Author Archives: markdowe1

Arts, Society

Don’t climb the wrong wall…

Where on the rung are you?

– Where on the rung are you? © A Knapsack Production 2015

Moral: Being good at climbing walls does not mean you’re climbing the right walls to achieve your goal.

Rudyard Kipling, 1865 – 1936, once reported that he kept six honest serving men, and said they taught him all he knew. Their names? What and Why and When and How and Where and Who.

These are the basic questions. As you apply them to the subject of “success”, they will clarify your understanding of what it is and how to achieve it. You will avoid climbing wrong walls.

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Science

Skimming pebbles on water. But how does it happen?

QUESTIONS OF SCIENCE

Intro: Neither medium seems especially elastic in nature, so how do the stones bounce on water? There are practical implications of the science, as this article reveals.

ONE

FOR the best results, a circular, flattish stone is best. It must be thrown so it is almost horizontal to the water’s surface, and also such that its trailing edge hits the water first. It is vital the action of the throwing imparts spin to the stone.

Any solid body moving through a liquid experiences forces that oppose its motion: these forces are proportional to the cross-sectional area of the body and the square of its speed. Although only a part of a skipping stone is actually moving through the water, with the rest travelling through air, these forces still have an effect on its forward motion.

There is a force exerted by the water at right angles to the spinning stone’s surface. It acts at the trailing edge of the spinning stone – because this is where the impact begins – and tends to turn the stone towards the horizontal. Because of its spin, though, the stone behaves like a gyroscope and refuses to change its orientation. Nevertheless, this force reduces the stone’s forward velocity somewhat.

There is also a force exerted by the water parallel to the stone’s surface, but this force is much smaller and so the stone’s velocity is barely changed by it on impact with the water. The net effect of these forces is that the stone flies from the water in a parabolic arc until it hits the water again and the whole process is repeated.

At each impact, the stone loses some of its kinetic energy, which is dissipated in the ripples that are created in the water. And as its velocity is gradually reduced, the impacts become closer together until the energy dissipated is greater than that lost in the impact and the stone sinks.

The minimum initial speed required by a stone varies with its inclination to the horizontal. Experiments show that skimming will not occur if the angle at which the flat surface of the stone hits the water is more than 45 degrees to the horizontal. The slowest speed for skimming is about 2.5 metres per second, when the inclination is about 20 degrees.

The fact that the water itself is not elastic is immaterial, but it is important that it gives way to the stone on impact.

Stones can also be skimmed on wet sand, and even on cloth-covered boards. In such cases, however, there is little or no give in the surface and the frictional grip at impact is sufficient to change the direction of the stone’s motion and also cause the stone to overcome the gyroscopic effect.

Skimming stones on water is an age-old pastime. The gunners of naval sailing ships worked out that it could be used to increase the range of cannonballs. These could be made to skip along the surface of the sea and hole enemy vessels near their waterline. However, to do this the cannon itself had to be near the sea surface, which meant the firing vessel needed perilously low ports. Indeed, some ships capsized after taking on water through those open ports. With cannonballs, the spin required by a skimming stone was unnecessary as their spherical symmetry precluded any gyroscopic effects.

Barnes Wallis’s famous bouncing bomb, created for the 1943 Dambusters raid on Germany in the Second World War, worked on the same principle. He had to use cylindrical bombs, though, so he figured out he could ensure their stability by giving them spin about a horizontal axis at right angles to their direction of motion on the water.

TWO

IN a paper published in the journal Nature, in 2004, the French physicist Lydéric Bocquet and his team of researchers revealed some of the secrets of successful stone skimming. They found that the optimum angle of attack is 20 degrees. So, even when the stone is thrown horizontally, the leading edge should be 20 degrees higher than the trailing edge. This maximises the number of jumps by limiting the contact time between the stone and the water, which is proportional to the energy dissipated.

The thrower also imparts spin to the pebble, providing a gyroscopic effect that stabilises its flight and preserves the original angle of attack when it bounces. In the absence of spin, the water would impart a torque (or turning force) on the stone and, because the trailing edge is the first to make contact with the water, this would tend to make it tumble.

The actual physics of stone skimming is not perfectly understood. However, the bounce could be understood as a result of the conservation of momentum and Newton’s third law: when the stone exerts a force on the water, the water exerts an equal and opposite force on the stone. This lifting force is proportional to the density of the water, the surface area that is wetted and the square of the forward speed of the stone. Also, the bow wave created ahead of the stone when it strikes the water might act like a water-ski jump – helping to launch the next hop. This minimises the contact time between the stone and the water, which in turn maximises the number of jumps.

Although ensuring the optimal angle of attack as the stone strikes the water, and imparting just enough spin to maintain stable flight are important, there are other factors. Selecting the correct size and shape of stone and having a fast throwing arm are examples.

Given that the urge to skim stones has been with us for thousands of years and the rules – getting the greatest distance or number of bounces – have remained unchanged since the ancient Greeks, perhaps this should become an Olympic sport. In the meantime, the current world record stands at 51 skips, set by Russell Byars in Pennsylvania on 19 July 2007.

