# Ball drop and rebound question =/?

• May 13th 2009, 08:19 AM
SarahGr
Ball drop and rebound question =/?
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• May 13th 2009, 08:41 AM
artvandalay11
This is an infinite geometric series question with r=.8 and the coefficient equal to 12. Since r<1, this series converges and the ball actually travels a finite distance.

$\Sigma_{k=0}^{k=\infty}r^k=\frac{1}{1-r}$ for |r|<1 that formula holds true, but in our case here, $\Sigma_{k=0}^{k=\infty}12(r)^k=\frac{12}{1-r}$ however I forgot that the ball bounces and falls so Soroban is correct
• May 13th 2009, 08:53 AM
Soroban
Hello, Sarah!

Quote:

A ball is dropped from a height of 12 feet.
The ball hits the ground and rebounds 80% of the distance it falls each time.
In theory, how far will the ball travel before coming to rest?

Let's baby-step our way through this . . .

It falls 12 feet.

$\text{Then it bounces up }80\%\text{ of }12\:=\:(0.8)12\text{ feet.}$
. . Then it falls that distance.

$\text{Then it bounces up }80\%\text{ of }(0.8)12 \:=\:(0.8)^212\text{ feet.}$
. . Then it falls that distance.

$\text{Then it bounces up }80\%\text{ of }(0.8)^212 \:=\:(0.8)^312\text{ feet.}$
. . Then it falls that distance.
And so on . . .

We can see that the total distance is:

. . $D \;=\;12 + 2(0.8)12 + 2(0.8)^212 + 2(0.8)^312 + 2(0.8)^412 + \hdots$

. . $D \;=\;12 + 2(0.8)12\underbrace{\bigg[1 + 0.8 + 0.8^2 + 0.8^3 + \hdots \bigg]}_{\text{geometric series}}$

The geometric series has first term $a = 1$, common ratio $r = 0.8$
. . Its sum is: . $\frac{1}{1-0.8} \:=\:\frac{1}{0.2} \:=\:5$

Therefore: . $D \;=\;12 + 2(0.8)12(5) \;=\;108\text{ feet}$

• May 13th 2009, 10:36 AM
SarahGr
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