James May's Things You Need to Know (2010) s02e04 Episode Script
...about Speed
I often feel the need for speed, preferably in well-considered moderate bursts.
But the thing is, do we even know what speed is? Or the answers to questions like Hold on tight, while I answer the Things You Need to Know about Speed.
Let's start with the basics.
You'd think measuring the speed of anything was kids' stuff.
It's the distance from A to B, divided by the time it takes to get there.
BEAR ROARS So a grizzly bear in a bad mood does about 30 miles an hour.
A bullet train travels at 186 miles an hour.
And Australia is heading for China at two inches a year, the same speed as your fingernails grow.
Speed is a concept that we're all quite familiar with.
But it can be described very simply by an equation.
But a simple equation.
Speed is simply the distance you've travelled, divided by the time its taken you to travel it.
But actually it becomes a bit more complicated than that.
If you say what's my speed, well I'm not moving.
But I am moving because the earth's moving.
It's very odd if you think about it.
There's never just one answer to the question what's my speed? The trouble is, you always need a frame of reference to measure your speed against.
If there was nothing else in the universe, you couldn't even tell whether you were moving or not.
Maybe you're going scarily fast.
Speed is always relative because it depends on the frame of reference.
A baby can throw his rattle and think it's going five miles an hour.
But if he's on a train doing 500 miles an hour, and you're on the platform, the speed of the rattle adds up.
Then again, if baby fires his machine gun backwards at 500 miles an hour, you could catch the bullets in your teeth, because the speeds cancel out.
In the right frame of reference, your speed can be unbelievably fast.
For watching aliens, everything on Earth, including us, is going around the Sun at over 67,000 miles an hour.
And the sun is moving through the milky way at nearly There's no point telling the judge that all speed is relative.
He's heard that one before.
Because as long as there have been drivers, they've been driving too fast.
And the police have been asking a question to which they already know the answer.
Even in the steam powered 1860s, there was a speed limit.
It was two miles an hour plus a man with a flag.
Well, initially, you needed three people in the car.
A driver, a stoker and someone to walk ahead with a red flag so you didn't scare the horses.
And often it was just much quicker to walk.
In 1896, petrol-crazed Walter Arnold of Peckham got the first speeding fine for doing eight miles an hour.
But then the law went mad, and upped the speed limit to 14 miles an hour, and got rid of the flag.
So in New England, the speed trap was born.
One of the first victims was the New York Police Commissioner William McAdoo.
We don't know if he paid his fine.
Speeding tickets were extortionate.
They were £5, which is equivalent to a month's salary, or you had to spend four weeks in jail.
The first speed cameras flashed in 1905 with a time stamp at each end of the trap, to work out your speed.
The Automobile Association hit back with cyclists, to warn their members of hidden speed traps.
But this was later deemed illegal.
So AA cycle scouts began to salute all their members instead.
If they didn't, it meant speed trap ahoy! But in the 50s, police technology overtook them.
A radar gun fires a beam of microwaves.
When they hit your car, they change frequency, depending on how fast you're going.
So the reflected beam tells the police your speed.
The down side is that drivers detect the radar before the radar detects them.
Die hards even try absorbent paint so the beam can't bounce back.
But this probably only really works if you're in a Stealth Bomber.
It pays to watch your speed at all times, but also to keep your ears open.
At 70 miles an hour, you might hear this.
POLICE SIREN But go eleven times faster, and you'll hear this.
SONIC BOOM A sound that means my next question has arrived.
You don't need a jet fighter to make a sonic boom.
Nearly 5,000 years ago we discovered them, along with the simple whip.
But no-one knew what made the crack, until Austrian scientist Ernst Mach figured it out in 1887.
He realised sound waves were like ripples in water.
When a boat goes faster than the ripples, they've got nowhere to go, so they bunch up in a wake.
The same thing happens with a whip.
Sound waves travel through the air, at 768 miles an hour.
But the tip of a whip goes faster, bunching up the sound waves, and producing a very loud shock wave.
A whip gets narrower and lighter all the way to the tip.
So when you snap it at the top, the wave travels faster and faster and faster and the tip can actually be going about 30 times as fast as that initial snap.
That means that the tip is going faster than the speed of sound so the sound waves can't propagate away from it, they kind of form a wake of sound, then you hear it in one big blast.
That's a sonic boom.
This is all good fun for pistol-packin', whip-crackin' cowboys, but bad news for 1920s pilots.
Planes were slow, but the propeller tips were spinning near the speed of sound.
In fact, parts of a plane can go supersonic, even when the whole plane is not supersonic.
To understand that, it's to do with the propellers.
In the middle it may not be spinning that fast.
But as you go further and further out along the propeller, it's going faster and faster.
In fact, if you double the distance you're going out, you double the speed that's going.
And so while most of the propeller may be below the speed of sound, the tip of it might break the sound barrier.
