Physics Help Forum Free Falling Object

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Sep 18th 2008, 08:46 PM #1 Junior Member   Join Date: Sep 2008 Posts: 11 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 18th 2008 at 11:31 PM.
 Sep 18th 2008, 09:22 PM #2 Junior Member   Join Date: Apr 2008 Posts: 21 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.
 Sep 18th 2008, 10:33 PM #3 Junior Member   Join Date: Sep 2008 Posts: 11 Thank you so much for the help

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