Question

If a ball is launched straight upwards from the ground with an initial velocity of 20.0 m/s, how long does the ball take to reach a height of 15.0 m on the way up?

For the same situation, at what time does the ball reach 15.0 m on the way back down?

Answer 1

h = 20 t - 4.9 t^2
15 = 20 t -4.9 t^2

4.9 t^2 -20 t + 15 = 0

solve quadratic for two values of t
little t on the way up
big t on the way down