MHB Where can the water jet spray?

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The discussion focuses on a water jet spraying from ground level at various angles in a vertical plane. The key problem is to derive an equation that defines the boundary of the spray reach based on the jet's initial velocity, v0, and the angle of spray, θ. Participants are seeking guidance on how to approach the problem mathematically. A hint is provided to assist in starting the solution process. The conversation emphasizes understanding the physics behind the water jet's trajectory.
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Consider a water jet that sprays from ground level on flat ground. The jet can spray at any angle $0<\theta<\pi$ along some horizontal axis, that is in one vertical plane. The jet sprays water at a velocity of $v_0$. Ignoring drag, find an equation that describes the boundary between where water can reach and where it cannot.
 
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Hi MarkFL,

I'm hoping to get some hint on how to at least start to work on the problem...:o
 
anemone said:
Hi MarkFL,

I'm hoping to get some hint on how to at least start to work on the problem...:o

Well, since you asked so nicely...sure no problem! (Sun)

Hint:

I would begin with the parametric equations of motion:

$$\tag{1}x=v_0\cos(\theta)t$$

$$\tag{2}y=-\frac{g}{2}t^2+v_0\sin(\theta)t$$

To eliminate the parameter $t$, we solve (1) for $t$ and substitute into (2) to get:

$$\tag{3}y=\tan(\theta)x-\frac{g}{2v_0^2\cos^2(\theta)}x^2$$

Using $$\frac{1}{ \cos^2(\theta)}= \sec^2(\theta)= \tan^2(\theta)+1$$ (3) becomes:

$$\tag{4}y=\tan(\theta)x-\frac{g}{2v_0^2}\left(\tan^2(\theta)+1 \right)x^2$$

Now, using the substitution:

$$u=\tan(\theta)$$

we get the parametrization:

$$\tag{5}y=ux-\frac{g}{2v_0^2}\left(u^2+1 \right)x^2$$

Now, let the boundary we seek be denoted by $$f(x)$$. What can we say about the intersection of $y$ and $f(x)$?
 
Solution:

Continuing from the hint I gave above, we will equate $y=f(x)$ and obtain the quadratic in $u$ in standard form:

$$u^2-\frac{2v_0^2}{gx}u+\left(1+\frac{2v_0^2f(x)}{gx^2} \right)=0$$

Because $y$ will only ever be tangent to $f(x)$ as it is the boundary, we know therefore that the discriminant must be zero, giving us:

$$\left(-\frac{2v_0^2}{gx} \right)^2-4\left(1+\frac{2v_0^2f(x)}{gx^2} \right)=0$$

$$\left(\frac{v_0^2}{gx} \right)^2=1+\frac{2v_0^2f(x)}{gx^2}$$

And then solving for $f(x)$, we find:

$$f(x)=-\frac{g}{2v_0^2}x^2+\frac{v_0^2}{2g}$$
 
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