# projectile

• Mar 9th 2009, 02:33 PM
william
projectile
a projectile is shot upwards with an initial velocity of $30m/s$ , its height at time t is given by $h=30t-4.9t^2$, during what period of time is the projectile more then $40m$ above the ground? express your answer rounded to the nearest hundredths

my thinking: $40=30t-4.9t^2$ but can't go anywhere from there
• Mar 9th 2009, 02:42 PM
JeWiSh
Quote:

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

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

So $4.9t^2-30t+40=0$
Use the quadratic formula: $t=\frac{-b\pm\sqrt{b^2-4ac}}{2a}$
$a=4.9$ $b=-30$ $c=40$
• Mar 9th 2009, 03:20 PM
william
Quote:

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

so it was above 40m between 1.9 and 4.1 seconds?
• Mar 9th 2009, 03:23 PM
JeWiSh
Quote:

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

Correct (Nod)