Tuesday, May 26, 2009

What force is necessary to lift a very thin wire ring 55 mm in diameter from a water surface at a temperature of 20 Centigrades? Neglect the weight of the wire.

OK, that was easy. Now, can you do this?

Anonymous said...

I put this problem to Koko, but she is going on holiday. She said she would work on it later, and would have a better chance of making it through the airport security without any "solutions".

However, we will propose that if weight of the ring is to be neglected, then there must be another downward force that we have to oppose in order to lift the ring; this must be something to do with surface tension or at least the force of electrostatic attraction between the metal and the water. If so, then this will be in proportion to the length of wire in contact with the surface, which is presumably the circumference: pi x 55mm. The other thing we can infer, if we are correct so far, is that surface tension/electrostatic attraction should be a function of temperature, else why give us the precise value. We need a formula; or is it possible to work it out from first principles?

At this point, I will have to wait for Koko's return.

Anon.

latté island said...

Problem A: get a man to do it.

Problem B: I'd coat the underside and sides of the coin with wax.

J said...

A romantic like Koko will know how to solve this, because it is about LOVE. It is about the fatal attraction between water and ring. It is about blissful intimate union and about tragic separation by force. LOVE is the solution to everything.

J said...

Latte,

I shall try it. I am for cheating if it works.

J said...

σ = 0.0728 N/m

L = 0.1413 m

F = L X σ = 0.0103 N

One of the students questioned the solution: You may suppose that the ring has no weight, but isnt it a bit too much to suppose that it is also uni-dimensional?

Another one: Does the solution mean that if the weight of the ring is, say, 1.0 gram, it would sink?

Next: Is the ring made of gold?

J said...

One for the research "How the Jewish mind works":

Doctor j, do you know anybody whose finger is 45 mm wide? Which finger is it??