- #1
monkeyboy590
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Homework Statement
A gun is fired straight up. Assuming that the air drag on the bullet varies quadratically with speed. Show that speed varies with height according to the equations
v^2 = Ae^(-2kx) - g/k (upward motion)
V^2 = g/k - Be^(2kx) (downward motion)
in which A and B are constants of integration, g is the acceleration of gravity, k=cm, where c is the drag constant and m is the mass of the bullet. (Note: x is measured positive upward, and the gravitational force is assumed to be constant.)
Homework Equations
m dv/dt = mg - cv^2 = mg(1 - cv^2/mg)
.'. dv/dt = g(1 - v^2/v(sub t)^2) where v(sub t) is the terminal speed, sqrt(mg/c)
dv/dt = dv/dx dx/dt = 1/2 dv^2/dx
So now we can write the equation as:
dv^2/dx = 2g(1 - v^2/v(sub t)^2)
The Attempt at a Solution
I have gotten as far as to say:
For downward motion:
v^2 = g/k(1exp^[2kx]) + v(naught)^2.exp^[2kx]
Likewise for upward, we get:v^2 = g/k(1-exp^[-2kx]) + v(naught)^2.exp^[-2kx]
I just don't know how to relate these to the original equation to show that speed varies with height.