An object is thrown vertically from a height of #9 m# at #4 m/s#. How long will it take for the object to hit the ground?

1 Answer
A08
Jan 14, 2016

#t approx 0.9 s#

Explanation:

Assuming no air resistance and using the equation
#s= ut + 1/2 g t^2#
Substituting given values and of #g = 9.8 m / s^2#
#9= 4t + 1/2 9.8 t^2#
Rearranging
#1/2 9.8 t^2 +4t -9 =0#

#implies# #4.9 t^2 +4t -9 =0#

Solving the quadratic equation using the formula
#t=(-b+-sqrt (b^2-4ac))/(2a)#

#t=(-4+-sqrt (4^2-4 times 4.9 times (-9)))/(2 times 4.9)#

and ignoring the #- ve# value of t

#t=(-4+13.28)/(9.8)#

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