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Old Sep 18th 2008, 09:46 PM   #1
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Post Free Falling Object

I have been trying to do this problem but I cant seem to get the answer. An object is thrown upwards with a speed of 16 m/s. how long does it take it reach a height of 7.0m on the way up? The answer is .52 sec but I am not getting that answer

Last edited by tweedydaf; Sep 19th 2008 at 12:31 AM.
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Old Sep 18th 2008, 10:22 PM   #2
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Well, first, you need to document what you know:

$\displaystyle v_{y0} = 16 \frac{m}{s}$

$\displaystyle v_{yf} = ?$

$\displaystyle y_0 = 0 m$

$\displaystyle y_f = 7 m$

$\displaystyle t = ?$

$\displaystyle a = -9.8 \frac{m}{s^2}$

Now, you have everything you need to solve for t:

The easiest way to do this would be to find the final velocity at 7 m and then solve for time, the first equation we need is a basic kinematic equation:

$\displaystyle v_{yf}^2 = v_{y0}^2 + 2(a)(y_f)$

Plug in what we know:

$\displaystyle v_{yf}^2 = 16^2 + 2(-9.8)(7)$

$\displaystyle = 256 - 137.2$

$\displaystyle = 119$

$\displaystyle v_{yf} = 10.9 \frac{m}{s}$

Now, we take another equation:

$\displaystyle v_{yf} = v_{y0} + at$

Plug what we know:

$\displaystyle 10.9 = 16 - 9.8t$

$\displaystyle -5.1 = -9.8t$

$\displaystyle 0.52 s = t$

And there you go.
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Old Sep 18th 2008, 11:33 PM   #3
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Thank you so much for the help
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