Well its been a while since i've done any physics problems and im having a problem with a real simple problem. The problem is:(adsbygoogle = window.adsbygoogle || []).push({});

You know that the height (in m) of a projectile is given by the formula

h = - g * t^2 / 2 + vi * t

where t is the time from launch, g is the acceleration due to gravity (9.8 m/sec2) and vi is the initial vertical velocity component (in m/sec).

Calculate the time at which a projectile will reach a certain height.

vi = 20

desired height = 10

So far i have this done, but i always get a negative number and the answers should be 0.58 sec and 3.50 sec.

10 = -9.8t^2/2 + 20t

10(2 + 20t) = -9.8t^2

20 + 200t = -9.8t^2

9.8t^2 + 200t + 20 = 0

(-200 + sqrt(200^2 - 4(9.8*20)) / 2(9.8)

-200 + sqrt(40000 - 784)) / 19.6

(-200 + 198.0303)/19.6

Im sure its just a stupid math mistake, but i cant find it. Also where we even given the correct equation?

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# Homework Help: Simple Projectile Question

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