Terminal Velocity of Body under Fall Retarded Proportional to Velocity
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Theorem
Let $B$ be a body falling in a gravitational field.
Let $B$ be falling through a medium which exerts a resisting force $k \mathbf v$ upon $B$ which is proportional to the velocity of $B$ relative to the medium.
Then the terminal velocity of $B$ is given by:
- $v = \dfrac {g m} k$
Proof
Let $B$ start from rest.
From Motion of Body Falling through Air, the differential equation governing the motion of $B$ is given by:
- $m \dfrac {\d^2 \mathbf s} {\d t^2} = m \mathbf g - k \dfrac {\d \mathbf s} {\d t}$
Dividing through by $m$ and setting $c = \dfrac k m$ gives:
- $\dfrac {\d^2 \mathbf s} {\d t^2} = \mathbf g - c \dfrac {\d \mathbf s} {\d t}$
By definition of velocity:
- $\dfrac {\d \mathbf v} {\d t} = \mathbf g - c \mathbf v$
and so:
\(\ds \int \dfrac {\d \mathbf v} {\mathbf g - c \mathbf v}\) | \(=\) | \(\ds \int \rd t\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds -\dfrac 1 c \map \ln {\mathbf g - c \mathbf v}\) | \(=\) | \(\ds t + c_1\) | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \mathbf g - c \mathbf v\) | \(=\) | \(\ds \mathbf c_2 e^{-c t}\) |
When $t = 0$ we have that $\mathbf v = 0$ and so:
- $\mathbf c_2 = \mathbf g$
Hence by taking magnitudes:
- $v = \dfrac g c \paren {1 - e^{-c t} }$
Since $c > 0$ it follows that $v \to \dfrac g c$ as $t \to \infty$.
Thus in the limit:
- $v = \dfrac g c = \dfrac {g m} k$
$\blacksquare$
Sources
- 1972: George F. Simmons: Differential Equations ... (previous) ... (next): $1$: The Nature of Differential Equations: $\S 5$: Falling Bodies and Other Rate Problems