Question

The height of an object dropped from the top of a 20-meter building is given byh(t)= -5t^2 +20. How long will it take the object to hit the ground?

Answer 1

It will take the object 2 seconds to hit the ground

Here, we want to get the time it will take the object to hit the ground

To get this, we know that the height at this point is zero

Thus, we have it that h is 0 at this point

Mathematically, we have it that;

$\begin{gathered} 0=-5t^2\text{ + 20} \\ 5t^2\text{ = 20} \\ \text{divide through by 5} \\ \frac{5t^2}{5}\text{ = }\frac{20}{5} \\ \\ t^2\text{ = 4} \\ t\text{ = }\sqrt[]{4} \\ t\text{ = }\pm\text{ 2} \end{gathered}$

But, since time cannot be negative, we have it that t = 2 seconds