Even with 1940s jet engines, no-one could figure out how to break the sound barrier.
Until they remembered Ernst Mach.
He said the perfect shape was like a long, thin cigar.
Streamline the nose, bend the wings to reduce the shock waves, and your jet fighter's ready to make a sonic boom.
Not just once when you break the sound barrier, but the whole time you're supersonic.
Everybody under the flight path is hit by the sonic boom, but at slightly different times as the plane passes overhead.
Now, to be honest, it's all rather unpleasant, unless you're in the aeroplane, then it's enormously good fun.
The boom presents itself on the ground like a giant red carpet.
But there's a second boom from the tail, when the air rushes in to fill the gap.
So while only the rich could afford to go supersonic on Concorde, everyone on the ground got the booms for free.
So now that we've broken through this so-called sound barrier, we can fly anywhere at top speed.
Brilliant! But have your ever stopped to wonder In the 17th century, architect, scientist and all-round genius Robert Hooke was working on new theories of springs and gravity.
That's how he got the bright idea that the fastest way round the world was through the middle.
We're used to thinking of gravity as down to the ground.
But in fact its acting like it's pulling us towards the centre of the earth, so if you have a mine shaft, it will pull you down the mine shaft.
Hooke's plan was to first drill a hole 20 feet wide and 8,000 miles deep.
Then, suck out all the air and do something useful with it.
All you've got to do now is drop like a stone.
According to Hooke, after 21 minutes and six seconds, you'll hit about 18,000 miles an hour.
But at the centre, down turns up and you begin to slow down.
So after exactly 42 minutes and 12 seconds, gravity brings you to a nice, gradual stop before it takes you home again, like a spring.
So how does it work? Well, if this is the earth, and this is your hole, gravity is still pulling you down as you travel through the hole.
So gravity is always working towards the centre.
Actually when you pass the centre there's more mass behind you so you're going to be slowed down and attracted back.
And then you go back down the tunnel again.
And you just keep doing it.
You just keep bouncing backwards and forwards and that's called simple harmonic motion.
Now, I know what some of you are thinking.
If Hooke's idea worked, and that's a pretty big if, it would only be useful for travelling between opposite sides of the world.
But here comes the really weird bit.
You'd think the biggest problem is that the centre of the earth is molten magma.
But Robert Hooke said you could miss it out completely.
A shorter shaft, say London to Los Angeles, works just as well.
You don't get pulled so much by gravity, so you go a bit slower.
But the crazy thing is it still takes 42 minutes and 12 seconds.
The reason is although the distance is shorter, the force is pulling you less.
And they cancel each other out and you get the same time wherever you go.
Zanzibar to Alaska, or Moscow to Washington, It's always 42 minutes 12 seconds, from anywhere to anywhere else.
The gravity express isn't just for humans.
You could order pizza from Italy and for once, it would take less than 45 minutes.
Right.
So if gravity is like a spring, then the basic Theory of Gravity, ie.
"What Goes Up, Must Come Down", should be correct.
Or is it? Because some of you would have just thought of an obvious question.
Trees fall down, Trousers fall down, And dead pigeons fall down.
So why not the Moon? Because the moon doesn't fall to the ground like everything else we see around us, you kind of imagine that there's something holding it up.
But that's not true.
It took a genius like Isaac Newton to realise that the same force that makes things fall down is the force that makes the moon go around the earth and the earth go around the sun.
In 1687, Isaac Newton explained the whole thing in his masterpiece, Principia Mathematica.
First, get rid of the atmosphere.
It just gets in the way.
Now we need a very high mountain and a volunteer as a human cannonball.
A decent bang, and he'll go right over the horizon before gravity brings him back to Earth.
But fire him fast enough, at five miles a second, and you get to the clever bit.
He falls towards the Earth at exactly the same rate the Earth curves away from him.
So he's falling, but he never comes down.
And that's what happens when you're in orbit.
The direction always changes because you go around the earth, but your speed stays the same.
It's as if gravity is swinging him around on a string.
With no air resistance to slow him down, he'll stay in orbit for ever.
The Moon does the same thing, but the string's a bit longer, about a quarter of a million miles.
The Apollo astronauts left mirrors on the Moon, so we can measure the distance using a laser.
We all know the Moon's gravity causes the tides, but get this.
The earth's rotation pulls the tidal bulge just ahead of the moon.
This bulge pulls back on the moon for a slingshot effect.
That makes the moon move an inch and a half further away every year.
So I'm sorry to report that the Moon is really falling up.
Mmm.
But let's get back to what gravity does best.
Making things fall from the sky, at very high speed.
Sooner or later, they reach something known as Terminal Velocity.
And if that sounds scary, it isn't necessarily fatal.
Just ask your cat.
In New York in 1987, it was practically raining cats.
