Example 2: A stone is thrown upward at 25 m/s from street level near a three story building. Find:

a) the speed at which it passes a person on a balcony 6 m up?

b) the time it takes to reach the person on the balcony?

Solution:

a) Taking the origin at the street level (s_o = 0) , and up as positive,

Unlike Example 2, both of these signs are meaningful. The positive sign applies to the stone moving up, whereas the negative sign refers to its direction on the way back down!

b) Method 1: This is the easiest way...

Again, the two answers refer to the two possibilities - the stone is on the way up or it is returning.

Method 2: While it is unnecessary to use this equation for this problem, sometimes it is the only way...

Both answers from the quadratic formula are physically meaningful. This is not always the case and you must sometimes decide whether to discard one of the answers (and which one).