Am I the only one who is beginning to think that the amount of work necessary to make this "work" may suggest that the physical principle it is designed to demonstrate is not that robust or important?
Lesley
________________________________________
From: psci-com: on public engagement with science [[log in to unmask]] On Behalf Of Richard Wiseman [[log in to unmask]]
Sent: 12 January 2009 12:43
To: [log in to unmask]
Subject: Re: [PSCI-COM] Breaking broomsticks
Hi
Thanks for all the online and offline posts about this. I have now found a paper saying that the ideal thing to do is to needlework pins into the ends of the wood and then cut the heads off. Apparently this, with a metal bar, means that it will work. I will film the next round and see what happens.
Cheers
Richard
on 12/1/09 12:27, Richard Ellam at [log in to unmask] wrote:
HI Richard (and All)
I suspect that there are a number of factors working against you: certainly hitting the broomstick as hard and fast as possible with something stiffer than another piece of broom handle - a baseball bat or a length of scaffolding tube would be good. The hitter doesn't have to be much heavier than the broom stave but stiffer is good because it will deflect less when it makes contact with the stick you're trying to break and therefore transfer energy more effectively.
You should also look carefully at the supports for the broom-stave. I can't see how it is supported on the wineglasses, as the video resolution isn't good enough, but it looks like you've driven pins or screws onto the ends of the stick and are resting these on the wineglasses. This is probably not a terribly good idea as the very low contact area of the pin on the glass will produce a huge contact stress when the broom-stave is hit. The contact stress will be easily big enough to start cracks in the glass, which being brittle will then shatter. This effect will be magnified if:
1. There are any sharp edges on the pin - some nails have ridges on them or are just badly made. Screws are definitely not a good idea here if the threads are in contact with the glass
2. There are any notches or sharp changes of section (such as the moulding flash you get on cheap blow-moulded wineglasses) in the area where the pin is in contact with the glass. If necessary moulding flash an be removed from the glass with fine wet or dry paper, used wet. Support the abrasive with a file or similar to hold it up to the work. Or make sure any imperfections in the glasses are well away from the area where the stick rests.
The use of pins to support the ends of the broom-stave can be avoided if you cut flats on the ends of the broom-stave so it rests on these on the wineglasses - but you need to get the flats accurately lined up which is not easy. Or you could just use a piece of square section softwood instead of the broom -stave.
This might be a good idea anyway, as softwood is easier to break than hardwood, and softwood in the sections you need (say 32 mm square) is probably cheaper per foot than broom-stave anyway.
It might also be worth getting in some stick-breaking practise with your stick supported between two unbreakable supports before you put it on the wineglasses.
Hope this helps
When you succeed, do post another video, and tell us all about it
Cheers
Richard Ellam
L M Interactive
Science Shows and Hands-On Stuff
tel/fax 01761 412 797
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