SOLUTION: An object is thrown downward from a plane which is 380 m above the ground. The object is thrown with an initial velocity of 20 m/sec. How long does it take to reach the ground?

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Question 386394: An object is thrown downward from a plane which is 380 m above the ground. The object is thrown with an initial velocity of 20 m/sec. How long does it take to reach the ground?
Found 2 solutions by richard1234, mananth:
Answer by richard1234(7193) About Me  (Show Source):
You can put this solution on YOUR website!
We must first choose a frame of reference. Here, I let the acceleration due to gravity be negative, and the ground is at 0 meters.
Now we have 0+=+%281%2F2%29%28-9.8t%5E2%29+-+20t+%2B+380 (Note that all units must be consistent, so I ignored them for now)

0+=+-4.9t%5E2+-+20t+%2B+380

Now, just solve using the quadratic formula:

t+=+%2820+%2B-+sqrt%287848%29%29%2F%28-9.8%29 = 6.99887...seconds (quite close to 7 s)

Answer by mananth(16946) About Me  (Show Source):
You can put this solution on YOUR website!
s= ut*1/2*g*t^2
s= distance
u= initial velocit
t= time
380=20t+1/2*9.8*t^2
380=20t+4.9t^2
4.9t^2+20t-380=0
multiply by 10
49t^2+200t-3800=0
find the roots by quadratic formula
b^2-4ac=784800
...
x1=%28-200%2Bsqrt%28784800%29%29%2F98
x1=6.998 seconds
7 seconds
..
x2 will be negative & time cannot be negative
...
m.ananth@hotmail.ca