0:00:02 > 0:00:03I often feel the need for speed,
0:00:03 > 0:00:07preferably in well-considered moderate bursts.
0:00:07 > 0:00:11But the thing is, do we even know what speed is?
0:00:11 > 0:00:13Or the answers to questions like...
0:00:24 > 0:00:25Hold on tight,
0:00:25 > 0:00:30while I answer the Things You Need to Know about Speed.
0:00:30 > 0:00:31Let's start with the basics.
0:00:34 > 0:00:38You'd think measuring the speed of anything was kids' stuff.
0:00:38 > 0:00:40It's the distance from A to B,
0:00:40 > 0:00:43divided by the time it takes to get there.
0:00:44 > 0:00:46BEAR ROARS
0:00:47 > 0:00:50So a grizzly bear in a bad mood does about 30 miles an hour.
0:00:52 > 0:00:55A bullet train travels at 186 miles an hour.
0:00:55 > 0:01:00And Australia is heading for China at two inches a year,
0:01:00 > 0:01:03the same speed as your fingernails grow.
0:01:06 > 0:01:09Speed is a concept that we're all quite familiar with.
0:01:09 > 0:01:12But it can be described very simply by an equation.
0:01:12 > 0:01:13But a simple equation.
0:01:13 > 0:01:16Speed is simply the distance you've travelled,
0:01:16 > 0:01:18divided by the time its taken you to travel it.
0:01:18 > 0:01:21But actually it becomes a bit more complicated than that.
0:01:21 > 0:01:24If you say what's my speed, well I'm not moving.
0:01:24 > 0:01:26But I am moving because the earth's moving.
0:01:26 > 0:01:28It's very odd if you think about it.
0:01:28 > 0:01:32There's never just one answer to the question what's my speed?
0:01:32 > 0:01:34The trouble is, you always need a frame of reference
0:01:34 > 0:01:36to measure your speed against.
0:01:36 > 0:01:39If there was nothing else in the universe,
0:01:39 > 0:01:42you couldn't even tell whether you were moving or not.
0:01:45 > 0:01:47Maybe you're going scarily fast.
0:01:48 > 0:01:53Speed is always relative because it depends on the frame of reference.
0:01:53 > 0:01:58A baby can throw his rattle and think it's going five miles an hour.
0:01:58 > 0:02:01But if he's on a train doing 500 miles an hour,
0:02:01 > 0:02:05and you're on the platform, the speed of the rattle adds up.
0:02:06 > 0:02:10Then again, if baby fires his machine gun backwards
0:02:10 > 0:02:13at 500 miles an hour, you could catch the bullets in your teeth,
0:02:13 > 0:02:16because the speeds cancel out.
0:02:16 > 0:02:20In the right frame of reference, your speed can be unbelievably fast.
0:02:22 > 0:02:26For watching aliens, everything on Earth, including us,
0:02:26 > 0:02:29is going around the Sun at over 67,000 miles an hour.
0:02:29 > 0:02:32And the sun is moving through the milky way at nearly
0:02:32 > 0:02:34half a million miles an hour.
0:02:36 > 0:02:39But don't worry, it's only aliens. Not the police.
0:02:40 > 0:02:43There's no point telling the judge that all speed is relative.
0:02:43 > 0:02:45He's heard that one before.
0:02:45 > 0:02:47Because as long as there have been drivers,
0:02:47 > 0:02:50they've been driving too fast.
0:02:50 > 0:02:52And the police have been asking a question to which
0:02:52 > 0:02:55they already know the answer.
0:02:59 > 0:03:04Even in the steam powered 1860s, there was a speed limit.
0:03:04 > 0:03:07It was two miles an hour plus a man with a flag.
0:03:09 > 0:03:11Well, initially, you needed three people in the car.
0:03:11 > 0:03:15A driver, a stoker and someone to walk ahead with a red flag
0:03:15 > 0:03:17so you didn't scare the horses.
0:03:17 > 0:03:20And often it was just much quicker to walk.
0:03:20 > 0:03:24In 1896, petrol-crazed Walter Arnold of Peckham
0:03:24 > 0:03:28got the first speeding fine for doing eight miles an hour.
0:03:29 > 0:03:31But then the law went mad,
0:03:31 > 0:03:34and upped the speed limit to 14 miles an hour,
0:03:34 > 0:03:36and got rid of the flag.
0:03:38 > 0:03:40So in New England, the speed trap was born.
