Tuesday 10 June 1997
Q Does a bullet still accelerate once it has left the barrel of a gun?
No - it begins to decelerate as soon as it leaves the barrel, or even before if the barrel is long, because there is no new impulse applied. Also, a bullet fired from a gun held one metre off the ground will hit the ground at the same time as a pebble dropped from someone's hand one metre above the ground - about half a second later.
Q Why does newspaper go yellow faster than other paper?
The main components of wood are cellulose and lignin. Cellulose is long- fibred and strong - so paper remains supple over a long time. Lignin is a polymer that makes wood hard, and is acidic. When making high-quality paper, the pulp is cooked, which removes the lignin. However, the newspaper publishers' main concern is to get the news to the reader as quickly and as cheaply as possible. Thus they use the cheapest papers and the cheapest inks. As newspapers have a very short shelf life, there isn't much point in making them out of posh paper. Newspaper is made from wood that hasn't been cooked, so most of the lignin remains. This is what turns yellow on exposure to sunlight.
Q Why doesn't dew form on a car parked in a car port, although it is open on all sides?
This is because of heat loss. The car port is insulated by the roof both from excessive heat build-up during the day, and from sudden heat loss at night. So during the day, moisture-heavy air doesn't collect under the port because the air there is cooler than that around it (and so can't absorb as much moisture as warm air). At night, when the earth starts to lose heat, the car port stays at relatively the same temperature as before; so there isn't such a sharp drop in temperature, nor so much moisture in the air there. The result: dew doesn't form.
Q How much rubbish do people recycle in the UK?
Not enough. Presently we only recycle about 5 per cent of our rubbish; the rest is disposed of in landfill sites or incinerated. We are trying to recycle more, with government targets to recycle 20 per cent of domestic waste by the year 2000, and packaging regulations to be introduced this year requiring retailers to recycle a percentage of the packaging they sell. The problem with landfills is that even the normally biodegradable stuff like paper, which takes up more than half of landfills, doesn't really break down because of the lack of oxygen: even 20-year-old sausages have been found intact. We are also running out of sites for potential landfills. Rubbish often has to be taken by truck to locations miles away from where it was generated.
Q Why does jelly set when you put tinned pineapples in it, but does not set when you put fresh pineapple in it?
Raw pineapples, like figs and papayas, contain an enzyme (known as a protease) that breaks proteins down into small fragments. If raw pineapple is put in gelatin for a dessert or fruit salad, this enzyme digests the gelatin molecules and liquefies the water-protein gel. Canned pineapple has been heated, which deactivates the enzyme, and so it co-operates quite well with gelatin.
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