Question 1188509: A ball is thrown vertically upward from the top of a building 64 feet tall with an initial velocity of 48 feet per second. The distances (in feet) of the ball from the ground
after t seconds is s(t) = 64 + 48t - 1612.
(a) After how many seconds does the ball strike the ground?
(b) After how many seconds will the ball pass the top of the building on its way down?
Found 2 solutions by ikleyn, Boreal:
Answer by ikleyn(52809)   (Show Source):
You can put this solution on YOUR website! .
A ball is thrown vertically upward from the top of a building 64 feet tall with an initial velocity of 48 feet per second.
The distances (in feet) of the ball from the ground
after t seconds is s(t) = 64 + 48t -  16t^2.
(a) After how many seconds does the ball strike the ground?
(b) After how many seconds will the ball pass the top of the building on its way down?
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Notice how I edited your post to make the formula correct.
(a) To answer question (a), solve this equation
-16t^2 + 48t + 64 = 0 (ground level)
First, simplify it by dividing both sides by -16
t^2 - 3t - 4 = 0.
To solve it, factor left side
(t-4)*(t+1) = 0.
Of two roots, t= 4 and t= -1, use only positive one t= 4 seconds. ANSWER
(b) To answer question (b), solve this equation
-16t^2 + 48t + 64 = 64 (top of the building level)
Simplify it
-16t^2 + 48t = 0
-16t(t - 3) = 0.
Of two roots, t= 0 and t= 3, only greater value t= 3 seconds is the solution to this part of the problem.
Solved and thoroughly explained.
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- Introductory lesson on a projectile thrown-shot-launched vertically up
- Problem on a projectile moving vertically up and down
- Problem on an arrow shot vertically upward
- Problem on a ball thrown vertically up from the top of a tower
- Problem on a toy rocket launched vertically up from a tall platform
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Answer by Boreal(15235)  (Show Source):
You can put this solution on YOUR website! The formula is s(t)=-16t^2+48t+64
the ground is 0.
so -16t^2+48t+64=0=t^2-3t-4, moving everything over and dividing by 16
(t-4)(t+1)=0
t=4 seconds.
-
want to know when s=64
so -16t^2+48t+64=64
or 16t^2=48t
t=3 seconds
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