Q When a person is floating (eg in a swimming bath) and tenses their muscles, the body begins to sink. Why? (asked by Irene Palmer)
The only factor that will affect buoyancy is the weight (or amount) of air in the person under water - inhalation makes you go up and exhalation makes you go down. This is standard practice in diving. Tensing your muscles without breathing will not affect your buoyancy per se, though it might perhaps change your centre of buoyancy (ie by moving your diaphragm), resulting in a small displacement. Or you might unconsciously tense one set of muscles slightly more than another - which would affect your orientation in the water, and so possibly your position. Or by tensing your muscles, you might actually move your arms and legs, albeit slightly and without your knowledge. This would change the relative positions of your centre of gravity and centre of buoyancy, causing a small change in position.
However, all the movements described above would be slight, and they would certainly not cause you to sink. Experts at the National Sports Medical Institute and several diving schools agree: it simply doesn't happen. So they wonder - are you sure you aren't breathing out when you tense your muscles?
Q What's actually inside a bacterium?
Bacteria are simple cells; they do not have a distinct nucleus and they do not have many of the organelles (such as mitochondria, chloroplasts, and endoplasmic reticulum) that a higher organism's cells have. The size and shape of bacteria varies according to their type, but they all have a cell wall, a cell membrane and a rather untidy mass of DNA in the middle of their cytoplasm. The cytoplasm itself is granular and not organised into compartments - enzymes and the protein-making ribosomes exist freely. Some bacteria have long hair-like structures on their outer surface called flagellae, which are important in movement: they can be rotated to act like a propeller, moving the bacterium about in the liquid in which it lives.
Q Are there sounds you can't hear?
Yes. A young child's ears can hear up to 25,000 hertz or more - the pitch (frequency) used by bats hunting for insects to eat. As we get older, we can only hear lower frequencies: by 13, most people can only hear sounds below 20,000 Hz. Adults, especially those who have abused their hearing by working in noisy places, lose even more of the top frequencies, and some retired people can only hear sounds below 5,000 Hz. Luckily, most of the energy in speech is in the range 300 - 3,000 Hz, so nearly everyone can still communicate. Most people's ears are most sensitive to sounds around 3,000 Hz. Maybe it's no coincidence that this is the pitch of a smoke alarm - and a baby crying.
Low-frequency sounds below about 100 Hz feel more like a vibration than a note. The transformers in electricity substations vibrate at 100 Hz; you'll need to listen carefully, as your ears don't respond so well to such low frequencies, which sound more like a buzz than a note. A bee's buzz, for example, is about 20 Hz.
Q Silicon is similar to carbon. Why are there no life forms based on silicon?
Silicon is unsuitable because, although it is a valency IV element like carbon, the Si-Si covalent bond is not strong enough for it to form long stable chains, unlike the C-C bond. Silicon cannot therefore form molecules of the complexity needed to make up cellsn
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