Question

A ball is thrown horizontally from a platform so that the initial height of the ball is 6.0 m above the level ground below. The ball lands 24 m from its original horizontal location. Find how fast the ball was thrown
>I don't know what formula to use

Answer 1

Can you figure out how long it would take for the ball to fall 6.0m if it were simply dropped?

Answer 2

The time of flight depends on the vertical displacement of the ball. First determine how long it will take the ball to fall vertically downward. You only need to know the vertical displacement and acceleration of gravity to perform this calculation (this implies that the initial vertical velocity is zero because the ball is not initially thrown in a downward direction.) Then, using this calculated time and the known horizontal displacement, determine the horizontal velocity of the ball. Remember, there is no acceleration in the horizontal plane as there is no external force being applied on the ball in this direction in your case so you will only need an equation that has acceleration (a = 0 m/s^2), time, displacement, and initial velocity. Look at your kinematic equations and see which variables match up to the described situation and try it from there.