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Question:

A tennis ball is dropped from $3$ meter drop and it bounces back upwards by $30\%$ of it's initial drop height. What is total distance traveled by tennis ball ?

Attempt to solve

Now we can measure distance traveled by tennis ball with sum defined as:

$$ \sum_{i=0}^{\infty}3*0.3^{i}$$ where $i=$ number of bounces. Since we do not want to limit number of maximum bounces we set it to $\infty$.

plot of first 100 bounces. We can also see that our series is convergent.

summation plot $$$$

Summation of 100 bounces will give us approximately:

$$ \sum_{i=0}^{100}3*0.3^i \approx 4.285714286 $$

There is a limit in what kind of total distance can be achieved which would be defined as:

$$\lim_{n\rightarrow \infty}\sum_{i=0}^{n}3*0.3^i=s$$

Now $s$ would be the value we want to know ? Now the problem is i don't know how to compute value for $s$.

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    $\begingroup$ You have that $\sum_{i=0}^\infty q^i = 1/(1-q)$ for every $q\in (-1, 1)$. $\endgroup$ – Rigel Nov 4 '17 at 16:46
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is a serie geometric So $$\sum_{i=0}^{\infty}3*0.3^{i}=3\sum_{i=0}^{\infty}0.3^{i}=3\cdot\frac{1}{1-0.3}$$ so is equal

$$\sum_{i=0}^{\infty}3*0.3^{i}=\frac{30}{7}\approx 4.28571428571$$

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  • $\begingroup$ How did you come up with $\frac{30}{7}$ ? $\endgroup$ – Tuki Nov 4 '17 at 17:02
  • $\begingroup$ I did not think of it, I just solved the equation knowing the formula of the geometric series. (i.stack.imgur.com/wtW9F.gif) $\endgroup$ – Juan Alfaro Nov 4 '17 at 17:05

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