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2009 BP03 Discussion

This is a thread for anyone taking BP0 3 this year. Talk about our practice, successes, failures(the majority, I presume) and generally have a good cry.

Also, anyone who has previously been through this round there advice and experiences would surely be appreciated.

Personally I'm happy to get through to the top 48 and am quite frankly screwed for getting any further. I got through BPO 2 by picking the mechanics questions out and that's impossible for this one. Anyone really want to get through to round 4?
Reply 1
Avatar for Shrub
Shrub
I'll start attempting questions soon (once the chem olympiad is out the way) but skimming through them I can't even begin to start most :s-smilie:
Would be nice to get through to round 4 but realistically for me it isn't going to happen - thats reserved for the genii!
Reply 2
Avatar for JoMo1
JoMo1
OP
12
Yeah, I thought i'd just get most of them wrong, turns out I can't bloody well start any of them, I just sit there drawing lots of diagrams and eventually give up and go do something else.
Reply 3
Avatar for Shrub
Shrub
Just to get the ball rolling, can anyone give me any hints for question 2.a) from this paper:
http://www.physics.ox.ac.uk/olympiad/Downloads/PastPapers/BPhO_Paper3_2003_QP.pdf

looks like a really nice question but I can't even begin to think how to start it :frown:
Reply 4
Avatar for Hellier
Hellier
8
Draw a circle
Pick any point inside this circle
Draw 2 lines through this point
You will notice that GMm/r*r gives the same pull in both directions
now that works with any 2 lines
so within a ring a mass feels no force
and what is a uniform spherical shell apart from a revolved ring?
Reply 5
Avatar for JoMo1
JoMo1
OP
12
Hellier
Draw a circle
Pick any point inside this circle
Draw 2 lines through this point
You will notice that GMm/r*r gives the same pull in both directions
now that works with any 2 lines
so within a ring a mass feels no force
and what is a uniform spherical shell apart from a revolved ring?


Either I'm horribly misinterpreting you're argument, or that's horribly wrong.

Considering lines through a point is basically worthless as the underlying argument is going to rely on the fact that there are lots of far away points on one side of the ring, but a few close points on the other. In considering the pull in both directions the only variable will be r, so unless r is constant, it will most certainly be different.
Reply 6
Avatar for Hellier
Hellier
8
look, get the 2 2 minor arcs formed by those lines. the smaller one has to be closer, and you can show that the distance of the centre of mass of that arc, d, from the point you chose , has d*d*(arc length)=k type proportionality. Therefore the factors exactly cancel. IM me if you want me to try and explain it more in depthly...
Reply 7
Avatar for Shrub
Shrub
Hm I see your argument but you could just post a bit of working to go through it, I'd like to see how you actually do it! Thanks

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