The Double Dangerous Book for Boys. Conn Iggulden
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Название: The Double Dangerous Book for Boys
Автор: Conn Iggulden
Издательство: HarperCollins
Жанр: Сделай Сам
isbn: 9780008299682
isbn:
Each one of those three expresses a relationship between two of the triangle lines and an angle. The mnemonic for all three is SOH-CAH-TOA (pronounced so-car-toe-a). The letter ‘O’ is for Opposite, ‘H’ is for Hypotenuse and ‘A’ is for Adjacent – the side closest to the angle.
Sine is the relationship or ratio between the Opposite and the Hypotenuse. You find the sine of the angle x by Opposite/Hypotenuse. The most famous example is the 3–4–5 triangle. (Pythagoras used it to prove the relationship between the sides on a right-angled triangle was a2 + b2 = c2, where c is the hypotenuse.) In this case that would be 3² (3 × 3) + 4² (4 × 4) = 5² (5 × 5).
In this example, if we wanted to find the angle x and had all three side lengths, we could use Sin, Cos or Tan to do it.
Sin = Opposite divided by Hypotenuse = 4/5 = 0.8
Cos = Adjacent divided by Hypotenuse = 3/5 = 0.6
Tan = Opposite divided by Adjacent = 4/3 = 1.33 recurring
It is possible to work out which angle would produce each of those ratios, but it’s pretty advanced. Before calculators, schoolboys used log books, where the answers had been worked out and could be checked or confirmed. Today, you’ll probably use the Sin−1 button – inverse sine – to turn that ratio back into an angle.
Sin x = 0.8
x = inverse sin (Sin−1) of 0.8 = 53°
Now that we’ve covered the basics of trig – back to our tree. We have the base of the triangle. However, we don’t know the height of the tree, nor the hypotenuse. We need to know an angle. For this, we use a protractor, a pencil and a blob of Blu Tack.
Lie on the grass and get as low as you can with the protractor. (We found we couldn’t lay it right on the ground because we couldn’t get an eye low enough to look along the pencil and see the top of the tree.) Holding it steady and just off the ground, raise the pencil until the tip points at the top of the tree. Read off the angle – in our example it was 50°.
We still couldn’t use sine or cosine (Sin or Cos) as we didn’t know the hypotenuse. However, we could use the Tan ratio to discover the missing height.
Tan 50° = Opposite (h for height) divided by Adjacent (60)
Tan of 50° is 1.19 (to two decimal places), which we can plug into the equation:
1.19 = h/60
To get h alone, we still have to do something about that ‘divided by 60’. You may know that an equation means two sides that are equal. You can double one side and, as long as you do the same to the other side, it’s still equal. So 2x = 4 is the same as 4x = 8.
If we multiply both sides by 60, that will make the ‘divided by 60’ disappear – and leave just h: the height of the tree.
1.19 × 60 = h
So 71.4 = h
That figure of 71.4 is in yards, of course. We’d multiply that by three to put it in feet – a mature Douglas fir that turned out to be 214ft tall.
Just to be clear, 60 yards is approximately 54m. (It’s actually 54.864m, so a little over.) To prove it works, we’ll use the metre figure here. If we plug that into the same equation, the answer comes out in metres.
Tan 50° = Opposite (h) divided by Adjacent (54.864)
1.19 = h/54.864
1.19 × 54.864 = h (in metres)
h = 65.288m
Or in feet: 1.19 × 180 = h
h = 214ft tall
Now, this wasn’t as precise as we’d have liked – that angle will always be a best estimate and it’s a key number. However, the basic idea – pace off 60 yards, estimate the angle and use Tan x = Opposite divided by Adjacent to find a height you can’t reach otherwise – is not beyond you.
Finally, you could estimate the key angle by kneeling and raising an outstretched arm. Remember to tell those around you that it’s not a Nazi salute. That could be really important.
HOW TO START A FIRE WITH A BATTERY
Quick and easy, this one. You’ll need a packet of chewing gum, a battery and ideally a pair of scissors, though at a pinch you could tear it carefully.
Chewing gum usually comes wrapped in a piece of paper-backed foil. You’ll have to try this a few times before you find the width that works, but you want to cut or tear the foil into this sort of shape. Foil on one side, paper on the other.
The electricity in an ordinary AA battery cannot overwhelm the full width of foil – but it can set a narrow strip of paper on fire, where it is in contact with the foil. Test it a few times first, then assemble something flammable, such as a shred of tissue paper or, you know, a small piece of firelighter.
Attach one end of the foil to either terminal of the battery, put it in position and touch the other end to the second terminal. With a little luck, it should burn through the paper layer. Alternatively, try a little wire wool rubbed across the terminals of a 9v battery. We teased out a bit of Brillo Pad.
You may be thinking, with some justification, that if you have the foresight to bring a battery and some chewing gum or steel wool with you, you might as well just bring a box of matches. Wire wool can be lit with those matches to help you start a fire, after all. However, that would be missing the point entirely. The nice thing about this is that you will have learned something interesting. The actual lighting of the fire is fiddly and secondary. And you never know, there might come a time when all you have is a battery and a bit of wrapper – and perhaps a firelighter. That is worth a little forward planning. This page is that forward planning.
QUESTIONS ABOUT THE LAW – PART ONE
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