Please help with this problem:
A ball is thrown vertically upward from the top of a building 96 feet tall with an initial velocity of 80 feet per second. The distance is s (in feet) of the ball from the ground after t seconds is s=96+80t-16t^2. After how many seconds does the ball strike the ground? After how many seconds will the ball pass the top of the building on its way down?

Question

Please help with this problem:
A ball is thrown vertically upward from the top of a building 96 feet tall with an initial velocity of 80 feet per second. The distance is s (in feet) of the ball from the ground after t seconds is s=96+80t-16t^2. After how many seconds does the ball strike the ground? After how many seconds will the ball pass the top of the building on its way down?

deepolisnoob · Answer

Use the quadratic formula to solve this question. The equation is: $$\frac{ -b \pm \sqrt{b-(4ac)} }{ 2a }$$ You will receive 2 answers using this equation; use the one that fits into this problem's situation.

anonymous · Answer

There's a typo in the quadratic formula, the b inside the square root should be b squared.

But first try to imagine the motion of the ball - draw a sketch of the motion - and think about what values of s you need to consider, then you can think about solving the equation for t.

The numbers have been chosen in this question to be kind to you, you should be able to solve without using the formula above, if you know how to do simple factorisations.