0:03:44 > 0:03:48One of the first victims was the New York Police Commissioner
0:03:48 > 0:03:51William McAdoo. We don't know if he paid his fine.
0:03:51 > 0:03:54Speeding tickets were extortionate.
0:03:54 > 0:03:57They were £5, which is equivalent to a month's salary,
0:03:57 > 0:04:00or you had to spend four weeks in jail.
0:04:00 > 0:04:04The first speed cameras flashed in 1905
0:04:04 > 0:04:08with a time stamp at each end of the trap, to work out your speed.
0:04:08 > 0:04:12The Automobile Association hit back with cyclists,
0:04:12 > 0:04:15to warn their members of hidden speed traps.
0:04:16 > 0:04:19But this was later deemed illegal.
0:04:20 > 0:04:25So AA cycle scouts began to salute all their members instead.
0:04:25 > 0:04:27If they didn't, it meant speed trap ahoy!
0:04:29 > 0:04:32But in the 50s, police technology overtook them.
0:04:32 > 0:04:35A radar gun fires a beam of microwaves.
0:04:35 > 0:04:38When they hit your car, they change frequency,
0:04:38 > 0:04:40depending on how fast you're going.
0:04:40 > 0:04:43So the reflected beam tells the police your speed.
0:04:43 > 0:04:46The down side is that drivers detect the radar
0:04:46 > 0:04:48before the radar detects them.
0:04:49 > 0:04:53Die hards even try absorbent paint so the beam can't bounce back.
0:04:55 > 0:04:58But this probably only really works if you're in a Stealth Bomber.
0:05:00 > 0:05:03It pays to watch your speed at all times,
0:05:03 > 0:05:05but also to keep your ears open.
0:05:05 > 0:05:09At 70 miles an hour, you might hear this.
0:05:09 > 0:05:11POLICE SIREN
0:05:11 > 0:05:15But go eleven times faster, and you'll hear this.
0:05:15 > 0:05:17SONIC BOOM
0:05:17 > 0:05:20A sound that means my next question has arrived.
0:05:23 > 0:05:26You don't need a jet fighter to make a sonic boom.
0:05:26 > 0:05:32Nearly 5,000 years ago we discovered them, along with the simple whip.
0:05:32 > 0:05:35But no-one knew what made the crack,
0:05:35 > 0:05:40until Austrian scientist Ernst Mach figured it out in 1887.
0:05:40 > 0:05:43He realised sound waves were like ripples in water.
0:05:43 > 0:05:45When a boat goes faster than the ripples,
0:05:45 > 0:05:48they've got nowhere to go, so they bunch up in a wake.
0:05:48 > 0:05:51The same thing happens with a whip.
0:05:51 > 0:05:56Sound waves travel through the air, at 768 miles an hour.
0:05:56 > 0:06:00But the tip of a whip goes faster, bunching up the sound waves,
0:06:00 > 0:06:02and producing a very loud shock wave.
0:06:02 > 0:06:06A whip gets narrower and lighter all the way to the tip.
0:06:06 > 0:06:11So when you snap it at the top, the wave travels faster and faster
0:06:11 > 0:06:13and faster and the tip can actually be going
0:06:13 > 0:06:16about 30 times as fast as that initial snap.
0:06:16 > 0:06:19That means that the tip is going faster than the speed of sound
0:06:19 > 0:06:22so the sound waves can't propagate away from it,
0:06:22 > 0:06:26they kind of form a wake of sound, then you hear it in one big blast.
0:06:26 > 0:06:28That's a sonic boom.
0:06:29 > 0:06:32This is all good fun for pistol-packin',
0:06:32 > 0:06:34whip-crackin' cowboys,
0:06:34 > 0:06:36but bad news for 1920s pilots.
0:06:37 > 0:06:39Planes were slow,
0:06:39 > 0:06:42but the propeller tips were spinning near the speed of sound.
0:06:46 > 0:06:49In fact, parts of a plane can go supersonic,
0:06:49 > 0:06:51even when the whole plane is not supersonic.
0:06:51 > 0:06:54To understand that, it's to do with the propellers.
0:06:54 > 0:06:57In the middle it may not be spinning that fast.
0:06:57 > 0:07:00But as you go further and further out along the propeller,
0:07:00 > 0:07:02it's going faster and faster.
0:07:02 > 0:07:04In fact, if you double the distance you're going out,
0:07:04 > 0:07:06you double the speed that's going.
