A Rebounding Problem

Calculus Level pending

A ball is dropped from a height h off the ground. After each bounce, the ball returns to a height of two thirds of the height of the previous bounce . What is the total distance travelled by the ball? Assume it moves only in the vertical plane and that it continues bouncing indefinitely.

6h 3h 5h 2h

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1 solution

Kyle Fleck
Mar 14, 2015

Each bounce is two thirds of previous height:

h n = 2 3 h n 1 { h }_{ n }=\frac { 2 }{ 3 } { h }_{ n-1 }

Total distance is the original height plus twice the height of each bounce:

d = h + 2 n = 1 ( 2 3 ) n h d=h+2\sum _{ n=1 }^{ \infty }{ { (\frac { 2 }{ 3 } ) }^{ n } } h

Simplifying:

d = h + 2 h n = 1 ( 2 3 ) n d=h+2h\sum _{ n=1 }^{ \infty }{ { (\frac { 2 }{ 3 } ) }^{ n } }

Sum to infinity:

S = a 1 r { S }_{ \infty }=\frac { a }{ 1-r } where a = 2 3 , r = 2 3 a=\frac { 2 }{ 3 } ,\quad r=\frac { 2 }{ 3 }

S = 2 3 ÷ ( 1 2 3 ) = 2 3 ÷ 1 3 = 2 { S }_{ \infty }=\frac { 2 }{ 3 } \div \left( 1-\frac { 2 }{ 3 } \right) =\frac { 2 }{ 3 } \div \frac { 1 }{ 3 } =2

Therefore:

d = h + 2 h × 2 = h + 4 h = 5 h d=h+2h\times 2=h+4h=5h

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