More than 100 fell from six storeys or more on to concrete sidewalks.
But cats really must have nine lives because nine out of ten of them survived.
So you got to be thinking, who is it that's throwing all these cats out of buildings? And it turns out it wasn't deliberate, some of the windows of the apartments kind of opened up into the apartment and if the owner didn't notice that their cat was asleep on the windowsill and they closed the window, it would have the side effect of ejecting the cat.
So, you've got a whole bunch of cats that have fallen 20 meters or so, and they're being taken to the vets with nothing more than a few bumps and bruises.
It's all down to the fact that cats have a non-fatal terminal velocity.
It sounds impossible, but for cats, it's basic physics.
Isaac Newton said gravity makes everything accelerate at the same rate, from apples to grand pianos.
But only if there's no air resistance or drag.
In the real world drag builds up until it cancels out gravity, and a falling object hits a constant speed - its terminal velocity.
It's different for different shapes and sizes.
About 200 miles an hour for a piano, a bit slower for Isaac Newton, and just 60 miles an hour for a cat.
Small things have relatively more surface area than large things, so air resistance has a greater effect.
The cat, essentially, has a built-in parachute.
Leonard da Vinci designed the first parachute back in 1483.
Bigger surface area means more air resistance to slow you down.
But with a cat, it's automatic.
His ears have a built in gyroscopic motion sensor which he uses to get Head Up, Paws Down.
At terminal velocity, he can't feel he's accelerating any more.
So he chills out, and stretches out.
Given time, he gets to a slower terminal velocity.
A cat needs to fall from the 7th floor or higher so it's got enough time in the air to fully rotate around, land on the ground, and just stroll off.
To paraphrase the great biologist J.
B.
S.
Haldane, a horse splashes, a man is broken, but a cat just walks away.
So the happy fact is, the bigger the fall, the better his chances.
Nine out of ten New York cats prove it.
Perhaps by landing on something less advanced.
Thanks to Sir Isaac Newton, we now understand Terminal Velocity.
Although next time you go outside, it's unlikely to be raining cats.
It's more likely to be raining rain.
And that leads us to a very important scientific question.
You should never leave a mathematician go out in the rain because they'll insist on calculating the best way to stay dry.
First things first, you need to break down the question into simple, easy chunks.
So, point number one, how much rain falls on your head? Point number two, how much rain do you collect on your front? You get the same amount of rain on your front - you collect it up over the path of your walk - no matter how fast you go.
So it makes no difference.
Unless there's a wind.
If there's wind, it gets a little bit tricky.
Raindrops fall at the terminal velocity of about 15 miles an hour.
But wind can blow them sideways, at around seven miles an hour.
You can walk through it at four miles an hour, or try running at ten.
But mathematically, humans are a difficult shape to deal with.
So let's keep it simple, with rectangles.
Now, please pay attention for the emergency rain procedure.
If there's no wind, you should run not walk.
You'll get exactly the same amount of rain on your front, you're just sweeping it up faster.
But you'll get home sooner, so less rain falls on your head.
It's a different story when the wind's blowing.
If the rain's coming right at you, bend double, so it has a smaller area to hit.
Brilliant, except you can't see where you're going.
The really clever bit is when the wind's behind you.
Now the trick is to match your speed to the wind so none of it hits your back or front.
Unfortunately the wind isn't always going the way you want it to.
And finally, there's bad news for large people.
You've got a lot of surface area, and that soaks up a lot of rain.
So, I'm sorry.
You should always run home, no matter what.
But don't forget the real world is more complicated.
You'll probably get soaked anyway.
Getting wet is annoying but it won't actually kill you.
But there's no escaping the fact that there are times when you will need to run away from things at very high speed.
So the answer to my next question should be particularly useful if you find yourself being pursued down a high street by a giant dinosaur.
To answer a silly question, you need a silly bird.
Ostrich legs are nothing like a human's, unless it's running backwards, of course, which is just weird.
But an ostrich is like a T-Rex, because birds evolved from dinosaurs and therefore have similar skeletons.
So scientists have worked out a formula that links the spacing of ostrich footprints to how fast they run.
And since birds are like dinosaurs, the same formula should tell us how fast T-Rex was.
Unfortunately, he must have covered his tracks, because we can't find any.
We do know small dinosaurs can run about eight miles per hour.
But we don't have those figures for the T-Rex.
So, scientists tried Plan B.
Imagine a bird pumped up to T-Rex size to make the world's first six tonne chicken.
But if you grow in size, you grow massively in weight, far more than you grow in strength.
King Kong would be lucky if he could lift his own finger, never mind climb the Empire State Building.
I hate it when science ruins a perfectly good movie.
Thing is, unlike King Kong, T-Rex really did exist, and he really could move.
So how? Where the movies get it wrong is they scale things up to these large sizes without taking into account what would actually happen if you did that.