0:07:06 > 0:07:09And so while most of the propeller may be below the speed of sound,
0:07:09 > 0:07:11the tip of it might break the sound barrier.
0:07:11 > 0:07:14Even with 1940s jet engines,
0:07:14 > 0:07:17no-one could figure out how to break the sound barrier.
0:07:17 > 0:07:20Until they remembered Ernst Mach.
0:07:20 > 0:07:24He said the perfect shape was like a long, thin cigar.
0:07:24 > 0:07:27Streamline the nose, bend the wings to reduce the shock waves,
0:07:27 > 0:07:30and your jet fighter's ready to make a sonic boom.
0:07:32 > 0:07:36Not just once when you break the sound barrier,
0:07:36 > 0:07:38but the whole time you're supersonic.
0:07:38 > 0:07:42Everybody under the flight path is hit by the sonic boom,
0:07:42 > 0:07:47but at slightly different times as the plane passes overhead.
0:07:47 > 0:07:50Now, to be honest, it's all rather unpleasant,
0:07:50 > 0:07:53unless you're in the aeroplane, then it's enormously good fun.
0:07:56 > 0:08:01The boom presents itself on the ground like a giant red carpet.
0:08:01 > 0:08:03But there's a second boom from the tail,
0:08:03 > 0:08:05when the air rushes in to fill the gap.
0:08:05 > 0:08:10So while only the rich could afford to go supersonic on Concorde,
0:08:10 > 0:08:13everyone on the ground got the booms for free.
0:08:17 > 0:08:21So now that we've broken through this so-called sound barrier,
0:08:21 > 0:08:25we can fly anywhere at top speed.
0:08:25 > 0:08:26Brilliant!
0:08:30 > 0:08:33But have your ever stopped to wonder...
0:08:37 > 0:08:40In the 17th century, architect, scientist and all-round genius
0:08:40 > 0:08:45Robert Hooke was working on new theories of springs and gravity.
0:08:47 > 0:08:50That's how he got the bright idea
0:08:50 > 0:08:53that the fastest way round the world was through the middle.
0:08:53 > 0:08:56We're used to thinking of gravity as down to the ground.
0:08:56 > 0:08:57But in fact its acting
0:08:57 > 0:08:59like it's pulling us towards the centre of the earth,
0:08:59 > 0:09:02so if you have a mine shaft, it will pull you down the mine shaft.
0:09:02 > 0:09:04Hooke's plan was to first drill a hole
0:09:04 > 0:09:0720 feet wide and 8,000 miles deep.
0:09:09 > 0:09:12Then, suck out all the air and do something useful with it.
0:09:15 > 0:09:18All you've got to do now is drop like a stone.
0:09:19 > 0:09:22According to Hooke, after 21 minutes and six seconds,
0:09:22 > 0:09:26you'll hit about 18,000 miles an hour.
0:09:26 > 0:09:32But at the centre, down turns up and you begin to slow down.
0:09:32 > 0:09:35So after exactly 42 minutes and 12 seconds, gravity brings you
0:09:35 > 0:09:40to a nice, gradual stop before it takes you home again, like a spring.
0:09:42 > 0:09:44So how does it work?
0:09:44 > 0:09:47Well, if this is the earth, and this is your hole,
0:09:47 > 0:09:51gravity is still pulling you down as you travel through the hole.
0:09:51 > 0:09:54So gravity is always working towards the centre.
0:09:54 > 0:09:57Actually when you pass the centre there's more mass behind you
0:09:57 > 0:10:00so you're going to be slowed down and attracted back.
0:10:00 > 0:10:03And then you go back down the tunnel again. And you just keep doing it.
0:10:03 > 0:10:05You just keep bouncing backwards and forwards
0:10:05 > 0:10:07and that's called simple harmonic motion.
0:10:09 > 0:10:12Now, I know what some of you are thinking.
0:10:12 > 0:10:16If Hooke's idea worked, and that's a pretty big if,
0:10:16 > 0:10:17it would only be useful
0:10:17 > 0:10:21for travelling between opposite sides of the world.
0:10:21 > 0:10:23But here comes the really weird bit.
0:10:24 > 0:10:27You'd think the biggest problem is that the centre of the earth
0:10:27 > 0:10:29is molten magma.
0:10:29 > 0:10:33But Robert Hooke said you could miss it out completely.
0:10:33 > 0:10:37A shorter shaft, say London to Los Angeles, works just as well.
