Problem 317
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Problem 317
For this question, can we assume that the firecracker exploded on earth? If so, since earth isn't perfectly spherical, it matters where on earth the firecracker exploded. So, what should we assume?
scio me nescire
Re: Problem 317
All assumptions are provided in the problem.A firecracker explodes at a height of 100 m above level ground... in a uniform gravitational field with g=9.81 m/s^(2).
Re: Problem 317
Ah, thanks TripleM. Seems that I didn't read the problem statement intently enough.
scio me nescire
Re: Problem 317
Hello, I believe I have solved the problem, but I'm getting a wrong answer.
I believe it's an error from rounding, because I figured out a formula for the solution volume(as a solid) and tested it by overlaying graphics of my formula and the "bruteforced" parametrized equation of the solid composed of all the firework particles. They seem to match pixel for pixel(i.e. my solution seems to at least be close to the answer).
Can someone who has solved 317 please PM me and confirm if I have a rounding error? I'll send my graphs, equations & answer
I believe it's an error from rounding, because I figured out a formula for the solution volume(as a solid) and tested it by overlaying graphics of my formula and the "bruteforced" parametrized equation of the solid composed of all the firework particles. They seem to match pixel for pixel(i.e. my solution seems to at least be close to the answer).
Can someone who has solved 317 please PM me and confirm if I have a rounding error? I'll send my graphs, equations & answer
Re: Problem 317
I am in a similar situation  I haven't modelled my result but I am very confident in it, except for the precision. Would someone PM me so I can check my result?
Re: Problem 317
You can both PM me, but I wouldn't think a precision error would be that likely.
Re: Problem 317
Yes, I can agree that a precision error is unlikely, because the rounding isn't anything special(i.e. the answer isn't X.100099999)
My error was that I had actually forgotten a certain formula and my recollection from memory lacked a certain term. I.e. my answer was (about) half of the real answer.
So, yeah, doublecheck your formulas and when in doubt  use Wikipedia, even for the really really simple things
My error was that I had actually forgotten a certain formula and my recollection from memory lacked a certain term. I.e. my answer was (about) half of the real answer.
So, yeah, doublecheck your formulas and when in doubt  use Wikipedia, even for the really really simple things
Re: Problem 317
So is it the volume the fragments move through before some fragment reaches the ground or the volume the fragments move through before all fragments reaches the ground which should be computed?
Re: Problem 317
Any point that any fragment passes through should be included in the volume.
Re: Problem 317
Used 10 instaed of 9.81 in my maths, that's probably what's going wrongdouble check

 Posts: 18
 Joined: Thu Jan 20, 2011 2:06 am
Re: Problem 317
Hello, I did this problem with pencil/paper and figured the final anwer with my TI83. I think there's a chance I lost accuracy by using the calculator. I think my method is correct, but I made one assumption in my calculus. That could also be the problem. Could I pm someone to see if my answer is close?
Edit: Nevermind, I never covered how to find the volume of a surface of revolution in high school. Anyway, I figured it out on my own. Very fun problem.
Edit Again: See my forum post for a generic formula. Also, could someone tell me how to get symbols like pi and theta in my posts?
Edit: Nevermind, I never covered how to find the volume of a surface of revolution in high school. Anyway, I figured it out on my own. Very fun problem.
Edit Again: See my forum post for a generic formula. Also, could someone tell me how to get symbols like pi and theta in my posts?

 Posts: 3
 Joined: Mon Jun 27, 2011 2:29 pm
 Location: India
Re: Problem 317
I am stuck in the math of this problem
I have solved it using normal math(volume of solid of revolution) and physics(equations of motion) but I am getting wrong answer
please can anyone see my solution and tell me where I have gone wrong
I am getting answer as
I have solved it using normal math(volume of solid of revolution) and physics(equations of motion) but I am getting wrong answer
please can anyone see my solution and tell me where I have gone wrong
I am getting answer as
Expand
Re: Problem 317
I got the fomula
and result
but I was wrong.
Where's my problem? I can send you my pdf file which I wrote how above formula is made.
Expand
Expand
Where's my problem? I can send you my pdf file which I wrote how above formula is made.

 Posts: 15
 Joined: Wed Aug 11, 2010 6:07 pm
 Location: Maryland, USA
Re: Problem 317
Can I get some clarification on a few things?
 Is air resistance something I need to take into account in this problem?
 And a slightly silly question: Does "It breaks into a large number of very small fragments, which move in every direction" (from the problem statement) mean that some of the particles will travel up? I would assume no, but just want to make sure...
 Marcus_Andrews
 Administrator
 Posts: 1531
 Joined: Wed Nov 09, 2011 5:23 pm
Re: Problem 317
matthewden:
1. From the problem description:
1. From the problem description:
2. The fragments move in every direction, including straight up.We assume that the fragments move without air resistance...

 Posts: 15
 Joined: Wed Aug 11, 2010 6:07 pm
 Location: Maryland, USA
Re: Problem 317
Oh, yeah. It does say that.... OK. Thank you.
Re: Problem 317
Ugh... So far, this problem has been a lot of ugly pencil and paper math. I hope I get this right the first time, because I am not going to want to comb back through it looking for a mistake.
I think I'm about one step away from solving this, but I see one interesting result which I have yet to determine whether or not it makes sense.
Update: crap... I'm hoping this is just going to be a simple mistake. I get the feeling my value isn't off by much.
I think I'm about one step away from solving this, but I see one interesting result which I have yet to determine whether or not it makes sense.
Update: crap... I'm hoping this is just going to be a simple mistake. I get the feeling my value isn't off by much.
Re: Problem 317
And it looks like everything is checking out. Not sure what to do at this point. As far as I can tell, I should have a correct answer. I'm stumped.
Update: Never mind. Good old unit checks on physics problems. Problem completed.
Update: Never mind. Good old unit checks on physics problems. Problem completed.
Re: Problem 317
If two particles pass through the same point in space, should that point be counted twice in the total volume calculation?
 Marcus_Andrews
 Administrator
 Posts: 1531
 Joined: Wed Nov 09, 2011 5:23 pm
Re: Problem 317
No, because the question is asking for the volume of the region through which the fragments move.