It's all about scale factors.
If you double the height of something, then its strength goes up squared, but its weight goes up cubed.
So this is why there are no giant ants around because the strategy they have for supporting their own weight would just not work if you scale them up to the size of an elephant.
To run like a six tonne chicken, T-Rex would need to be more than 100% muscle.
In other words, impossible.
So forget giant chickens.
Let's try some real giants.
Elephants never lift all four feet, because the impact's too big for their bones.
This is known as Groucho-Running, after comedy legend Groucho Marx.
So it's possible T-Rex did the same thing, to take the strain off his legs.
He didn't sprint like the fastest humans, at 27 miles an hour.
But he could still do about 15 miles an hour.
Try outrunning T-Rex yourself, and see how far you get.
At least you can see T-Rex coming.
How do you avoid invisible, microscopic nasties like viruses? I mean, when I was at school they used to say "Coughs and Sneezes Spread Diseases".
But unfortunately, it looks like the flu can get around quite a bit quicker than that.
Congratulations! You're the first to catch a new and horrible, mutant flu virus.
So after you've called the doctor, call a mathematician, because now it's all about numbers.
The key number is how many people you pass it on to.
If that's exactly one, then you get better, and your friend takes your place, so the outbreak isn't growing.
He won't be as happy about that as you are.
If it's more than one, it's an epidemic, and now everyone can get it.
Its number is known as the basic reproductive ratio.
The higher the number, the more infectious the disease.
Contact is one of the ways these things spread.
If you're on a desert island on your own, you're not going to be infecting anybody, whereas if you're on a packed train there's lots of potential people you could infect.
So that's why you really need to take a tissue with you.
For measles, every victim potentially infects another 14.
But for flu, the average is only 1.
8.
And we can stamp it out completely by making that number less than one.
The bad news is that means vaccination.
It's from the Latin for cow, because the first vaccines were for cowpox.
But the strange thing is you don't need to vaccinate them all.
Just enough and even unvaccinated cows are safe in a bubble of vaccinated cows.
And it's the same for us humans too, although the bubble probably smells a little bit better.
You see, you reach a point where every individual benefits from so-called Herd Immunity.
Which is just as well, really, because vaccinating everybody would be terribly expensive.
Chicken or fish? But these days, flu is getting its numbers back up by hijacking airliners.
20 million flights a year, each able to carry flu at 600 miles an hour.
In 2009, the H1N1 virus went from one tourist in Mexico to one million Americans in six weeks.
Maybe you should just stay at home for a bit.
So dinosaurs come on big and slow.
Viruses come on small and fast.
But what happens when you get hit by something big and fast? Your only hope then is quick thinking.
So When Mother Nature turns nasty, a few handy hints make all the difference.
If a hurricane's coming, the wrong place to be is out at sea.
As it spins, it sucks energy from the ocean, whipping up winds of 160 miles an hour.
But the hurricane itself never moves faster than 50 miles an hour, so you can get out of the way.
A hurricane spins mighty fast.
In fact, the winds have to be going over 74 miles an hour for it to be classified as a hurricane and not just a tropical storm.
But if a tsunami's coming, forget about it.
Ocean waves go faster in deeper water.
So, out at sea, a tsunami beats a Jumbo Jet.
But the strange thing is the wave's only a few feet high, and it's hundreds of miles from one peak to the next.
You'll bob up and down so slowly that you won't even feel it.
It's hard to imagine you wouldn't notice a 600 mile an hour wave.
But, of course, if the peaks of the wave are 600 miles apart then it takes a whole hour to go past.
So it's safer there than on the beach.
In shallow water, a tsunami slows down to 30 miles an hour, but grows into a wall of water 100 feet high.
This sucks all the water from the shore.
If you see that, run, and don't stop until you reach the mountains.
But don't jump for joy.
Nine out of ten people who die in avalanches start it themselves.
A slope can be covered in layers of snow.
You can have really strong layers that are inter-connected big, strong snow crystals.
On top of that, you may have a layer of snow that melted and refrozen.
That's a weak layer.
If you get a big dump of powder on top of that, that is a ticking time bomb and you, as a skier or a boarder, could be the straw that breaks the camel's back.
You could cause that whole layer just to break off and go flying down the slope.
This is actually a phenomenon that's all over science in that you get an unstable equilibrium, we call it.
You've got something balanced essentially on a fine pin and the slightest disturbance will release a huge amount of potential energy.
Ten million tons of snow dropping at 80 miles an hour, and your only chance is swimming for the surface before the snow sets like concrete.
You'll suffocate if you get buried.
Yes! Nothing to worry about now, except a comet impact, of course, at 25,000 mph, with the possible extinction of life on earth.
We're still working on a handy hint for that one.
Well, we've covered a lot of ground in a very short time.