0:10:37 > 0:10:42You don't get pulled so much by gravity, so you go a bit slower.
0:10:42 > 0:10:47But the crazy thing is it still takes 42 minutes and 12 seconds.
0:10:47 > 0:10:50The reason is although the distance is shorter,
0:10:50 > 0:10:51the force is pulling you less.
0:10:51 > 0:10:53And they cancel each other out and you get the same time
0:10:53 > 0:10:55wherever you go.
0:10:55 > 0:10:58Zanzibar to Alaska,
0:10:58 > 0:11:00or Moscow to Washington,
0:11:00 > 0:11:06It's always 42 minutes 12 seconds, from anywhere to anywhere else.
0:11:06 > 0:11:07The gravity express isn't just for humans.
0:11:07 > 0:11:12You could order pizza from Italy and for once,
0:11:12 > 0:11:15it would take less than 45 minutes.
0:11:15 > 0:11:16Right.
0:11:16 > 0:11:21So if gravity is like a spring, then the basic Theory of Gravity,
0:11:21 > 0:11:25ie. "What Goes Up, Must Come Down", should be correct.
0:11:27 > 0:11:28Or is it?
0:11:28 > 0:11:32Because some of you would have just thought of an obvious question.
0:11:38 > 0:11:40Trees fall down,
0:11:40 > 0:11:43Trousers fall down,
0:11:43 > 0:11:45And dead pigeons fall down.
0:11:46 > 0:11:48So why not the Moon?
0:11:49 > 0:11:51Because the moon doesn't fall to the ground
0:11:51 > 0:11:53like everything else we see around us,
0:11:53 > 0:11:56you kind of imagine that there's something holding it up. But that's not true.
0:11:56 > 0:12:00It took a genius like Isaac Newton to realise that the same force
0:12:00 > 0:12:02that makes things fall down is the force that makes the moon
0:12:02 > 0:12:05go around the earth and the earth go around the sun.
0:12:07 > 0:12:11In 1687, Isaac Newton explained the whole thing in his masterpiece,
0:12:11 > 0:12:13Principia Mathematica.
0:12:13 > 0:12:17First, get rid of the atmosphere. It just gets in the way.
0:12:17 > 0:12:22Now we need a very high mountain and a volunteer as a human cannonball.
0:12:22 > 0:12:27A decent bang, and he'll go right over the horizon
0:12:27 > 0:12:30before gravity brings him back to Earth.
0:12:30 > 0:12:33But fire him fast enough, at five miles a second,
0:12:33 > 0:12:36and you get to the clever bit.
0:12:36 > 0:12:40He falls towards the Earth at exactly the same rate
0:12:40 > 0:12:42the Earth curves away from him.
0:12:42 > 0:12:46So he's falling, but he never comes down.
0:12:46 > 0:12:48And that's what happens when you're in orbit.
0:12:48 > 0:12:51The direction always changes because you go around the earth,
0:12:51 > 0:12:52but your speed stays the same.
0:12:52 > 0:12:55It's as if gravity is swinging him around on a string.
0:12:55 > 0:13:00With no air resistance to slow him down, he'll stay in orbit for ever.
0:13:02 > 0:13:05The Moon does the same thing, but the string's a bit longer,
0:13:05 > 0:13:07about a quarter of a million miles.
0:13:09 > 0:13:12The Apollo astronauts left mirrors on the Moon,
0:13:12 > 0:13:14so we can measure the distance using a laser.
0:13:15 > 0:13:21We all know the Moon's gravity causes the tides, but get this.
0:13:21 > 0:13:25The earth's rotation pulls the tidal bulge just ahead of the moon.
0:13:25 > 0:13:29This bulge pulls back on the moon for a slingshot effect.
0:13:29 > 0:13:34That makes the moon move an inch and a half further away every year.
0:13:34 > 0:13:39So I'm sorry to report that the Moon is really falling up.
0:13:42 > 0:13:44Mmm.
0:13:44 > 0:13:47But let's get back to what gravity does best.
0:13:47 > 0:13:50Making things fall from the sky, at very high speed.
0:13:50 > 0:13:51Sooner or later,
0:13:51 > 0:13:55they reach something known as Terminal Velocity.
0:13:55 > 0:13:59And if that sounds scary, it isn't necessarily fatal.
0:14:00 > 0:14:02Just ask your cat.
0:14:08 > 0:14:13In New York in 1987, it was practically raining cats.