. Science Book

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Asia, China, Economic, Europe, Government, Intelligence, Middle East, Military, National Security, NATO, Society, United States

US Defence Strategy…

(From the archives) Originally posted on January 7, 2012 by markdowe

SHIFT IN AMERICA’S DEFENCE STRATEGY

On Saturday, 07 January, 2012, the Editorial of the Daily Telegraph focused on America’s shift in defence strategy, following Thursday’s announcement by President Barack Obama. The US is to focus less on Europe and more on Asia following the rising threat of China.

The Editorial states:

‘The Pentagon briefing room rarely hosts all of America’s service chiefs, let alone the president. Its use by Barack Obama to announce the conclusions of his defence review was designed to add a sense of drama – and the occasion certainly lived up to its billing. Future historians will probably conclude that this was the week when America’s entire foreign and defence strategy pivoted decisively away from Europe and towards the Pacific. More ominously, it might also mark the onset of a new, if concealed, arms race between the US and its aspiring rival, China.

First things first: America’s military dominance will remain unchallenged for the foreseeable future. Mr Obama might have announced spending cuts of almost $500 billion over the next decade, but this amounts to a light trim for a defence machine with an annual budget of $650 billion, amounting to 45 per cent of all military expenditure in the world. America is not axing capabilities in the foolish fashion of British governments; rather, its power is being focused on the great strategic challenges of the next century. These can be simply summarised: the struggle for mastery in Asia, home of the world’s most populous countries and fastest-growing economies, and responding to sudden crises. To this end, the US will reduce its presence in Europe, cut 90,000 soldiers and bulk up in the Pacific, with new bases in Australia and elsewhere. As for other flashpoints, few will be surprised that the US policy stresses the goals of containing Iran and guaranteeing free passage through the Strait of Hormuz.

On a purely military level, two points stand out. The US might be cutting its army, but it has ruled out reducing its fleet of 11 aircraft carriers, each of which packs more punch than the entire air forces of most countries. While China’s defence budget has recorded double-digit increases for the past decade, it has still launched only one carrier – an old Russian model of doubtful combat value. Second, Mr Obama stressed his determination to invest in “intelligence, surveillance and reconnaissance”. Put simply, the US will seek to extend its lead in the most advanced combat systems: where scores of troops – and hundreds of support staff – might once have been required to dispatch a senior al-Qaeda operative, now one unmanned drone can do the job.

America’s new course could well be shifted by a strategic shock akin to the September 11 attacks. Nevertheless, this plan will have momentous consequences for Europe and Asia alike. For decades, the US has underwritten the security of the Atlantic as well as the Pacific, effectively allowing Europe a free ride and permitting a string of Nato members the luxury of running down their defence budgets. This era is rapidly coming to a close. Yet with a few honourable exceptions, such as Britain and France, European powers have failed to fund their armed forces adequately, or deploy them when needed. Germany, in particular, must overcome the burden of its history and face up to the responsibilities that go with being the Continent’s leading economic power.

Mr Obama’s address studiously refrained from mentioning China, the country that probably has most at stake. Beijing’s leaders will now have to make far-reaching choices of their own. As events in Burma have shown, China’s “peaceful rise” has alarmed many of its neighbours: for most countries in the region, American power and values remain far more appealing. Moreover, China has grown rich largely thanks to trade, not least with the US. Faced with the net of containment that America is quietly laying across the Pacific, China will search for the Achilles’ heel of the US Navy, perfecting a new generation of missiles capable of destroying aircraft carriers from hundreds of miles away, working out how to cripple the internet, and how to blind the US satellite network, on which all its military assets now depend.

The world will pay a bitter price, however, if this veiled arms race between America and China escalates. History shows that free trade and military rivalry – however disguised – make for uncomfortable bedfellows. Beijing has gained rapidly in both wealth and power. The manner in which it chooses to pursue them now will have consequences for us all.’ [sic]

 

MD responded:

Whilst the US has declared China as a threat and announcing Asia as a priority, America is also to invest in a long-term strategic partnership with India. India will become the new powerful Asian ally of the United States in the region. In rolling out its new strategy, the Pentagon has made clear that the fronts for potential conflicts are shifting towards China. The US says that all of the trends – whether that is demographic, geopolitical, economic or military – are shifting towards the Pacific and, that over the long-term, China’s emergence as a regional power will have the potential to affect the US economy and security in a variety of ways.

It shouldn’t be in any doubt that China has unsettled its neighbours over several years with the expansion of its navy and improvements in missile and surveillance capabilities. The Pentagon is anxious about China’s strategic goals as it begins to search for a new generation of weapons.

The US defence strategy followed a major diplomatic push by Washington to expand security partnerships with its allies in the region. Last month, the US, India and Japan held their first trilateral meeting in an attempt to counter China’s rising influence in the Asia-Pacific.

China has advanced its influence in the region, along with allies like North Korea, Pakistan, Myanmar and Sri Lanka. Over recent years it has established itself as a growing, and sometimes bullying power in the Pacific, particularly in East Asia. Most of the countries, though, in the Association of Southeast Asian Nations (ASEAN) have festering territorial disputes with China. America’s new emphasis on Asia and the containment of China also stems from the fact that the Asia-Pacific region now constitutes the centre of gravity of world economic activity.

But is America’s new stance the beginning of something that could fan Cold war-style antagonism?

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