Physics Help Forum speed of a feather falling.
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 Jan 8th 2009, 10:42 AM #1 Junior Member   Join Date: Jan 2009 Posts: 7 speed of a feather falling. A feather is placed in a 2 m long vacuum tube that has all the air removed from it. If you flip the tube upside-down how long will it take for the feather to fall to the bottom of the tube?
Jan 8th 2009, 09:54 PM   #2
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 Originally Posted by ariol A feather is placed in a 2 m long vacuum tube that has all the air removed from it. If you flip the tube upside-down how long will it take for the feather to fall to the bottom of the tube?
Taking downward as positive
$\displaystyle s=ut+gt^2/2$
$\displaystyle 2=0+10t^2$

 Jan 9th 2009, 06:17 AM #3 Junior Member   Join Date: Oct 2008 Posts: 12 Since the air has been removed, there are no opposing forces acting on the feather. The feather moves in a straight line. So, we can use one of the equations of linear motion. We know $\displaystyle s = 2\ m$. We know that the initial velocity of the feather is $\displaystyle 0$, i.e. $\displaystyle u = 0$. $\displaystyle a = g = 10\ \ or \ \ 9.81 ms^{-2}$. $\displaystyle s = ut + \frac {1}{2} at^{2}$ $\displaystyle 2 = 0 \times t + \frac {1}{2} \times 9.81 \times t^{2}$ $\displaystyle 2 = \frac {1}{2} \times 9.81 \times t^{2}$ $\displaystyle t = \sqrt {\frac {2}{\frac {1}{2} \times 9.81}}$ $\displaystyle t = 0.639 s$ (correct to 3 sig. fig.) With $\displaystyle g = 10$, you get $\displaystyle t = 0.632 s$. I hope that helps. ILoveMaths07. Last edited by ILoveMaths07; Jan 9th 2009 at 06:20 AM.

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