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    Topics || Problems

    If an object is shot vertically from the surface of the earth with an initial velocity of v feet per second and if the air resistance and other disturbing factors are neglected, it is found that \(s = vt -0.5gt^2\), where s feet is the height of the object above the surface at the end of t seconds and g = 32 feet per second square. (a) Solve t in terms of s. (b) If v = 300 feet per second use (a) to find when s =450 and s= 0. Solution:

    (a) Solve t in terms of s.

    \(s = vt - 0.5gt^2\)

    \(-\frac{-2s}{g} = \frac{-2vt}{g} + t^2\)

    \(\frac{v^2}{g^2}-\frac{2s}{g} = t^2 - \frac{2vt}{g} + \frac{v^2}{g^2} \)

    \(\frac{v^2 - 2sg}{g^2} = (t - \frac{v}{g})^2\)

    \(\sqrt{\frac{v^2 - 2sg}{g^2}} = t- \frac{v}{g}\)

    \(\frac{\sqrt{v^2-2sg}}{g} + \frac{v}{g}= t \)

    \(\frac{v \pm \sqrt{v^2-2sg}}{g}= t \)

    If \(v =300 \text{fps}\) solve \(t\) at \(s = 450\)

    \(t = \frac{300 + \sqrt{300^2-2(450)(32)}}{32} \)

    \(t = 17.11 \text{sec} \)

    \(t = \frac{300 - \sqrt{300^2-2(450)(32)}}{32} \)

    \(t = 1.64 \text{sec} \)


    If \(v =300 \text{fps}\) solve \(t\) at \(s = 0\)

    \(t = \frac{300 + \sqrt{300^2-2(0)(32)}}{32} \)

    \(t = 18.75 \text{sec} \)

    \(t = \frac{300 - \sqrt{300^2-2(0)(32)}}{32} \)

    \(t = 0 \text{sec} \)