The function s(t)=t^2-3t-4 describes how the speed of a ball (S, measured in feet per second) changes with respect to time (t, measured in seconds) as it rolls down a hill. How many seconds does it take for the ball to stop?

Question

The function s(t)=t^2-3t-4 describes how the speed of a ball (S, measured in feet per second) changes with respect to time (t, measured in seconds) as it rolls down a hill.  How many seconds does it take for the ball to stop?

jim766 · Answer

you need to factor it, do you know how?

campbell_st · Answer

well you need to let s(t) = 0.... no speed

and then solve for t

so 

$$t^2 -3t -4 = 0$$

only consider the positive solution for t... as time can't be negative. 

and the quadratic can be factored.

anonymous · Answer

-4 and 1

campbell_st · Answer

not quite 

$$t^2 -3t -4 = 0... factors ..\to... (t -4)(t + 1)=0$$

so you need to now solve 

t - 4 = 0

and 

t + 1 = 0

anonymous · Answer

That's was I said. I just gave you the factors

anonymous · Answer

It took the ball 4 seconds to stop then.

campbell_st · Answer

that seems to be the answer if you substitute it into your initial equation you should find s(t)  = 0

anonymous · Answer

Yes. Thanks a bunch!