An object is thrown vertically from a height of #5 m# at # 8 ms^-1#. How long will it take for the object to hit the ground?

1 Answer
Jan 15, 2016

Answer:

Assuming the object is thrown downward, the time taken will be #0.48 s#.

Explanation:

The question doesn't specify whether the object is thrown upward or downward. I will assume it is thrown downward. This means that the initial velocity, #u=8 ms^-1#, is in the same direction as the acceleration due to gravity, #a=9.8 ms^-2#.

Use the formula #d=ut+1/2at^2#, where #d=5m#

#5 = 8t + 1/2*9.8*t^2 = 8t + 4.9t^2#

Rearranging into standard form for quadratic equations:

#4.9t^2 +8t - 5 = 0#

Use the quadratic formula or your preferred method of solving quadratic equations and take only the positive root:

#t=0.48s#

(If the assumption was that the object was thrown upward instead, the initial velocity and the acceleration would have opposite signs: the initial velocity would be #-8ms^-1#.)