# DE problem: Dog chasing a rabbit

1. May 1, 2008

### camilus

http://i159.photobucket.com/albums/t121/camilus23/SCAN0873.jpg" [Broken]
Can anyone help me with this? Thanks.

Last edited by a moderator: May 3, 2017
2. May 1, 2008

### HallsofIvy

Here's a starter: Since dog and rabbit run at the same speed, assume a time scale so that is 1. At time t, the rabbit is at (0, t). If the dog is at (x,y), it runs directly at the rabbit so it runs on a curve whose tangent line is the line from (x,y) to (0, t): the slope is (y-t)/x.

dy/dx= (y-t)/x. That has the problem that it still involves "t". Try to eliminate t by by using the fact that length of the vector $x'(t)\vec{i}+ y'(t)\vec{j}$ is 1.

3. May 1, 2008

### camilus

I made a small effort and got stuck lol. I wanted to write the DE in linear form but in order to isolate the dy/dx, i would have to square both sides to get it out of the square root. But that means the left side would be x^2 times (d^2y/dx^2)^2, and the second derivative squared messes up the linearity. If I could get rid of it, like if it was x^2 times y-double-prime, i could bring the right over and solve the homogenous equation easily.

but the y-double-prime squared got me stuck!!

4. May 2, 2008

### HallsofIvy

Try this. Multiply
$$\frac{dy}{dx}= \frac{y-vt}{x}$$
through by x to get
$$x\frac{dy}{dx}= y- vt$$

Now differentiate both sides of that, with respect to x, to get
$$x\frac{d^2y}{dx^2}+ \frac{dy}{dx}= \frac{dy}{dx}- v\frac{dt}{dx}$$
$$x\frac{d^2y}{dx^2}= -v\frac{dt}{dx}$$

The fact that the "speed", or length of the velocity vector, is 1 means
$$\sqrt{\left(\frac{dx}{dt}\right)^2+ \left(\frac{dy}{dt}\right)^2}= 1$$
so
$$dx^2+ dy^2= dt^2$$
or
$$1+ \left(\frac{dy}{dx}\right)^2= \left(\frac{dt}{dx}\right)^2$$
$$\frac{dt}{dx}= \sqrt{1+ \left(\frac{dy}{dx}\right)^2}$$
so the differential equation becomes
$$x\frac{d^2y}{dx^2}= -\sqrt{1+\left(\frac{dy}{dx}\right)^2}$$

Oops! I think I just did part a for you!

That's a complicated equation but notice that "y" itself does not appear in it. Let u= dy/dx and you have a first order, separable, differential equation for u.

Last edited by a moderator: Apr 17, 2012
5. May 2, 2008

### Defennder

There's something I don't follow here. Why is the magnitude of the velocity vector 1? Shouldn't that apply only to v/|v|, which is T, the unit tangent vector?

6. May 3, 2008

### HallsofIvy

The problem says dog and rabbit run with the same speed and that is a constant. Since one of the conditions is that the dog starts at (L, 0), we are given some distance scale but we are free to choose the time scale so that speed is 1 "distance unit/time unit".

If you prefer, you could call the speed "v" but it will cancel out.

7. May 3, 2008

### Defennder

Oh okay thanks. Got it.

8. May 18, 2008

### RuDyTaBoOtIe

Hi, i have a similar question that deals with the above problem! I already solved the problem but there is a question that corresponds with it and i am not sure how to do it...

The question i am having problem with is:

a) Suppose that the dog in problem 7 runs twice as fast as the rabbit. find a differential equation for the path of the dog. then solve it to find the point where the dog catches the rabbit.

b) Suppose the dog runs half as fast as the rabbit. how close does the dog get to the rabbit? what are their positions when they are closest?

what i have done so far is use the same process in question 7 by implementing the "twice as fast" part, but i am not sure if this is right...

9. May 19, 2008

### RuDyTaBoOtIe

nevermind, i got it

10. May 20, 2008

### sennyk

I don't see anywhere in that problem in which it states that the speed of the rabbit, or the dog, is constant. As a matter of fact, there is no negative in front of the radical either. I think you're missing something here.

Last edited: May 20, 2008
11. May 20, 2008

### sennyk

If you look at the DE that was derived, you notice that it says that f(x) is concave down. It is obviously concave up.

12. May 20, 2008

### sennyk

Those darn negative signs.