0:14:13 > 0:14:18More than 100 fell from six storeys or more on to concrete sidewalks.
0:14:18 > 0:14:20But cats really must have nine lives
0:14:20 > 0:14:23because nine out of ten of them survived.
0:14:23 > 0:14:24So you got to be thinking,
0:14:24 > 0:14:26who is it that's throwing all these cats out of buildings?
0:14:26 > 0:14:30And it turns out it wasn't deliberate, some of the windows
0:14:30 > 0:14:33of the apartments kind of opened up into the apartment and
0:14:33 > 0:14:37if the owner didn't notice that their cat was asleep on the windowsill and
0:14:37 > 0:14:41they closed the window, it would have the side effect of ejecting the cat.
0:14:41 > 0:14:45So, you've got a whole bunch of cats that have fallen 20 meters or so,
0:14:45 > 0:14:46and they're being taken to the vets
0:14:46 > 0:14:50with nothing more than a few bumps and bruises.
0:14:50 > 0:14:54It's all down to the fact that cats have a non-fatal terminal velocity.
0:14:54 > 0:15:00It sounds impossible, but for cats, it's basic physics.
0:15:00 > 0:15:03Isaac Newton said gravity makes everything accelerate
0:15:03 > 0:15:05at the same rate, from apples to grand pianos.
0:15:06 > 0:15:09But only if there's no air resistance or drag.
0:15:10 > 0:15:15In the real world drag builds up until it cancels out gravity,
0:15:15 > 0:15:20and a falling object hits a constant speed - its terminal velocity.
0:15:20 > 0:15:24It's different for different shapes and sizes.
0:15:24 > 0:15:26About 200 miles an hour for a piano,
0:15:26 > 0:15:29a bit slower for Isaac Newton,
0:15:29 > 0:15:30and just 60 miles an hour for a cat.
0:15:33 > 0:15:38Small things have relatively more surface area than large things,
0:15:38 > 0:15:41so air resistance has a greater effect.
0:15:41 > 0:15:45The cat, essentially, has a built-in parachute.
0:15:45 > 0:15:50Leonard da Vinci designed the first parachute back in 1483.
0:15:50 > 0:15:55Bigger surface area means more air resistance to slow you down.
0:15:56 > 0:16:00But with a cat, it's automatic.
0:16:00 > 0:16:03His ears have a built in gyroscopic motion sensor
0:16:03 > 0:16:06which he uses to get Head Up, Paws Down.
0:16:08 > 0:16:12At terminal velocity, he can't feel he's accelerating any more.
0:16:12 > 0:16:14So he chills out, and stretches out.
0:16:14 > 0:16:18Given time, he gets to a slower terminal velocity.
0:16:18 > 0:16:22A cat needs to fall from the 7th floor or higher
0:16:22 > 0:16:25so it's got enough time in the air to fully rotate around,
0:16:25 > 0:16:28land on the ground, and just stroll off.
0:16:28 > 0:16:31To paraphrase the great biologist J.B.S. Haldane,
0:16:31 > 0:16:36a horse splashes, a man is broken, but a cat just walks away.
0:16:36 > 0:16:41So the happy fact is, the bigger the fall, the better his chances.
0:16:41 > 0:16:45Nine out of ten New York cats prove it.
0:16:45 > 0:16:48Perhaps by landing on something less advanced.
0:16:49 > 0:16:53Thanks to Sir Isaac Newton, we now understand Terminal Velocity.
0:16:53 > 0:16:58Although next time you go outside, it's unlikely to be raining cats.
0:16:58 > 0:17:00It's more likely to be raining rain.
0:17:03 > 0:17:07And that leads us to a very important scientific question.
0:17:13 > 0:17:15You should never leave a mathematician go out in the rain
0:17:15 > 0:17:18because they'll insist on calculating the best way
0:17:18 > 0:17:20to stay dry.
0:17:20 > 0:17:21First things first,
0:17:21 > 0:17:24you need to break down the question into simple, easy chunks.
0:17:24 > 0:17:27So, point number one, how much rain falls on your head?
0:17:27 > 0:17:30Point number two, how much rain do you collect on your front?
0:17:30 > 0:17:33You get the same amount of rain on your front - you collect it up
0:17:33 > 0:17:35over the path of your walk - no matter how fast you go.
0:17:35 > 0:17:37So it makes no difference.
0:17:37 > 0:17:41Unless there's a wind. If there's wind, it gets a little bit tricky.
