You throw a baseball at a 45 angle to the horizontal, aiming at a friend who's sitting in a tree a distance above level ground. At the instant you throw your ball, your friend drops another ball. The two balls will collide, no matter what your ball's initial speed, provided it's greater than some minimum value.

a) Find an expression for that minimum speed.

Express your answer in terms of the variable hand appropriate constants.

In a chase scene, a movie stuntman runs horizontally off the flat roof of one building and lands on another roof 2.1m lower.

a) If the gap between the buildings is 4.9m wide, how fast must he run to cross the gap?