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Q How fast would a coin dropped from the top of the Eiffel Tower be travelling by the time it reached the bottom? Could it hurt someone if it hit them?

A falling object doesn't accelerate indefinitely. It speeds up until it reaches its "terminal velocity", after which it will continue to fall at the same speed. A coin dropped from the Eiffel Tower has a terminal velocity of about 45 m/s (about 100 miles an hour) - which would certainly cause some damage to anyone it hit.

Q How much heat does the human body give off?

Curiously, men typically give off more heat than women - between 158 and 167 joules per square metre of skin in men and 150 to 158 joules per square metre in women. Adult men have about two square metres of skin, and so give out about 326 joules from their bodies each hour - a rather faint 0.09 watts.

Q When will the new millennium start? Why can't anyone agree on it?

The new millennium will not start until the year 2001. A man called Denys the Little can be blamed for the confusion. In the 6th century AD, he was making a table giving the dates of Easter when he decided to re- jig how the number of years was counted. Instead of counting years since the beginning of the reign of the Roman emperor Diocletian, he decided to start with the birth of Christ. However, because the number zero had yet to be invented, he called the first year of his era year one. When the use of the terms AD and BC came into use in the 11th century, the years went from 1BC straight to 1AD.

The effect on the start of the new millennium is that if the year one is the first year of the century, then the year two is the second, and so on up to the year 99, which is the 99th year of the century and the year 100 which is the 100th year. The start of the next century isn't until the year 101. It's the same with the millennium. The year 2000 is the 2000th year of the millennium while the year 2001 is the first year of the next. But that probably won't put a lot of people off celebrating on 31 December 1999.

Q When were fire alarms invented?

In 1852, D L Price filed the first patent for a fire alarm. It had a bi-metallic strip (two pieces of metal that expand at different rates when heated) that bent as the temperature rose until it made a connection in a circuit containing a bell. In the 1880s, a system was developed using a network of mercury thermometers around a house. As the temperature rose, the mercury rose until it made contact with two platinum electrodes which completed the circuit and set off the alarm. Smoke detectors weren't invented until the 20th century. In 1941, W Jaeger and E Meili from Switzerland developed a system very similar to that used today. They had a radioactive source whose radiation passed through an ionisation chamber creating ions and generating a current. As smoke entered the chamber, the number of radioactive particles decreased so the current decreased. This set off the alarm.

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