Physics
posted by Tarra .
A ball is thrown straight up and rises to a maximum height of 20 m above the ground. At what height is the speed of the ball equal to half of its initial value? Assume that the ball starts at a height of 2.1 m above the ground.

When the speed is 1/2 of the initial value, 1/4 of the initial kinetic energy will be lost to potential energy, and the ball will be at 3/4 of its maximum distance above where it was thrown from. It rises a total of 17.9 m. 3/4 of that is 14.43 m. Add that to 2.1 m for the answer.