Example 3: When a football is kicked with a vertical speed of 20m/s, its height, h
metres, after t
seconds is given by the formula:
How long after the kick is the football at a height of 15m?
Substitute 15 for h
in the formula:
This produces a quadratic equation. The solution of the equation gives the times when the height is 15m.
Collect all the terms on one side of the equation.
The football is at a height of 15m twice: first on the way up, 1 s after the kick, then on the way down, 3 s after the kick.