Iqra N.

asked • 10/20/17

Rock thrown vertically down. Find the time taken

A girl on a bridge throws a rock straight down at -12.5 m/s. It hits the ground 38.4 m below.
How much time did it take ?

1 Expert Answer

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Inactive Tutor answered • 10/20/17

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Δy = (1/2)gt2 + vt = 4.9t2 + 12.5t
 
Δy = 38.4
 
4.9t2 + 12.5t - 38.4 = 0
 
t = 1.80 s
 
 
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Iqra N.

Thank you very much for the solution, but the answer given is 1.08 s.
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10/23/17

Inactive Tutor

Solve the quadratic equation and see that 1.80 is the correct solution, rounded to 2 decimal places.  I suspect that 1.08 is a typo.  Where did that number come from?
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10/23/17

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