And as you can see, the road to understanding speed is long and dangerous.
I think it's time I make a speedy exit.
But the thing is, do we even know what speed is? Or the answers to questions like Hold on tight, while I answer the Things You Need to Know about Speed.
Let's start with the basics.
You'd think measuring the speed of anything was kids' stuff.
It's the distance from A to B, divided by the time it takes to get there.
BEAR ROARS So a grizzly bear in a bad mood does about 30 miles an hour.
A bullet train travels at 186 miles an hour.
And Australia is heading for China at two inches a year, the same speed as your fingernails grow.
Speed is a concept that we're all quite familiar with.
But it can be described very simply by an equation.
But a simple equation.
Speed is simply the distance you've travelled, divided by the time its taken you to travel it.
But actually it becomes a bit more complicated than that.
If you say what's my speed, well I'm not moving.
But I am moving because the earth's moving.
It's very odd if you think about it.
There's never just one answer to the question what's my speed? The trouble is, you always need a frame of reference to measure your speed against.
If there was nothing else in the universe, you couldn't even tell whether you were moving or not.
Maybe you're going scarily fast.
Speed is always relative because it depends on the frame of reference.
A baby can throw his rattle and think it's going five miles an hour.
But if he's on a train doing 500 miles an hour, and you're on the platform, the speed of the rattle adds up.
Then again, if baby fires his machine gun backwards at 500 miles an hour, you could catch the bullets in your teeth, because the speeds cancel out.
In the right frame of reference, your speed can be unbelievably fast.
For watching aliens, everything on Earth, including us, is going around the Sun at over 67,000 miles an hour.
And the sun is moving through the milky way at nearly There's no point telling the judge that all speed is relative.
He's heard that one before.
Because as long as there have been drivers, they've been driving too fast.
And the police have been asking a question to which they already know the answer.
Even in the steam powered 1860s, there was a speed limit.
It was two miles an hour plus a man with a flag.
Well, initially, you needed three people in the car.
A driver, a stoker and someone to walk ahead with a red flag so you didn't scare the horses.
And often it was just much quicker to walk.
In 1896, petrol-crazed Walter Arnold of Peckham got the first speeding fine for doing eight miles an hour.
But then the law went mad, and upped the speed limit to 14 miles an hour, and got rid of the flag.
So in New England, the speed trap was born.
One of the first victims was the New York Police Commissioner William McAdoo.
We don't know if he paid his fine.
Speeding tickets were extortionate.
They were £5, which is equivalent to a month's salary, or you had to spend four weeks in jail.
The first speed cameras flashed in 1905 with a time stamp at each end of the trap, to work out your speed.
The Automobile Association hit back with cyclists, to warn their members of hidden speed traps.
But this was later deemed illegal.
So AA cycle scouts began to salute all their members instead.
If they didn't, it meant speed trap ahoy! But in the 50s, police technology overtook them.
A radar gun fires a beam of microwaves.
When they hit your car, they change frequency, depending on how fast you're going.
So the reflected beam tells the police your speed.
The down side is that drivers detect the radar before the radar detects them.
Die hards even try absorbent paint so the beam can't bounce back.
But this probably only really works if you're in a Stealth Bomber.
It pays to watch your speed at all times, but also to keep your ears open.
At 70 miles an hour, you might hear this.
POLICE SIREN But go eleven times faster, and you'll hear this.
SONIC BOOM A sound that means my next question has arrived.
You don't need a jet fighter to make a sonic boom.
Nearly 5,000 years ago we discovered them, along with the simple whip.
But no-one knew what made the crack, until Austrian scientist Ernst Mach figured it out in 1887.
He realised sound waves were like ripples in water.
When a boat goes faster than the ripples, they've got nowhere to go, so they bunch up in a wake.
The same thing happens with a whip.
Sound waves travel through the air, at 768 miles an hour.
But the tip of a whip goes faster, bunching up the sound waves, and producing a very loud shock wave.
A whip gets narrower and lighter all the way to the tip.
So when you snap it at the top, the wave travels faster and faster and faster and the tip can actually be going about 30 times as fast as that initial snap.
That means that the tip is going faster than the speed of sound so the sound waves can't propagate away from it, they kind of form a wake of sound, then you hear it in one big blast.
That's a sonic boom.
This is all good fun for pistol-packin', whip-crackin' cowboys, but bad news for 1920s pilots.
Planes were slow, but the propeller tips were spinning near the speed of sound.
In fact, parts of a plane can go supersonic, even when the whole plane is not supersonic.
To understand that, it's to do with the propellers.
In the middle it may not be spinning that fast.
But as you go further and further out along the propeller, it's going faster and faster.
In fact, if you double the distance you're going out, you double the speed that's going.
And so while most of the propeller may be below the speed of sound, the tip of it might break the sound barrier.