0:17:41 > 0:17:47Raindrops fall at the terminal velocity of about 15 miles an hour.
0:17:47 > 0:17:51But wind can blow them sideways, at around seven miles an hour.
0:17:51 > 0:17:53You can walk through it at four miles an hour,
0:17:53 > 0:17:57or try running at ten.
0:17:57 > 0:18:01But mathematically, humans are a difficult shape to deal with.
0:18:01 > 0:18:04So let's keep it simple, with rectangles.
0:18:04 > 0:18:08Now, please pay attention for the emergency rain procedure.
0:18:09 > 0:18:12If there's no wind, you should run not walk.
0:18:14 > 0:18:16You'll get exactly the same amount of rain on your front,
0:18:16 > 0:18:20you're just sweeping it up faster.
0:18:20 > 0:18:24But you'll get home sooner, so less rain falls on your head.
0:18:24 > 0:18:27It's a different story when the wind's blowing.
0:18:27 > 0:18:30If the rain's coming right at you,
0:18:30 > 0:18:33bend double, so it has a smaller area to hit.
0:18:33 > 0:18:37Brilliant, except you can't see where you're going.
0:18:38 > 0:18:42The really clever bit is when the wind's behind you.
0:18:42 > 0:18:45Now the trick is to match your speed to the wind
0:18:45 > 0:18:48so none of it hits your back or front.
0:18:48 > 0:18:52Unfortunately the wind isn't always going the way you want it to.
0:18:53 > 0:18:57And finally, there's bad news for large people.
0:18:57 > 0:19:03You've got a lot of surface area, and that soaks up a lot of rain.
0:19:03 > 0:19:08So, I'm sorry. You should always run home, no matter what.
0:19:08 > 0:19:12But don't forget the real world is more complicated.
0:19:12 > 0:19:13You'll probably get soaked anyway.
0:19:19 > 0:19:23Getting wet is annoying but it won't actually kill you.
0:19:23 > 0:19:26But there's no escaping the fact that there are times
0:19:26 > 0:19:30when you will need to run away from things at very high speed.
0:19:30 > 0:19:34So the answer to my next question should be particularly useful if
0:19:34 > 0:19:38you find yourself being pursued down a high street by a giant dinosaur.
0:19:43 > 0:19:47To answer a silly question, you need a silly bird.
0:19:48 > 0:19:50Ostrich legs are nothing like a human's,
0:19:50 > 0:19:53unless it's running backwards, of course,
0:19:53 > 0:19:55which is just weird.
0:19:55 > 0:20:01But an ostrich is like a T-Rex, because birds evolved from dinosaurs
0:20:01 > 0:20:05and therefore have similar skeletons.
0:20:05 > 0:20:08So scientists have worked out a formula that links
0:20:08 > 0:20:12the spacing of ostrich footprints to how fast they run.
0:20:12 > 0:20:15And since birds are like dinosaurs,
0:20:15 > 0:20:18the same formula should tell us how fast T-Rex was.
0:20:18 > 0:20:21Unfortunately, he must have covered his tracks,
0:20:21 > 0:20:22because we can't find any.
0:20:24 > 0:20:28We do know small dinosaurs can run about eight miles per hour.
0:20:28 > 0:20:31But we don't have those figures for the T-Rex.
0:20:31 > 0:20:33So, scientists tried Plan B.
0:20:33 > 0:20:36Imagine a bird pumped up to T-Rex size
0:20:36 > 0:20:39to make the world's first six tonne chicken.
0:20:40 > 0:20:45But if you grow in size, you grow massively in weight,
0:20:45 > 0:20:47far more than you grow in strength.
0:20:47 > 0:20:50King Kong would be lucky if he could lift his own finger,
0:20:50 > 0:20:53never mind climb the Empire State Building.
0:20:54 > 0:20:58I hate it when science ruins a perfectly good movie.
0:20:58 > 0:21:02Thing is, unlike King Kong, T-Rex really did exist,
0:21:02 > 0:21:04and he really could move.
0:21:06 > 0:21:08So how?
0:21:08 > 0:21:11Where the movies get it wrong is they scale things up to these
0:21:11 > 0:21:14large sizes without taking into account what would actually
0:21:14 > 0:21:15happen if you did that.
0:21:15 > 0:21:18It's all about scale factors.
0:21:18 > 0:21:20If you double the height of something,
0:21:20 > 0:21:25then its strength goes up squared, but its weight goes up cubed.
