Question 1087912: Hello, I am having trouble answering two questions on projectiles using formulas: d=16t^2 where d=distance in feet that an object falls, in t seconds, regardless of weight. And, if a projectile is launched upward from the ground with an initial velocity of 73.5 m per second, neglecting air resistance, it's height, in meters, above the ground t seconds after projection is given by the formula: d= -4.9t^2+73.5t.
*After how many seconds will the projectile be 100 m above the ground? I took 100 and divided by 73.5 and got 1.36 seconds?
*How long will it take for the projectile to return to the ground? I converted the 100 m to 328.084 ft and used d=16t^2 to get 4.53 seconds? Could someone please tell me if these are correct? If not, could you please help me figure this out? Any help would be most appreciated! Thanks!
Found 3 solutions by solver91311, ikleyn, josmiceli:
Answer by solver91311(24713)   (Show Source):
Answer by ikleyn(52750)  (Show Source):
Answer by josmiceli(19441)  (Show Source):
You can put this solution on YOUR website! For the question:
" After how many seconds will the projectile be 100 m above the ground? "
the units are meters and seconds, so all you need is the formula
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The formula is a parabola with a peak, so unless the projectile is at its
peak at 100 m above ground, there will be 2 solutions, one when the projectile is
on the way up, and another on the way down.
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 m
Plug this value into the formula
Use the quadratic formula
 sec
and, the other solution is:
 sec
These are the 2 solutions
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" How long will it take for the projectile to return to the ground? "
At ground level, 
 This is when the projectile is launched
In 15 sec, the projectile hits the ground.
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I can use this answer to check my 1st answer.
The peak will be at 1/2 this time, or  sec
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My solutions for the height of  m must be
equally spaced on either side of sec, so I can say:
Right on the money!
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