Even with 1940s jet engines, no-one could figure out how to break the sound barrier.
Until they remembered Ernst Mach.
He said the perfect shape was like a long, thin cigar.
Streamline the nose, bend the wings to reduce the shock waves, and your jet fighter's ready to make a sonic boom.
Not just once when you break the sound barrier, but the whole time you're supersonic.
Everybody under the flight path is hit by the sonic boom, but at slightly different times as the plane passes overhead.
Now, to be honest, it's all rather unpleasant, unless you're in the aeroplane, then it's enormously good fun.
The boom presents itself on the ground like a giant red carpet.
But there's a second boom from the tail, when the air rushes in to fill the gap.
So while only the rich could afford to go supersonic on Concorde, everyone on the ground got the booms for free.
So now that we've broken through this so-called sound barrier, we can fly anywhere at top speed.
Brilliant! But have your ever stopped to wonder In the 17th century, architect, scientist and all-round genius Robert Hooke was working on new theories of springs and gravity.
That's how he got the bright idea that the fastest way round the world was through the middle.
We're used to thinking of gravity as down to the ground.
But in fact its acting like it's pulling us towards the centre of the earth, so if you have a mine shaft, it will pull you down the mine shaft.
Hooke's plan was to first drill a hole 20 feet wide and 8,000 miles deep.
Then, suck out all the air and do something useful with it.
All you've got to do now is drop like a stone.
According to Hooke, after 21 minutes and six seconds, you'll hit about 18,000 miles an hour.
But at the centre, down turns up and you begin to slow down.
So after exactly 42 minutes and 12 seconds, gravity brings you to a nice, gradual stop before it takes you home again, like a spring.
So how does it work? Well, if this is the earth, and this is your hole, gravity is still pulling you down as you travel through the hole.
So gravity is always working towards the centre.
Actually when you pass the centre there's more mass behind you so you're going to be slowed down and attracted back.
And then you go back down the tunnel again.
And you just keep doing it.
You just keep bouncing backwards and forwards and that's called simple harmonic motion.
Now, I know what some of you are thinking.
If Hooke's idea worked, and that's a pretty big if, it would only be useful for travelling between opposite sides of the world.
But here comes the really weird bit.
You'd think the biggest problem is that the centre of the earth is molten magma.
But Robert Hooke said you could miss it out completely.
A shorter shaft, say London to Los Angeles, works just as well.
You don't get pulled so much by gravity, so you go a bit slower.
But the crazy thing is it still takes 42 minutes and 12 seconds.
The reason is although the distance is shorter, the force is pulling you less.
And they cancel each other out and you get the same time wherever you go.
Zanzibar to Alaska, or Moscow to Washington, It's always 42 minutes 12 seconds, from anywhere to anywhere else.
The gravity express isn't just for humans.
You could order pizza from Italy and for once, it would take less than 45 minutes.
Right.
So if gravity is like a spring, then the basic Theory of Gravity, ie.
"What Goes Up, Must Come Down", should be correct.
Or is it? Because some of you would have just thought of an obvious question.
Trees fall down, Trousers fall down, And dead pigeons fall down.
So why not the Moon? Because the moon doesn't fall to the ground like everything else we see around us, you kind of imagine that there's something holding it up.
But that's not true.
It took a genius like Isaac Newton to realise that the same force that makes things fall down is the force that makes the moon go around the earth and the earth go around the sun.
In 1687, Isaac Newton explained the whole thing in his masterpiece, Principia Mathematica.
First, get rid of the atmosphere.
It just gets in the way.
Now we need a very high mountain and a volunteer as a human cannonball.
A decent bang, and he'll go right over the horizon before gravity brings him back to Earth.
But fire him fast enough, at five miles a second, and you get to the clever bit.
He falls towards the Earth at exactly the same rate the Earth curves away from him.
So he's falling, but he never comes down.
And that's what happens when you're in orbit.
The direction always changes because you go around the earth, but your speed stays the same.
It's as if gravity is swinging him around on a string.
With no air resistance to slow him down, he'll stay in orbit for ever.
The Moon does the same thing, but the string's a bit longer, about a quarter of a million miles.
The Apollo astronauts left mirrors on the Moon, so we can measure the distance using a laser.
We all know the Moon's gravity causes the tides, but get this.
The earth's rotation pulls the tidal bulge just ahead of the moon.
This bulge pulls back on the moon for a slingshot effect.
That makes the moon move an inch and a half further away every year.
So I'm sorry to report that the Moon is really falling up.
Mmm.
But let's get back to what gravity does best.
Making things fall from the sky, at very high speed.
Sooner or later, they reach something known as Terminal Velocity.
And if that sounds scary, it isn't necessarily fatal.
Just ask your cat.
In New York in 1987, it was practically raining cats.
More than 100 fell from six storeys or more on to concrete sidewalks.
