Say you have two people holding either end of a very long (several light seconds), taut string. The first person gives the string a pull (supposing he is able to pull a string of that mass), and the second person holds the other end (whilst watching the first person through a powerful telescope). Does the second person feel the pull before, at the same time or after he sees the first person pulling?

Nothing is supposed to travel faster than light, so what happens to the string to allow the force to be transmitted slower than it?

I have been told that the wind has more force on a yacht's sail in conditions of high humidity, such as in the tropics, because the higher water content of the air increases its mass, and therefore the force, on the sail at any given wind speed.

Is this true? If so, is there an equation I can use to calculate the increased force?

Mike Stovold, London, UK

(Image: johnnyberg, stock.xchng)