Author Archives: Eann Patterson

Slowing down time to think [about strain energy]

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161-6167_imgLet me take you bungee jumping.  I should declare that I am not qualified to do so, unless you count an instructor’s certificate for rock-climbing and abseiling, obtained about forty years ago.  For our imaginary jump, pick a bridge with a good view and a big drop to the water below and I’ll meet you there with the ropes and safety gear.

It’s a clear early morning and the air is crisp and fresh – ideal for throwing yourself off a bridge attached to a rope.  The rope is the star of this event.  It’s brand new, which is reassuring, and arrived coiled over my shoulder.  A few days ago, I asked you how much you weigh – that’s your real weight fully clothed, at least I hope that’s the number you gave me otherwise my calculations will be wrong and you’ll get wet this morning!  I have calculated how much the rope will stretch when it arrests your free-fall from the bridge parapet; so, now I am measuring out enough rope to give you an exciting fall but to stop you short of the water.  I’m a professor of structural materials and mechanics so I feel confident of getting this bit right; but it’s a long time since I worked as an abseiling instructor so I suggest you check those knots and that harness that we’ve just tightened around you.

You’ve swung yourself over the parapet and you’re standing on the ledge that the civil engineers conveniently left for bridge jumpers.  The rope is loosely coiled ready with its end secured to a solid chunk of parapet.  As you alternate between soaking up the beautiful view and contemplating the chasm at your feet, you wonder why you agreed to come with me.  At this moment, you have a lot of potential energy due to your height above the sparkling water [potential energy is your mass multiplied by your height and gravitational acceleration], but no kinetic energy because you are standing motionless.  The rope is relaxed or undeformed and has zero strain energy.

Finally, you jump and time seems to stand still for you as the fall appears to be happening in slow motion.  The air begins to rush past your ears in a whoosh as you build up speed and gain kinetic energy [equal to one half your mass multiplied by your velocity squared].  The bridge disappeared quickly but the water below seems only to be approaching slowly as you lose height and potential energy.  In reality, your brain is playing tricks on you because you are being accelerated towards the water by gravity [at about 10 metres per second squared] but your total energy is constant [potential plus kinetic energy unchanged].  Suddenly, your speed becomes very apparent.  The water seems very close and you cry out in surprise.  But the rope is beginning to stretch converting your kinetic energy into strain energy stored by stretching its fibres [at a molecular level work is being done to move molecules apart and away from their equilibrium position].  Suddenly, you stop moving downwards and just before you hit the water surface, the rope hurls you upwards – your potential energy reached a minimum and you ran out of kinetic energy to give the rope; so now it’s giving you back that stored strain energy [and the molecules are relaxing to their equilibrium position].  You are gaining height and speed so both your kinetic and potential energy are rising with that squeal that just escaped from you.

Now, you’ve noticed that the rope has gone slack and you’re passing a loop of it as you continue upwards but more slowly.  The rope ran out of strain energy and you’re converting kinetic energy into potential energy.  Just as you work out that’s happening, you run out of kinetic energy and you start to free-fall again.

Time no longer appears to stationary and your brain is working more normally.  You begin to wonder how many times you’ll bounce [quite a lot because the energy losses due to frictional heating in the rope and drag on your body are relatively small] and why you didn’t ask me what happens at the end.  You probably didn’t ask because you were more worried about jumping and were confident that I knew what I was doing, which was foolish because, didn’t I tell you, I’ve never been bungee jumping and I have no idea how to get you back up onto the bridge.  How good were you at rope-climbing in the gym at school?

When eventually you stop oscillating, the rope will still be stretched due to the force on it generated by your weight.  However, we can show mathematically that the strain energy and deformation under this static load will be half the values experienced under the dynamic loading caused by your fall from the bridge parapet.  That means you’ll have a little less distance to climb to the parapet!

Today’s post is a preview for my new MOOC on ‘Understanding Super Structures’, which is scheduled to start on May 22nd, 2017.  This is the script for a step in week 2 of the five-week course, unless the director decides it’s too dangerous.  By the way, don’t try this home or on a bridge anywhere.

More violent storms

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I made a mistake last week by initially publishing two posts.  My apologies for confusing you or tantalising you with the prospect of going bungee jumping and then postponing the trip.  We’ll go bungee jumping next week.  I postponed it because it’s a preview of the new MOOC on ‘Understanding Super Structures‘ that I am writing and there was a delay in publishing the registration page for the MOOC.

When I posted my comment about postponing the bungee jump due to rain, I didn’t realize that, the following day Liverpool would be battered by Storm Doris, with 90 miles per hour winds that closed the Port of Liverpool.  As I sat writing week 4 of the new MOOC, the wind was swirling around our house causing the windows to rattle; and, on the top storey of our narrow but tall house, you could feel the house moving in the gusts of wind.  Across the street, people visiting Liverpool Cathedral were hanging onto the railings as they made their way to the entrance, and the trees were being bent over to an angle that made you think there would be a loud cracking and splintering of wood at any moment.  Fortunately, the storm was short-lived in Liverpool and moved on to wreak havoc inland.  Bungee jumping would have been very hazardous!

