An object is thrown straight up into the air with an initial speed of 8 m/s, and reaches a greatest height of 15 m before it falls back to the ground. a. What height is the object thrown from? b. How long does it take for the object to hit the ground?

The value is    

Explanation:

From the we are told that  

         The initial speed of the object is  

         The greatest height it reached is  

Generally from kinematic equation we have that

At maximum height v  =  0 m/s

So

=>    

Here H is the height from the initial height to the maximum height

So the initial height is mathematically represented as  

=>    

=>    

Generally the time taken for the object to reach maximum height is mathematically evaluated using kinematic equation as follows

At maximum height v  =  0 m/s

=>        

Generally the time taken for the object to move from the maximum height to the ground is mathematically using kinematic equation as follows

Here the initial velocity is  0 m/s given that its the velocity at maximum height

Also  g is positive because we are moving in the direction of gravity  

So

=>      

Generally the total time taken is mathematically represented as

=>        

=>                    

the answer is b ) square root of 5