\"A body is projected upward with velocity 20m/s from the top of a cliff of height 160m. in what times will it reach the ground? (g=10m/s)\"

for upward journey, initial velocity = 20m/s and final is 0  v= u + at gives you the time for upward journey.

Now calculate the distance travelled by the body during this upward journey. The distance s plus 160 will be the distance covered by the body during downward journey and the initial velocity for the downward journey shall be zero. using s = 1/2at² calculate time for downward journey and add the two. 

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u=20m/s
v=0m/s
displacement = -160m
g = 10m/s

 

-160 = 20t – 5t²

5t² – 20t – 160 = 0

 

solving the above equation we get

t = 8 or t = -4

 

since t cannot be negative

therefore, t = 8 sec

 

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