0:21:25 > 0:21:28So this is why there are no giant ants around
0:21:28 > 0:21:31because the strategy they have for supporting their own weight
0:21:31 > 0:21:34would just not work if you scale them up to the size of an elephant.
0:21:34 > 0:21:38To run like a six tonne chicken, T-Rex would need to be more
0:21:38 > 0:21:40than 100% muscle.
0:21:40 > 0:21:42In other words, impossible.
0:21:43 > 0:21:47So forget giant chickens. Let's try some real giants.
0:21:47 > 0:21:49Elephants never lift all four feet,
0:21:49 > 0:21:53because the impact's too big for their bones.
0:21:53 > 0:21:57This is known as Groucho-Running, after comedy legend Groucho Marx.
0:21:57 > 0:22:00So it's possible T-Rex did the same thing,
0:22:00 > 0:22:02to take the strain off his legs.
0:22:02 > 0:22:06He didn't sprint like the fastest humans, at 27 miles an hour.
0:22:06 > 0:22:09But he could still do about 15 miles an hour.
0:22:09 > 0:22:14Try outrunning T-Rex yourself, and see how far you get.
0:22:14 > 0:22:17At least you can see T-Rex coming.
0:22:17 > 0:22:22How do you avoid invisible, microscopic nasties like viruses?
0:22:22 > 0:22:24I mean, when I was at school they used to say
0:22:24 > 0:22:27"Coughs and Sneezes Spread Diseases".
0:22:27 > 0:22:31But unfortunately, it looks like the flu can get around
0:22:31 > 0:22:32quite a bit quicker than that.
0:22:40 > 0:22:41Congratulations!
0:22:41 > 0:22:45You're the first to catch a new and horrible, mutant flu virus.
0:22:45 > 0:22:50So after you've called the doctor, call a mathematician,
0:22:50 > 0:22:52because now it's all about numbers.
0:22:52 > 0:22:56The key number is how many people you pass it on to.
0:22:56 > 0:22:59If that's exactly one, then you get better,
0:22:59 > 0:23:02and your friend takes your place, so the outbreak isn't growing.
0:23:02 > 0:23:05He won't be as happy about that as you are.
0:23:05 > 0:23:08If it's more than one, it's an epidemic,
0:23:08 > 0:23:09and now everyone can get it.
0:23:13 > 0:23:17Its number is known as the basic reproductive ratio.
0:23:17 > 0:23:20The higher the number, the more infectious the disease.
0:23:20 > 0:23:22Contact is one of the ways these things spread.
0:23:22 > 0:23:24If you're on a desert island on your own,
0:23:24 > 0:23:26you're not going to be infecting anybody,
0:23:26 > 0:23:27whereas if you're on a packed train
0:23:27 > 0:23:30there's lots of potential people you could infect.
0:23:30 > 0:23:33So that's why you really need to take a tissue with you.
0:23:33 > 0:23:38For measles, every victim potentially infects another 14.
0:23:38 > 0:23:41But for flu, the average is only 1.8.
0:23:41 > 0:23:45And we can stamp it out completely by making that number less than one.
0:23:45 > 0:23:48The bad news is that means vaccination.
0:23:48 > 0:23:54It's from the Latin for cow, because the first vaccines were for cowpox.
0:23:54 > 0:23:58But the strange thing is you don't need to vaccinate them all.
0:23:58 > 0:24:01Just enough and even unvaccinated cows are safe
0:24:01 > 0:24:04in a bubble of vaccinated cows.
0:24:04 > 0:24:07And it's the same for us humans too,
0:24:07 > 0:24:11although the bubble probably smells a little bit better.
0:24:13 > 0:24:17You see, you reach a point where every individual benefits from
0:24:17 > 0:24:19so-called Herd Immunity.
0:24:19 > 0:24:22Which is just as well, really,
0:24:22 > 0:24:25because vaccinating everybody would be terribly expensive.
0:24:26 > 0:24:27Chicken or fish?
0:24:28 > 0:24:30But these days,
0:24:30 > 0:24:34flu is getting its numbers back up by hijacking airliners.
0:24:34 > 0:24:4020 million flights a year, each able to carry flu at 600 miles an hour.
0:24:40 > 0:24:45In 2009, the H1N1 virus went from one tourist in Mexico
0:24:45 > 0:24:49to one million Americans in six weeks.