The number of violent storms appears to be increasing and the graphic shows the number of storms in the Atlantic basin since 1850.  Although there is a lot of scatter in the data, there is a clear concentration in the last couple of decades of years with fifteen of more named storms, which suggests there has been more energy in the weather systems in recent years.  The primary source of this energy is the temperature of the oceans and atmosphere.  There is a good account of the development of storms cells in Manuel Delanda’s book ‘Philosophy and Simulation: The Emergence of Synthetic Reason‘, see chapter 1 – The Storm in the Computer, which is available via Google Preview.

The increased frequency of high-energy storm systems is a very apparent manifestation of climate change that is having an impact on many people.  Yet, some governments refuse to even consider the possibility that our climate is changing and that they need to lead our society in discussing and planning strategies to mitigate the impacts.  It reminds me of the saying, attributed to Henri Poincare: ‘To doubt everything, or, to believe everything, are two equally convenient solutions; both dispense with the necessity of reflection.’

‘Rain bringing out the contours in everything’

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16blindness9-custom1Last October I cited John Hull’s audio diary in which he said ‘Cognition is beautiful.  It is beautiful to know.’ [See my post entitled ‘Cognition is beautiful‘ on October 19th, 2016]  Last week, I watched the film ‘Notes on Blindness‘ based on his book ‘Touching the Rock‘.  We found it a moving and life-enriching experience. At one point, John Hull, after he has lost all of his sight, opens his front door during a rain storm and describes the beauty of the rain.  “Rain has a way of bringing out the contours of everything; it throws a coloured blanket over previously invisible things; instead of an intermittent and thus fragmented world, the steadily falling rain creates continuity of acoustic experience.”  You can read more of this extract at www.johnmhull.biz/Touching the Rock.html in which John wishes that rain could fall inside a room to give him a sense of the things in the room.  This seemed particularly poignant to me, a sighted person, who benefits from photons raining down on everything around us during daylight or when the light is switched on.  The photons cause light waves to radiate from every surface in a similar way that the rain drops cause sound waves to radiate from everything as John experienced. Our eyes are amazing with 137 million separate ‘seeing’ elements on the retina, or in digital camera terms, that’s 137 megapixels.  But to quote the Roman poet, Lucretius who in his poem ‘De Rerum Natura’ wrote “Nothing in the body is made that we may use it.  What happens to exist is the cause of its use.”  In other words, we do not have eyes so that we can see but we see because we have eyes.  John Hull discovered new ways to experience the world using what was available to him although he struggled with what he had lost.  It is difficult to imagine losing one’s sight but his diary and the film bring us considerably closer to an appreciation of the loss.

Yes, I know I switched from a particle to wave description of light but I wanted to emphasize that the photons don’t just bounce off surfaces, otherwise all surfaces would look the colour of the illuminating light.

 

Sources:

John M. Hull, Touching the Rock: an experience of blindness, London: SPCK, 2016

Lucretius (author), Alicia Stallings (translator), The Nature of Things, London: Penguin Classics, 2007.

Charles Sherrington, Making of the eye, in The Faber Book of Science, John Carey (ed), London: Faber & Faber, 1995.

Picture: A production still from the film, Notes on Blindness, from NY Times on January 16th, 2016.

 

Digital limits analogue future

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Feet on Holiday I 1979 Henry Moore OM, CH 1898-1986 Presented by the Henry Moore Foundation 1982 http://www.tate.org.uk/art/work/P02699

Feet on Holiday I 1979 Henry Moore OM, CH 1898-1986 Presented by the Henry Moore Foundation 1982 http://www.tate.org.uk/art/work/P02699

Digital everything is trendy at the moment.  I am as guilty as everyone else: my research group is using digital cameras to monitor the displacement and deformation of structural components using a technique called digital image correlation (see my post on 256 Shades of grey on January 22nd, 2014) .  Some years ago, in a similar vein, I pioneered a technique known as ‘digital photoelasticity’ (se my post on ‘Cow bladders lead to strain measurement‘ on January 7th, 2015..  But, what do we mean by ‘digital’?  Originally it meant related to, resembling or operated by a digit or finger.  However, electronic engineers will refer us to A-to-D and D-to-A converters that transform analogue signals into digital signals and vice versa.  In this sense, digital means ‘expressed in discrete numerical form’ as opposed to analogue which means something that can vary continuously .  Digital signals are ubiquitous because computers can handle digital information easily.  Computers could be described as very, very large series of switches that can be either on or off, which allows numbers to be represented in binary.  The world’s second largest computer, Tianhe-2, which I visited in Guangzhou a couple of years ago, has about 12.4 petabytes (about 1016 bytes) of memory which compares to 100 billion (1012) neurons an average human brain.  There’s lots of tasks at which the world’s largest computers are excellent but none of them can drive a car, ride a bicycle, tutor a group of engineering students and write a blog post on the limits of digital technology all in a few hours.  Ok, we could connect specialized computers together wirelessly under the command of one supercomputer but that’s incomparable to the 1.4 kilograms of brain cells in an engineering professor’s skull doing all of this without being reprogrammed or requiring significant cooling.

So, what’s our brain got that the world latest computer hasn’t?  Well, it appears to be analogue and not digital.  Our consciousness appears to arise from assemblies of millions of neurons firing in synchrony and because each neuron can fire at an infinite number of levels, then our conscious thoughts can take on a multiplicity of forms that a digital computer can never hope to emulate because its finite number of switches have only two positions each: on and off.

I suspect that the future is not digital but analogue; we just don’t know how to get there, yet.  We need to stop counting with our digits and start thinking with our brains.