Little to say for myself
Wednesday, April 30, 2003
Physics is FunThat title is to save you the trouble of reading any further if you disagree.
One of the things I am is a physics graduate, but, like, it doesn't define me OK? I'm not one of your standard nerds with a clip-on tie and a shirt pocket crammed full of pens. There can't be more than two or three of them in there at any one time. Well, never more than five anyway. Eleven, tops.
How do things stay in orbit (round planets etc)?You know that gravity is a force of attraction between things, right? Gravity attracts the Earth to the Sun, and it also attracts us to the Earth (it's what keeps our feet on the ground and stops us from floating away). When you throw a stone, it is also attracted to the Earth and falls back down again.
Now, imagine you could throw a stone so hard that it went way over the horizon. Because the Earth is ball-shaped, the further over the horizon you go, the more the land curves "down" from where you are. If you could throw the stone far enough, it would still fall towards the surface, but the surface itself would also be curving down away from it, so the stone would never hit it. It would then be in orbit. In other words, the Earth is constantly falling towards the sun, but it's also going sideways so fast that it keeps missing. It just so happens that our sideways speed closely matches the strength of the Sun's attraction at this distance, so we travel along a stable, almost circular path.
Another way of looking at it is to think of gravity as being a "pull". When you swing a ball on a piece of elastic around your head, you can feel the "pull" that stretches the elastic. If you let go, the ball whizzes off. If you slow down, the elastic goes limp and the ball drops. But if you keep it spinning (i.e. keep the ball going sideways fast enough) the elastic stays stretched and the ball keeps going round in a circle. The trick is maintaining the sideways speed of the ball. That's why the space shuttle has to be so powerful - not only does it have to get up to a height of a few hundred miles (to get out of the atmosphere that would otherwise slow it down too much), it also has to get up to a sideways speed of about 17,000 mph - otherwise it won't miss.
Why is the Sky Blue?You know that sunlight is made up of all the colours of the rainbow, right? Well, that sunlight gets to us through our atmosphere, which is a roughly 50-mile-thick layer of air covering the whole planet (held there by, you guessed it, the Earth's gravity). This air is made up of about 80% Nitrogen gas.
Now, normally light travels in straight lines unless it hits something. When it hits something it scatters. It just so happens that Nitrogen gas molecules scatter the blue part of the spectrum more than any of the other colours.
When you're looking up at the sky, but away from the sun, the light that enters your eyes hasn't come directly from the sun - it's light that was going to miss you completely and go over your head, except that some nitrogen molecules got in the way. While most of the light carried on in a straight line, some of the blue part got scattered in all directions - and some of that scattered blue light headed straight into your eyes. So the atmosphere (the sky) looks blue.
In the film of the Apollo astronauts on the Moon's surface, the sky is very dark. This is because there is no atmosphere to scatter the light, so there's no light coming directly from the sky, so it looks black.
So why do clouds look white? Well, they're made up of tiny droplets of water floating about in the air. I say tiny, but in fact compared with the gas molecules they're enormous. Billions of times bigger. Even though that water is clear and colourless, the light changes direction when it crosses the boundary from air into water and back out again. This scatters not just one colour, but all of them - so the light that is scattered towards your eyes is a mixture of all the colours - in other words, white. It's the same story with snow, sugar and salt. If you look at them under a magnifying glass, they're all made of clear crystals, but they've got so many surface boundaries scattering the light that they look white.
Why does the sun look red when it rises and sets?Like I said, the Earth's atmosphere is a roughly even layer of gas clinging to the planet surface. When the sun is high in the sky, the light travels only a short distance through the air layer to the ground. The lower the sun is in the sky, the shallower the angle at which the light crosses the air layer, so the greater the distance it travels through the air to get to you.
As I said before, air scatters some of the blue part of the light. The more air the light travels through to get to you, the more of the blue part gets scattered away and doesn't make it into your eye. If you subtract the blue part from white sunlight, i.e. the part from one side of the rainbow, you're left with light which is dominated by the colours on the other side of the rainbow - i.e. the red side. So the unscattered light coming straight from the sun to your eye looks red.
Stone the crows - I never realised it would be so long-winded.
posted by Plig | 17:01 |
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