# projectile

• Mar 9th 2009, 01:33 PM
william
projectile
a projectile is shot upwards with an initial velocity of $\displaystyle 30m/s$ , its height at time t is given by $\displaystyle h=30t-4.9t^2$, during what period of time is the projectile more then $\displaystyle 40m$ above the ground? express your answer rounded to the nearest hundredths

my thinking: $\displaystyle 40=30t-4.9t^2$ but can't go anywhere from there
• Mar 9th 2009, 01:42 PM
JeWiSh
Quote:

Originally Posted by william
a projectile is shot upwards with an initial velocity of $\displaystyle 30m/s$ , its height at time t is given by $\displaystyle h=30t-4.9t^2$, during what period of time is the projectile more then $\displaystyle 40m$ above the ground? express your answer rounded to the nearest hundredths

my thinking: $\displaystyle 40=30t-4.9t^2$ but can't go anywhere from there

So $\displaystyle 4.9t^2-30t+40=0$
Use the quadratic formula:$\displaystyle t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$
$\displaystyle a=4.9$ $\displaystyle b=-30$ $\displaystyle c=40$
• Mar 9th 2009, 02:20 PM
william
Quote:

Originally Posted by JeWiSh
So $\displaystyle 4.9t^2-30t+40=0$
Use the quadratic formula:$\displaystyle t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$
$\displaystyle a=4.9$ $\displaystyle b=-30$ $\displaystyle c=40$

so it was above 40m between 1.9 and 4.1 seconds?
• Mar 9th 2009, 02:23 PM
JeWiSh
Quote:

Originally Posted by william
so it was above 40m between 1.9 and 4.1 seconds?

Correct (Nod)