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gelfand
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Homework Statement
A submarine engine provides maximum constant force ##F## to propel it through the
water.
Assume that the magnitude of the resistive drag force of the water experienced
by the submarine is ##kv##, where ##k## is the drag coefficient and ##v## is the
instantaneous speed of the boat.
When the engine is switched on to full power, the submarine starts from rest and
moves horizontally in a straight line.
If the mass of the submarine is ##M##
1) Write the differential equation of motion of the submarine in terms of its
speed ##v##
2) Solve the equation of motion from part (1) to find the speed ##v(t)## as a
function of time ##t##
3) What is the maximum possible speed ##v_{max}## the submarine can reach with
this engine?
Homework Equations
Newtons second states ##F = ma##. Then given force ##F## I have ##F = ma##.
The Attempt at a Solution
For the drag force ##-kv## I can write this as ##-Kmv## where ##K## is some constant
such that ##-Kmv = -kv##.
This gives me
$$
ma = -Kmv
$$
Dividing by ##m## and noting that ##a = \frac{dv}{dt}## gives
$$
\frac{dv}{dt} = -Kv
$$
Which is a differential equation of motion in terms of the speed ##v##.
Solving this differential equation as
$$
\frac{1}{v} dv = -K dt
$$
Then integrate both sides for
$$
\ln(v) = -K t + C_1
$$
Here I can take the exponential of each side and note that ##e^{x + b} = Ae^x##
where ##A,b## are constants.
So I have
$$
v = Ae^{-Kt}
$$For the maximum speed I need to consider where the force of motion by the engine
is balanced by the force of the drag.
At this point I will have
$$
kv_{max} = ma
$$
Using the solution from part (2) we can sub for ##v## as
$$
k\left( A e^{-K t} \right) = ma
$$I'm not sure how to obtain the maximum speed here though?
The expression can be rewritten as
$$
\frac{kA}{ma} = e^{Kt}
$$
Given ##K## is some constant this is just (where ##D## is some constant)
$$
\frac{kA}{ma} = e^{t} e^{K} = De^{t}
$$
So
$$
\frac{kA}{Dma} =
e^{t}
$$
I don't think that this is correct, or even know if it makes sense.
It's suggesting that the constant ##k## multiplied by the constant ##A## divided by
the product of mass, acceleration and ##D## is equal to ##e^t##.
So I'm confused
Thanks
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