A ball is kicked into the air at time t=0. While the ball is in the air, its height above the ground after t seconds is -16t^2+40t feet. How many seconds after the ball is kicked will it hit the ground?

Question

kropot72 · Answer

Factorise the expression as follows:
-16t^2 + 40t = 8t(-2t + 5) = 0
Can you see for what values of t the equation is true?

anonymous · Answer

Thank you for responding. What exactly do you mean by that?

kropot72 · Answer

What values of t will make 8t(-2t + 5) equal to zero. One value of t that solves this equation is obviously t = 0. Can you work out what the other value is?

kropot72 · Answer

The other value for t is found by solving the following:
-2t + 5 = 0
Can you do this?

anonymous · Answer

Yes, I got it. Thank you very much!

anonymous · Answer

t=5/2

kropot72 · Answer

-2t + 5 = 0
Subtract 5 from each side:
-2t + 5 - 5 = 0 - 5
-2t = -5
t = -5/-2 = 2.5 seconds
The ball will hit the ground 2.5 seconds after it is kicked into the air.

kropot72 · Answer

Well done katlvb21 :)