0:24:49 > 0:24:52Maybe you should just stay at home for a bit.
0:24:53 > 0:24:59So dinosaurs come on big and slow. Viruses come on small and fast.
0:24:59 > 0:25:03But what happens when you get hit by something big and fast?
0:25:03 > 0:25:06Your only hope then is quick thinking.
0:25:06 > 0:25:07So...
0:25:11 > 0:25:13When Mother Nature turns nasty,
0:25:13 > 0:25:16a few handy hints make all the difference.
0:25:16 > 0:25:20If a hurricane's coming, the wrong place to be is out at sea.
0:25:21 > 0:25:25As it spins, it sucks energy from the ocean,
0:25:25 > 0:25:29whipping up winds of 160 miles an hour.
0:25:29 > 0:25:33But the hurricane itself never moves faster than 50 miles an hour,
0:25:33 > 0:25:35so you can get out of the way.
0:25:35 > 0:25:39A hurricane spins mighty fast.
0:25:39 > 0:25:42In fact, the winds have to be going over 74 miles an hour for it
0:25:42 > 0:25:45to be classified as a hurricane and not just a tropical storm.
0:25:45 > 0:25:48But if a tsunami's coming, forget about it.
0:25:48 > 0:25:50Ocean waves go faster in deeper water.
0:25:50 > 0:25:53So, out at sea, a tsunami beats a Jumbo Jet.
0:25:56 > 0:26:00But the strange thing is the wave's only a few feet high,
0:26:00 > 0:26:03and it's hundreds of miles from one peak to the next.
0:26:03 > 0:26:07You'll bob up and down so slowly that you won't even feel it.
0:26:09 > 0:26:15It's hard to imagine you wouldn't notice a 600 mile an hour wave.
0:26:15 > 0:26:18But, of course, if the peaks of the wave are 600 miles apart
0:26:18 > 0:26:21then it takes a whole hour to go past.
0:26:22 > 0:26:26So it's safer there than on the beach.
0:26:26 > 0:26:30In shallow water, a tsunami slows down to 30 miles an hour,
0:26:30 > 0:26:34but grows into a wall of water 100 feet high.
0:26:34 > 0:26:37This sucks all the water from the shore.
0:26:37 > 0:26:42If you see that, run, and don't stop until you reach the mountains.
0:26:42 > 0:26:44But don't jump for joy.
0:26:44 > 0:26:48Nine out of ten people who die in avalanches start it themselves.
0:26:49 > 0:26:52A slope can be covered in layers of snow.
0:26:52 > 0:26:55You can have really strong layers that are inter-connected
0:26:55 > 0:26:57big, strong snow crystals.
0:26:57 > 0:27:00On top of that, you may have a layer of snow that melted and refrozen.
0:27:00 > 0:27:01That's a weak layer.
0:27:01 > 0:27:04If you get a big dump of powder on top of that,
0:27:04 > 0:27:07that is a ticking time bomb and you, as a skier or a boarder,
0:27:07 > 0:27:10could be the straw that breaks the camel's back.
0:27:10 > 0:27:12You could cause that whole layer just to break off
0:27:12 > 0:27:14and go flying down the slope.
0:27:14 > 0:27:17This is actually a phenomenon that's all over science
0:27:17 > 0:27:19in that you get an unstable equilibrium, we call it.
0:27:19 > 0:27:22You've got something balanced essentially on a fine pin
0:27:22 > 0:27:23and the slightest disturbance
0:27:23 > 0:27:26will release a huge amount of potential energy.
0:27:27 > 0:27:31Ten million tons of snow dropping at 80 miles an hour,
0:27:31 > 0:27:34and your only chance is swimming for the surface
0:27:34 > 0:27:37before the snow sets like concrete.
0:27:39 > 0:27:41You'll suffocate if you get buried.
0:27:41 > 0:27:42Yes!
0:27:42 > 0:27:46Nothing to worry about now, except a comet impact, of course,
0:27:46 > 0:27:50at 25,000 mph, with the possible extinction of life on earth.
0:27:50 > 0:27:54We're still working on a handy hint for that one.
0:27:58 > 0:28:01Well, we've covered a lot of ground in a very short time.
0:28:01 > 0:28:04And as you can see, the road to understanding speed
0:28:04 > 0:28:06is long and dangerous.
0:28:12 > 0:28:15I think it's time I make a speedy exit.
0:28:50 > 0:28:53Subtitles by Red Bee Media Ltd