A projectile is thrown upward so that its distance above the ground after t seconds is h = -16t2 + 672t. After how many seconds does it reach its maximum height?

21 s

31.5 s

42 s

10 s

Question

A projectile is thrown upward so that its distance above the ground after t seconds is h = -16t2 + 672t. After how many seconds does it reach its maximum height?

 21 s

 31.5 s

 42 s

 10 s

superdavesuper · Answer

OUCH Calculus :P

OK so the height is at max when dh/dt = 0.  Do u know how to differentiate h by t?

anonymous · Answer

Ouch indeed. 
I'm a bit clueless on this one. Should I be using a formula such as dv=4.9t^2
or dh=Vht?

superdavesuper · Answer

h = -16t2 + 672t
so what is dh/dt =?

com'on i know u can do it :)

anonymous · Answer

I came up with 42 seconds.
Mind telling me if that's correct?

superdavesuper · Answer

Sorry...try the differentiation again plz? u are very very close @hales :)

anonymous · Answer

I'm completely perplexed. 
My best guess would be 21 seconds.

superdavesuper · Answer

BINGO :)

Hahaa u got me concerned n thought u giving up lol

dh/dt = -32t + 672 = 0

but u got it urself - good job!!

anonymous · Answer

Thanks so much! I appreciate the help and encouragement. (:

agent0smith · Answer

You don't need calculus for this, you might've learned how to do it in algebra 2... remember x= -b/(2a)? (it comes from calculus though)