Object Thrown Vertically Upward

[marvolo1300]

1. Homework Statement :

A ball is thrown vertically upward with a speed of 20.0m/s.

i) How fast is it moving when it reaches 16m
ii) How long is required to reach this height?
iii) Explain why there are two answers.
2. The attempt at a solution
I assume there are two vectors. The force of the ball going upwards and the force of gravity pulling the ball downwards.


Thanks in advance for helping.

 

[jacsamell]

There is no force upwards because the ball is in free flight. Therefore the only force is due to gravity.

Using a=g you can derive all the formula you need by integrating wrt time:

a=g
v=v0+g*t
d=v0t+(1/2)*g*t^2