Halls,

It took me a while, but I finally figured out what is missing. Although, v could be variable, you are correct in saying that it will "cancel" out. Here's the issue with the negative in the final DE.

$$\sqrt{\left(\frac{dx}{dt}\right)^2+ \left(\frac{dy}{dt}\right)^2}= v$$
$$\left|\frac{dx}{dt}\right|\sqrt{\left(\frac{dy}{dx}\right)^2+1}=v$$

Since dx/dt is negative, the following is true.

$$-\frac{dx}{dt}\sqrt{\left(\frac{dy}{dx}\right)^2+1}=v$$

13. May 21, 2008

### Defennder

If we don't assume the speed is constant, but varies as a function of t, then we would have $$-x\frac{d^2y}{dx^2} = v \frac{dt}{dx} + t \frac{dv}{dx}$$ Any way to simplify this to the required DE?

14. May 21, 2008

### sennyk

Yeah. "v" is not dependent upon x, so dv/dx is zero. "v" is only dependent upon time.

15. May 21, 2008

### Defennder

v is a function of t, and x is also a function of t. Doesn't this mean that v can be regarded as a function of x?

16. May 21, 2008

### sennyk

In a pure math sense that is correct. Since the DE is given in the problem statement, one would have to assume that the problem writer implies that the speed of the rabbit is not influenced by the x position of the dog; otherwise, another DE would be given. And another constraint would have to be given. You could chide the problem writer for his/her "ambiguity".

Last edited: May 21, 2008
17. May 21, 2008

### D H

Staff Emeritus
There is no ambiguity here. The dog's speed at any time is equal to that of the rabbit.

A few auxiliary variables makes this problem a lot easier. Let
• $r(t)$ be the rabbit's position on the y-axis as a function of time. Vectorially, $\mathbf r(t) = r(t) \hat {\mathbf y}$
• $\mathbf d(t)$ be the dog's position as a function of time: $\mathbf d(t)=x(t)\hat {\mathbf x} + y(t)\hat {\mathbf y}$
• $\mathbf l(t)$ be the displacement vector from the dog to the rabbit: $\mathbf l(t) = \mathbf r(t) - \mathbf d(t)$
• $u(t)$ be the y-component of $\mathbf l(t)$: $u(t) = r(t) -y(t)$
• $l(t)$ be the distance between dog and rabbit: $l(t) = ||\mathbf l(t)|| = \sqrt{x^2(t)+u^2(t)}$

Per the problem statement, the dog's velocity vector at some time t is
$$\dot {\mathbf d}(t) = \dot r(t) \frac {\mathbf l(t)}{l(t)} = \frac{\dot r(t)}{l(t)} (-x(t)\hat {\mathbf x} + u(t)\hat {\mathbf y})$$
Thus
\begin{aligned} \dot x(t) &= -\dot r(t)\,\frac{x(t)}{l(t)} \\ \dot y(t) &= \dot r(t)\frac{u(t)}{l(t)} \\ \dot u(t) &= \dot r(t) - \dot y(t) = \dot r(t)\left(1-\frac{u(t)}{l(t)}\right) \end{aligned}

Applying the chain rule,
$$\frac {dy}{dx} = \frac {\dot y(t)}{\dot x(t)} = -\,\frac {u(t)}{x(t)}$$

The chain rule yields the second derivative of y wrt x,
$$\frac {d^2y}{dx^2} = \frac 1 {\dot x(t)} \frac d{dt}\left(\frac {dy}{dx}\right) = \frac 1 {\dot x(t)} \frac {\dot x(t) u(t) - \dot u(t) x(t)}{x^2(t)} = \frac{l(t)}{x^2(t)}$$

Note that
$$1 + \left(\frac {dy}{dx}\right)^2 = 1 + \frac {u^2(t)}{x^2(t)} = \frac {l^2(t)}{x^2(t)}$$

Thus
$$x(t) \frac {d^2y}{dx^2} = \sqrt{1 + \left(\frac {dy}{dx}\right)^2}$$

Last edited: Apr 17, 2012
18. May 22, 2008

### Defennder

That was brilliant, thanks.

19. Oct 27, 2010

### tdude

Could someone also derive the equation in relation to y?

I'm having huge problems with a question of the sort. Hopefully with the steps laid out it would become clear. Thanks!

20. Apr 16, 2012

### corey2014

So how would I do this if the rabbit has 1/2 the velocity? how would i set up an equation for how X, and Y will change?