A picture of the problem is given in Fig. where we’ve drawn the coordinate axis. The
penny begins its motion at y = 0 and since it falls down, its coordinate upon striking the ground is −427 m. Since we drop the penny its initial velocity is v0 = 0 and its acceleration during the fall is a = −g = −9.8 m/s2.
We are looking for the final velocity v but we don’t have the time of the fall.
Taking the square root of this number gives 91.5 m/ but there are really two answers for v, namely ±91.5 m/, and since the penny is falling downward when it hit the ground we want the negative one:
v = −91.5 m/*
But the answer to the question is that the penny’ speed (the absolute value of v) was 91.5 m/ when it hit the ground.
