Falling in Steps

A block is initially positioned at height y = 1 m y = 1 \, \text{m} above the ground. There are infinitely many ladder rungs below the block.

Rung 1 1 is a distance of 1 2 m \frac{1}{2} \, \text{m} below the block's starting position. Rung 2 2 is a distance of 1 4 m \frac{1}{4} \, \text{m} below Rung 1 1 . Rung 3 3 is a distance of 1 8 m \frac{1}{8} \, \text{m} below Rung 2 2 , etc, etc.

Each time the block hits a rung, it comes to a stop and immediately begins falling again under the gravitational acceleration g = 10 m/s 2 g = 10 \, \text{m/s}^2 . The block initially starts falling from its starting position ( y = 1 m ) (y = 1 \, \text{m}) at time t = 0 t = 0 .

At what time does the block reach the ground?

Note: Assume that the block's size is negligible


The answer is 1.079669.

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1 solution

Parth Sankhe
Jan 2, 2019

Time taken to fall h h metres = 2 h g \sqrt {\frac {2h}{g}}

Hence total time :-

t = 2 1 2 g + 2 1 4 g + 2 1 8 g . . . t=\sqrt {\frac {2\cdot \frac {1}{2}}{g}} + \sqrt {\frac {2\cdot \frac {1}{4}}{g}}+\sqrt {\frac {2\cdot \frac {1}{8}}{g}}...

This can be converted into an infinite G.P sum.

The final answer is t = 1 5 ( 2 1 ) t=\frac {1}{√5(√2-1)}

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