrt{mg/k},\lambda=\sqrt{gk/m}$, and thereafter work with $f'=1-f^2$, which is perhaps preferable