But cats really must have nine lives because nine out of ten of them survived.
So you got to be thinking, who is it that's throwing all these cats out of buildings? And it turns out it wasn't deliberate, some of the windows of the apartments kind of opened up into the apartment and if the owner didn't notice that their cat was asleep on the windowsill and they closed the window, it would have the side effect of ejecting the cat.
So, you've got a whole bunch of cats that have fallen 20 meters or so, and they're being taken to the vets with nothing more than a few bumps and bruises.
It's all down to the fact that cats have a non-fatal terminal velocity.
It sounds impossible, but for cats, it's basic physics.
Isaac Newton said gravity makes everything accelerate at the same rate, from apples to grand pianos.
But only if there's no air resistance or drag.
In the real world drag builds up until it cancels out gravity, and a falling object hits a constant speed - its terminal velocity.
It's different for different shapes and sizes.
About 200 miles an hour for a piano, a bit slower for Isaac Newton, and just 60 miles an hour for a cat.
Small things have relatively more surface area than large things, so air resistance has a greater effect.
The cat, essentially, has a built-in parachute.
Leonard da Vinci designed the first parachute back in 1483.
Bigger surface area means more air resistance to slow you down.
But with a cat, it's automatic.
His ears have a built in gyroscopic motion sensor which he uses to get Head Up, Paws Down.
At terminal velocity, he can't feel he's accelerating any more.
So he chills out, and stretches out.
Given time, he gets to a slower terminal velocity.
A cat needs to fall from the 7th floor or higher so it's got enough time in the air to fully rotate around, land on the ground, and just stroll off.
To paraphrase the great biologist J.
B.
S.
Haldane, a horse splashes, a man is broken, but a cat just walks away.
So the happy fact is, the bigger the fall, the better his chances.
Nine out of ten New York cats prove it.
Perhaps by landing on something less advanced.
Thanks to Sir Isaac Newton, we now understand Terminal Velocity.
Although next time you go outside, it's unlikely to be raining cats.
It's more likely to be raining rain.
And that leads us to a very important scientific question.
You should never leave a mathematician go out in the rain because they'll insist on calculating the best way to stay dry.
First things first, you need to break down the question into simple, easy chunks.
So, point number one, how much rain falls on your head? Point number two, how much rain do you collect on your front? You get the same amount of rain on your front - you collect it up over the path of your walk - no matter how fast you go.
So it makes no difference.
Unless there's a wind.
If there's wind, it gets a little bit tricky.
Raindrops fall at the terminal velocity of about 15 miles an hour.
But wind can blow them sideways, at around seven miles an hour.
You can walk through it at four miles an hour, or try running at ten.
But mathematically, humans are a difficult shape to deal with.
So let's keep it simple, with rectangles.
Now, please pay attention for the emergency rain procedure.
If there's no wind, you should run not walk.
You'll get exactly the same amount of rain on your front, you're just sweeping it up faster.
But you'll get home sooner, so less rain falls on your head.
It's a different story when the wind's blowing.
If the rain's coming right at you, bend double, so it has a smaller area to hit.
Brilliant, except you can't see where you're going.
The really clever bit is when the wind's behind you.
Now the trick is to match your speed to the wind so none of it hits your back or front.
Unfortunately the wind isn't always going the way you want it to.
And finally, there's bad news for large people.
You've got a lot of surface area, and that soaks up a lot of rain.
So, I'm sorry.
You should always run home, no matter what.
But don't forget the real world is more complicated.
You'll probably get soaked anyway.
Getting wet is annoying but it won't actually kill you.
But there's no escaping the fact that there are times when you will need to run away from things at very high speed.
So the answer to my next question should be particularly useful if you find yourself being pursued down a high street by a giant dinosaur.
To answer a silly question, you need a silly bird.
Ostrich legs are nothing like a human's, unless it's running backwards, of course, which is just weird.
But an ostrich is like a T-Rex, because birds evolved from dinosaurs and therefore have similar skeletons.
So scientists have worked out a formula that links the spacing of ostrich footprints to how fast they run.
And since birds are like dinosaurs, the same formula should tell us how fast T-Rex was.
Unfortunately, he must have covered his tracks, because we can't find any.
We do know small dinosaurs can run about eight miles per hour.
But we don't have those figures for the T-Rex.
So, scientists tried Plan B.
Imagine a bird pumped up to T-Rex size to make the world's first six tonne chicken.
But if you grow in size, you grow massively in weight, far more than you grow in strength.
King Kong would be lucky if he could lift his own finger, never mind climb the Empire State Building.
I hate it when science ruins a perfectly good movie.
Thing is, unlike King Kong, T-Rex really did exist, and he really could move.
So how? Where the movies get it wrong is they scale things up to these large sizes without taking into account what would actually happen if you did that.
It's all about scale factors.
