Answer:
The ball will return to the ground after 1.25 seconds
Explanation:
Given
f(t) = -16t² + 20t
Required
After what time (in seconds) does the ball return to the ground?
Function f(t) is a function of time and it represents the displacement of the ball.
For the ball to return to the ground, it means the displacement is 0.
So, we have f(t) = 0.
Substitute 0 for f(t) in the above, we have
0 = -16t² + 20t ---- Factorize
0 = -4t(4t - 5) ---- Reorder
-4t(4t - 5) = 0
This above expression can be splitted to;
-4t = 0 or 4t - 5 = 0
For -4t = 0,
divide through by -4
This will give t = 0
For 4t - 5 = 0,
Collect like terms
4t = 5 + 0
4t = 5
Divide through by 4
t = 5/4
t = 1.25
So, we have that
t = 0 or t = 1.25
But t can't be 0 because the ball has already traveled some distance.
So, t = 1.25
Hence, the ball will return to the ground after 1.25 seconds