Question

A ball is thrown directly downward, with an initial speed of 8.45 m/s, from a height of 29.4 m. After what time interval does the ball strike the ground?

Answer 1

The relevant equation is:\[y=y _{O}+v _{O}t+\frac{ 1 }{ 2 }at ^{2}\]where y is the final distance above the ground; yO is the starting distance above the ground; vO is the starting velocity; t is time; and a is the acceleration. Note that the distance above the ground is generally considered positive; that a velocity directed upward is positive; and that an acceleration directed upward is positive.

Answer 2

Now consider this problem. The velocity is directed downward, so it must be negative. Like the acceleration--in this case the acceleration of gravity--is also directed downward and must also be negative.

Answer 3

Do you see how to solve the problem?