If you double the height of something, then its strength goes up squared, but its weight goes up cubed.
So this is why there are no giant ants around because the strategy they have for supporting their own weight would just not work if you scale them up to the size of an elephant.
To run like a six tonne chicken, T-Rex would need to be more than 100% muscle.
In other words, impossible.
So forget giant chickens.
Let's try some real giants.
Elephants never lift all four feet, because the impact's too big for their bones.
This is known as Groucho-Running, after comedy legend Groucho Marx.
So it's possible T-Rex did the same thing, to take the strain off his legs.
He didn't sprint like the fastest humans, at 27 miles an hour.
But he could still do about 15 miles an hour.
Try outrunning T-Rex yourself, and see how far you get.
At least you can see T-Rex coming.
How do you avoid invisible, microscopic nasties like viruses? I mean, when I was at school they used to say "Coughs and Sneezes Spread Diseases".
But unfortunately, it looks like the flu can get around quite a bit quicker than that.
Congratulations! You're the first to catch a new and horrible, mutant flu virus.
So after you've called the doctor, call a mathematician, because now it's all about numbers.
The key number is how many people you pass it on to.
If that's exactly one, then you get better, and your friend takes your place, so the outbreak isn't growing.
He won't be as happy about that as you are.
If it's more than one, it's an epidemic, and now everyone can get it.
Its number is known as the basic reproductive ratio.
The higher the number, the more infectious the disease.
Contact is one of the ways these things spread.
If you're on a desert island on your own, you're not going to be infecting anybody, whereas if you're on a packed train there's lots of potential people you could infect.
So that's why you really need to take a tissue with you.
For measles, every victim potentially infects another 14.
But for flu, the average is only 1.
8.
And we can stamp it out completely by making that number less than one.
The bad news is that means vaccination.
It's from the Latin for cow, because the first vaccines were for cowpox.
But the strange thing is you don't need to vaccinate them all.
Just enough and even unvaccinated cows are safe in a bubble of vaccinated cows.
And it's the same for us humans too, although the bubble probably smells a little bit better.
You see, you reach a point where every individual benefits from so-called Herd Immunity.
Which is just as well, really, because vaccinating everybody would be terribly expensive.
Chicken or fish? But these days, flu is getting its numbers back up by hijacking airliners.
20 million flights a year, each able to carry flu at 600 miles an hour.
In 2009, the H1N1 virus went from one tourist in Mexico to one million Americans in six weeks.
Maybe you should just stay at home for a bit.
So dinosaurs come on big and slow.
Viruses come on small and fast.
But what happens when you get hit by something big and fast? Your only hope then is quick thinking.
So When Mother Nature turns nasty, a few handy hints make all the difference.
If a hurricane's coming, the wrong place to be is out at sea.
As it spins, it sucks energy from the ocean, whipping up winds of 160 miles an hour.
But the hurricane itself never moves faster than 50 miles an hour, so you can get out of the way.
A hurricane spins mighty fast.
In fact, the winds have to be going over 74 miles an hour for it to be classified as a hurricane and not just a tropical storm.
But if a tsunami's coming, forget about it.
Ocean waves go faster in deeper water.
So, out at sea, a tsunami beats a Jumbo Jet.
But the strange thing is the wave's only a few feet high, and it's hundreds of miles from one peak to the next.
You'll bob up and down so slowly that you won't even feel it.
It's hard to imagine you wouldn't notice a 600 mile an hour wave.
But, of course, if the peaks of the wave are 600 miles apart then it takes a whole hour to go past.
So it's safer there than on the beach.
In shallow water, a tsunami slows down to 30 miles an hour, but grows into a wall of water 100 feet high.
This sucks all the water from the shore.
If you see that, run, and don't stop until you reach the mountains.
But don't jump for joy.
Nine out of ten people who die in avalanches start it themselves.
A slope can be covered in layers of snow.
You can have really strong layers that are inter-connected big, strong snow crystals.
On top of that, you may have a layer of snow that melted and refrozen.
That's a weak layer.
If you get a big dump of powder on top of that, that is a ticking time bomb and you, as a skier or a boarder, could be the straw that breaks the camel's back.
You could cause that whole layer just to break off and go flying down the slope.
This is actually a phenomenon that's all over science in that you get an unstable equilibrium, we call it.
You've got something balanced essentially on a fine pin and the slightest disturbance will release a huge amount of potential energy.
Ten million tons of snow dropping at 80 miles an hour, and your only chance is swimming for the surface before the snow sets like concrete.
You'll suffocate if you get buried.
Yes! Nothing to worry about now, except a comet impact, of course, at 25,000 mph, with the possible extinction of life on earth.
We're still working on a handy hint for that one.
Well, we've covered a lot of ground in a very short time.
And as you can see, the road to understanding speed is long and dangerous.
I think it's time I make a speedy exit.