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Manic mould

When I was a child I grew a mould garden from mashed potato, cheese and vegetable soup, all of it placed in a tightly sealed jar. I was disappointed when it grew only a boring monoculture of short, grey fur.

Now that I’m grown up, I’d like to repeat the experiment, but can anyone tell me what I should put in the jar to ensure a more interesting variety of colours and growth?

•The most exciting fungal culture I ever had was grown on fresh cow dung under a bell jar. I don’t recall all the different types of fungi that appeared, but I do remember that there is a succession of different species as a consequence of the changing levels of sugars, water and complex polysaccharides over the changing course of the garden’s life.

One of the fungi that I do recall was Pilobolus, which produces a fluid-filled sporangium on a stalk with the spore perched on the very top. The whole thing is highly sensitive to light and, when it bursts, it fires the mature spores off in the direction of the light. They remain stuck to the side of the bell jar. I also recall that some of the fungi were orange and that one of them produced tall, delicate, mushroom-like bodies.

Putting the words cow dung and fungi into ain Internet search produces loads of stuff; take a look.

Ronald Monroe

Isle of Man

•In my capacity as a bad housekeeper I have found that the best media for growing interesting moulds are cooked rice (basmati works well) and Greek yogurt. Both of these give a very interesting range of colours, my personal favourite being a bright and violent pink.

These moulds appear after some weeks sitting in a domestic fridge, but I presume that at room temperature they are quickly outcompeted by the boring grey stuff your correspondent mentions and that I frequently find in my bread bin.

Clare Byrne

St Albans, Hertfordshire

•I suggest that your correspondent moves into student accommodation for a couple of weeks. Students certainly have no problem cultivating an amazingly diverse and spectacularly colourful collection of fungi in their communal fridges. Tomatoes, leftover pizza and coagulated yogurt seem especially effective.

Mick Wilson

Nairobi, Kenya

Ship shifting

Suppose a large ship, such as the QE2, is floating freely alongside a quay and no forces such as wind or sea currents are acting on it. If I stand on the quay and push the side of the ship, will it move, even very slowly and slightly? Or is there some sort of limiting friction caused by all those water molecules around the hull that can only be overcome by a much larger threshold force?

•While I was a conscript in the service of King George V, on several occasions I moved a destroyer under the circumstances described by your correspondent.

At slack tide in Harwich harbour in Essex, and with a slack breeze, I leaned my belly against a stanchion on one ship, stretched with both hands across the narrow gap to a similar stanchion on the ship that was lying alongside and pulled hard.

For perhaps half a minute there seemed to be no result, but slowly the gap between them began to diminish until the two ships came quietly, and without fuss or noise, into contact. And, left alone, they remained in contact. Then, by reversing the process over a similar timescale, and substituting a push for a pull, the two ships returned to their starting positions. The process was remarkably simple.

The QE2 is just a trifle larger than a Royal Navy destroyer but I believe that the only difference would be in the timescale required to move the ship. Should your correspondent find an, admittedly unlikely, opportunity to try this experiment with such a large vessel I would advise that he takes care not to hold his breath while pulling.

Ken Green

Tintagel, Cornwall

•There is no threshold force that needs to be overcome to move a ship in the absence of wind or current. In fact it is remarkably easy for an unassisted person to move a large ship. This can be explained in terms of kinetic energy (E) and momentum.

Consider a ship with a mass of 20,000 tonnes (m = 2 × 10⁷ kilograms). If the ship is given a velocity of 1 centimetre per second (v = 10^−2 metres per second) then its energy, E = 1/2 mv² = 1/2 × 2 × 10⁷ × (10^−2)² = 1000 joules. A thousand joules is a very modest amount of energy. It is the energy expended by a 51-kilogram man climbing up a 2-metre-high flight of stairs.

At 1 centimetre per second the ship’s momentum (mass × velocity) = 2 × 10⁷ × 10^−2 = 2 × 10⁵ newton-seconds. The 51-kilogram man can impart this to the ship by applying his full weight for 400 seconds: 51 × g × 400 = 2 × 10⁵ newton-seconds, where g is the acceleration due to gravity, 9.8 ms^−2. If he moves the ship by standing with full weight on one of the mooring lines, he will have descended by 2 metres by the time the ship is moving at 1 centimetre per second.

Actually when a ship is set in motion, a comparable mass of water is also set in motion at a comparable speed. Consequently the kinetic energy and momentum calculated above have been underestimated by a factor of two or so. However, the main conclusion stands: an unaided person can easily move a ship.

John Ponsonby

Wilmslow, Cheshire

•The ship will move. Fluid forces don’t have a limiting static friction. We can think of these frictional fluid forces as being directly proportional to the speed of the ship. They are close to zero when the speed is close to zero, and so on.

So push, and good luck!

Marco Venturini Autieri
Pisa, Italy

This week’s question

Extra strong mints

If you drink a glass of water immediately after eating a strong mint, you will notice a cold, almost burning sensation in your mouth.

Why does this happen?

Emma Down

Hemel Hempstead, Hertfordshire