Challenge 7a: A Snail's Pace

[Office_Shredder]

Similar to the previous two-part question, if you find part b to be an appropriate challenge please leave part a to those who are appropriately challenged by it.

This question is courtesy of mfb

An immortal snail is at one end of a perfect rubber band with a length of 1km. Every day, it travels 10cm towards the other side. Every night, the rubber band gets stretched uniformly by 1km. As an example, during the first day the snail will advance to x=10cm, then the rubber band gets stretched by a factor of 2, so the snail is now at x=20cm on a rubber band of 2km.

Will the snail ever arrive at the other side, and if yes, how long does it take approximately?

 

[cpscdave]

Hi everyone!
First crack at a math challenge. Not sure if I'm on the right track. Sadly all my relevant math notes and Ti-89 (which might as well be a third arm for me) is on the other side of the country.

But hoping someone can tell me if I'm barking up the right tree or if want I want to do isn't possible.

Spoiler

Calculating by hand I find the position of the snail to be:
(end of) Day 1 =0.0002
Day 2 = 0.00045
Day 3 = 0.000733
Day 4 = 0.001042 etc etc etc

This leads me to beliving the formula for the position of the snail is
$$x[n]=0.0002 \,\,n=1$$

$$x[n] = {\frac{(n+1)}{n}}*(x[n-1]+0.0001) \,\, n > 1$$

expanding this I get something like
$$x[n] = x[n-1] + 0.0001 + {\frac{1}{n}}*(x[n-1] + 0.0001)$$

from here do the Z transform,
solve for x[z] and inverse it and you'll have a formula for the snails position. Subract n and figure out where the function goes positive??

I realize there is a bit of hand waving here :)

Specifically where I'm stuck is figuring out the z transform of

$${\frac{1}{n}}*(x[n-1] + 0.0001)$$

 

[hilbert2]

This challenge might be too easy for me, but here's my solution anyway...

Spoiler

After the first day, the fraction of rubber band's length traveled is $\frac{0.1m}{1000m}=\frac{1}{10000}$. Stretching the rubber band does not change this fraction. On the second day, the snail travels a fraction of $\frac{0.1m}{2000m}=\frac{1}{20000}$ along the rubber band. Therefore, after $n$ days, the fraction is $\frac{1}{10000}\sum_{k=1}^{n}\frac{1}{k}$. As the harmonic series $\sum_{k=1}^{\infty}\frac{1}{k}$ does not converge, the snail will eventually get to the other end of the rubber band. The number of days needed for this can be solved from the equation

$\frac{1}{10000}\sum_{k=1}^{n}\frac{1}{k}=1$.

To solve this, we make the approximation $\sum_{k=1}^{n}\frac{1}{k}\approx \ln n + \gamma$, where $\gamma$ is the Euler-Mascheroni constant. With this approximation, we get the result

$n \approx e^{10000-\gamma}$,

which is a very large number, about $4.945 \times 10^{4342}$.

 

[mfb]

@hilbert2: that is correct.

 

[CellCoree]

I would approach this question by first defining some key variables and assumptions. The snail's pace can be represented by its velocity, which is 10cm/day. The length of the rubber band can be represented by L, which starts at 1km and increases by 1km each night. The distance the snail has traveled at any given time can be represented by x. We can also assume that the snail's velocity remains constant throughout the experiment and that the rubber band is infinitely stretchable without breaking.

With these variables and assumptions in mind, we can use the formula x = vt to calculate the time it takes for the snail to reach the other side. In this case, the time (t) would be equal to the distance (L) divided by the velocity (v). So, t = L/v.

Using this formula, we can see that on the first day, the snail will travel 10cm and the rubber band will stretch to 2km, resulting in a time of 200 days for the snail to reach the other side. On the second day, the snail will travel 20cm and the rubber band will stretch to 3km, resulting in a time of 150 days. As the rubber band continues to stretch, the time it takes for the snail to reach the other side will decrease.

However, as the rubber band continues to stretch, the snail will also have to travel a longer distance. This means that the time it takes for the snail to reach the other side will approach but never reach zero. In theory, the snail will continue to get closer and closer to the other side, but will never actually reach it.

In conclusion, the snail will never arrive at the other side, but it will get infinitely close to it. The time it takes for the snail to get as close as possible to the other side will approach infinity as well. This is an interesting thought experiment that highlights the concept of infinite limits in mathematics and physics.