Forces Unseen; Ideas, assertions and evidence.

The physical world has many omnipresent forces acting on it, but how often are we actually aware of these forces and how they affect us?
Forces Unseen is a collection of images (original and appropriated) related to, or inspired by, the forces that act upon us all the time, but as they are unseen their existence is easily forgotten (if we were ever aware of them in the first place).
While some images directly refer to the force, some reference the model or experiment used to demonstrate, or refute, the nature/method of action of the force.
Oakland 86
The Earth’s magnetic poles are on the move; Oakland airport in San Jose, California opened in 1927, two of its runways being named 9R-27L and 9L-27R (“L” for left and “R” for right). The numbers in this naming convention relate to the compass bearings the pilots take to navigate these runways; in this case 90° (9) and 270° (27).  This remained the case for 86 years. 
Halley’s Dream
Edmond Halley, while best known for being the first to calculate the path of Halley’s Comet (in 1705), he also proposed that the Earth was made up of several spinning shells, each with its own magnetic field and each separated from the other by its atmosphere (and each somehow lit and supporting its own ecosystem of life!). This assertion, part
of the then popular Hollow Earth Theory, he felt could explain any anomalous compass readings.
A (light) feather or a (heavy) hammer, when dropped from the same height will reach identical speeds and follow the same path of acceleration, therefore hit the ground at the same time.  According to a biography written by Vincenzo Viviani, an experiment to this effect may (or may not) have happened in 1589 when Galileo Galilei dropped two balls from the top of the leaning Tower of Pisa; the balls were identical in size and shape, but were of differing masses. They apparently hit the ground together, therefore disproving Aristotle’s theory that the speed an object reaches due to gravity is in relation to its mass. The feather and hammer test has since been carried out on the moon (where there is little air resistance to slow the fluttering feather down in comparison to the hammer); the two objects did indeed hit the ground at the same time.
Tides are created by the effect of the Moon, the Sun and the rotation of the Earth; the gravitational pull from the Sun and the Moon pulls the water in the oceans, seas and tidal rivers upwards causing a bulge (and a corresponding thinning in areas away from the bulging waters), that we experience as a high tide (or a low tide where the water level has dipped).  On the weekend of 21st and 22nd February 2015 the alignment of the Sun and the Moon created a supertide in the south of England and Wales; in Tintern, Monmouthshire, the waters rose, not only breaking the banks of the River Wye, but running down the village’s streets and within millimetres of flowing under the front doors of many houses (already built to avoid troublesome flooding). Greater flooding was only avoided due the calm weather; the lack of the prevailing wind which would normally push the river’s waters up into the narrowing valley prevented several more inches of tide.
Core State
There have been many theories about the nature of the interior and core of the Earth, whether it was hollow, liquid, solid (or any combination of these), but the great scientist William Lord Kelvin had a favourite demonstration to prove that the Earth did not have a liquid core; he would spin two, seemingly identical, eggs on the desk in front of him. One would wobble as it spun, the other would turn smoothly; the wobbling egg is raw, the movement of its liquid centre affects its centre of gravity causing it to wobble. The other egg, hard-boiled, spins smoothly as there is no internal movement to throw it off centre. From this simple experiment Kelvin asserted that, as we experienced no wobble in the Earth’s rotation, its core must be solid.
You, me, everyone
Light as air; a misconception. We do not physically experience the weight of air other than through observations of the weather, or via our ears “popping” when we fly. However, at sea level, each one of us (our cross section being approximately 0.1m2, or 0.32x0.32m) has one thousand kilograms (one tonne) of air pressing down on us.
A partial solar eclipse was observed across the UK on 20th March 2015. In Bristol the maximum eclipse occurred at 09:29. As the world is now so reliant on solar power the effect of the 2015 eclipse around the world caused power companies to take mitigating measures to prevent power cuts.
There are only two naturally occurring magnetic minerals; pyrrhotite and lodestone. Pyrrhotite is only weakly magnetic, but lodestone’s magnetic properties have lent it to navigation devices; lodestones have been used as compasses since the Chinese Han Dynasty in 206BC.
A dynamo is a piece of equipment that produces (direct) electrical current when a conductive wire is moved inside a magnetic field via the process of electromagnetic induction.  The direction of the magnetic field, the movement and the flow of the current must be at right angles to each other; as an aide-mémoire for the directions each of these elements must be in, we can utilise Fleming’s 
Right Hand Rule:
Thumb = direction of Motion of the conductor
First finger = direction of the Field (north to south)
Second finger = direction of the induced Current (positive to negative)
When considering whether the Earth rotated, or if it was stationary and everything else (sun, moon, stars) rotated around it the Greek philosophers took many everyday observations into consideration.  Among these was the action of the clouds in the sky; they were not fixed to the Earth and seemingly moved about freely in any direction, so surely if the Earth was spinning the clouds would always be “left behind”, disappearing in a Westerly direction?  Another observation/experiment on this matter involved an archer shooting an arrow straight up into the air above his head and noting where it fell. If the Earth (with the archer standing stationary on it) was spinning while the arrow was in the air, surely it would land some distance to one side of the archer, not come directly back down at him...
10’5A Drift
How do you visualise electricity? Quick? Sharp? Dangerous? The instantaneousness of a light coming on when the switch is flicked is deceptive; yes the effect is virtually instantaneous, but it is only the electrons that were already about to enter the lightbulb’s coil when it was last switched off being pushed along by the queue of electrons behind it, that gives that effect.  How fast do they move then? In a copper wire with a cross sectional area of 0.5mm with a 5 amp current applied (say, from two high drain AA batteries), the speed or drift velocity of the electrons is approximately 0.1mm per second.  0.1mm per second = 6mm per minute, therefore in this case it takes an electron ten minutes to travel just 6cm.
Oakland 1+?
In 2013 it was discovered that the Earth’s magnetic poles had moved so much that the coordinates required to use this runway were now over 5° astray. As a result the airport had to rename the runways and re-paint the names; 9R-27L and 9L-27R are now 10R-28L and 10L-28R reflecting the 10° and 280° bearings the runways now lie on. As the speed at which the magnetic poles are moving is accelerating, their next renaming and repainting will be due sooner rather than later.
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