A ball's thrown upwards from the roof of a building with a speed of 15 m/s. Velocity of the ball when it's 5 m above the roof?
We can use:
Applying the quadratic formula:
This gives 2 values for
These 2 times refer to the object moving up and then returning back down.
Using the convention "up is +ve" we can use:
This is the velocity on the way up.
This is the velocity on the way down.
What goes up, must come down.