Letters in STS issue 69, May 2011- committee structures and limp lettuces!

Committees or no committees? (this topic is further discussed here)

Dear Chris

At a recent committee meeting a fellow member said she thought that there were legal reasons for our dance group being a 'group' rather than a 'club' with members paying a fee to belong. There are people who consider themselves 'members' of our group when they only attend about 3 times out of 20 meetings a year. Another local club we attend charges an annual membership fee, provides membership cards and reduced prices for Saturday night hops. Only people with paid-up membership can vote at their AGM but at ours anyone who thinks they're a member can. How much does it matter anyway? What do STS readers or any legal advisers out there think? Advice gratefully received.

Maureen Knight

Physics, swinging and limp lettuces.

Dear Chris

I have been revising my nuclear physics: people need to learn that we must live within an energy budget or face unpleasant consequences. Either we boil the Earth by burning remaining reserves of coal, oil and gas, build more nuclear stations or abandon energy guzzling lifestyles. I have worked on renewable energy - but it can realistically only supply a fraction of what we now consume - especially for transport.

However, this is easier than explaining what is wrong with John Sweeney's ideas of what happens in a circle or swing (STS 68).

First, a body travelling in a circle is accelerating towards the centre at all times. This is despite that it may be travelling at a constant speed: its direction (and therefore its velocity) is changing. Therefore it is accelerating. The force needed for this acceleration is an inward centripetal force. If the force is suddenly removed, the body will travel not backwards out of the circle but along a tangent. The trebuchet, an ancient weapon of war, used this principle. After release from the device on a tangential trajectory, its projectile (sometimes a diseased corpse) would continue in a straight line in the absence of any imposed force. In practice, it curved downwards back to Earth, with air viscosity and gravity providing forces. Cruise missiles have a greater range but are more expensive.

The mistake that John makes is in saying 'Centrifugal force is pulling you away from the centre of the circle'. This reactive force only exists for the time that any inward centripetal force operates. Once the string is cut or the swinging dancers part company, centripetal force vanishes and with it the reactive centrifugal force. Any tendency for dancers to fly outwards is psychological - or maybe physiological as a consequence of muscular tension! Some dancers may believe that the centrifugal reaction force still exists and move accordingly. Of course, we have so called centrifuges - used for separating cream from milk and various isotopes of uranium - but these utilise centripetal acceleration and the physics is quite complicated. If you find this easy - try understanding the Coriolis effect.

However, I couldn't agree more that dancers should learn to 'connect' and not be limp lettuces. In strip the willow ceilidh dances such as Drops of Brandy there are lots of fast half circles with bodies briefly yet strongly connected and then flying off at tangents. A mistake some fast dancers make is to hang on to the elbow turns for too long. Just let go and let physics do the rest!

Steve Wozniak