Water can exist in three states (or three phases):

• Solid phase:  The particles in a solid are strongly bonded to one another. Ice cubes maintain their form regardless of the container that holds them.

• Liquid phase: The particles are no longer in an ordered state. The bonds between molecules are broken, and the liquid water takes the shape of its container.  The particles are very close to one another, and so a liquid is incompressible.

• Gaseous phase: Agitation and disorder are at the maximum level. Water vapor occupies all of the space in a container. The distances between molecules are large. A gas in compressible.

Note that water vapor is invisible.

The principal agents of erosion of the littoral remain waves, tidal effects and marine currents. The masses of moving water act like "bumper collisons" that alter the rock.

One should not underestimate the importance of chemical degradation of the rock by spray (causing dissolution of material), as is the case for calcereous rock.

The form of the littoral depends on a great many parameters. It could be  something like simple beaches composed of sand or pebble beaches, or it could be a matter of cliffs that have been fashioned by successive rockfalls.

Frozen water occupies a volume 10% greater than when it is in its liquid state.

For this simple reason, water that infiltrates faults or fissures is able to break even the hardest rock during  several cycles of freeze-thaw.  Rocks that have been damaged in this way display breakages that are clean and straight.

This type of degradation of rock is common in mountain regions, where the temperature oscillates regularly around 0° C.

The U-shaped valley is chraracteristic of glacial erosion. The passage of the   gigantic mass of a glacier marks the landscape with imposing tracks. Its abrasive power tears away the walls of blocks of rock. These are crushed and carried downstream.  These are the moraines.

Glaciers are numerous during glacial periods (the last one ended 10,000 years ago). Numerous lakes and fjords bear witness to their presence, long after they have receded.

Thick  sedimentary layers that have accumulated at the bottoms of fjordsand lakes conceal the true depth of the original valley.

In 1869 the Russian chemist D. I. Mendeleev discovered that pure chemical elements (containing a single kind of atom) could be classified in a table,  in families,  according to the number of protons in their nuclei. This number, called the atomic number, increases from top to bottom and from left to right on this table. The elements arranged in the same column have similar chemical properties.

This animation shows which elements were known at different periods of history:

Transformers are powerful electrical components which enable the conversion of a sinusoidal voltage into another sinusoidal voltage of the same frequency, but with a different amplitude.

The physical principle of induction, which is the basis of the transformer, does not work with direct current (DC).

A chopper is analogous to a transformer, but it does work with direct current.  A voltage, V, is reduced (stepped down) or increased (stepped up) to a different level of direct current.

We call this a DC-DC converter, as opposed to the transformer, which converts from alternating current (AC) to a different alternating current (AC-AC).

The chopper  underlies  switching power supplies  and the speed controllers for direct current machines.

Transformers are powerful electrical components which enable the conversion of a sinusoidal voltage into another sinusoidal voltage of the same frequency, but with a different amplitude.

The physical principle of induction, which is the basis of the transformer, does not work with direct current (DC).

A chopper is analogous to a transformer, but it does work with direct current.  A voltage, V, is reduced (stepped down) or increased (stepped up) to a different level of direct current.

We call this a DC-DC converter, as opposed to the transformer, which converts from alternating current (AC) to a different alternating current (AC-AC).

The chopper  underlies  switching power supplies  and the speed controllers for direct current machines.

This animation presents  the structural changes in the uterine mucous membrane in the course of the menstrual cycle.

The V-shaped valley is typical of one that has been carved by flowing water. The erosion is more pronounced when the water flow is a heavy one, and the water carries suspended particles (sedimentary load).

Sediments torn away from the walls are carried away by the water, downstream, where the speed of the flow is too weak to carry them any further.

When a force tends to make an object turn on an axis (around a pivot point) one can define the moment (or torque) of this force in relation to the pivot point.

In Mechanics , the study of the moment of a rotating body is the equivalent of the study of forces for bodies undergoing translation.

The vector moment is a function of the force and the distance seperating the pivot point of the axis from that force  (called the lever arm or moment arm – not shown in the animation).  The mathematical formula that enables us to determine the moment (torque) brings into play the operator vector product (note “X”).

The particularity of the resulting vector moment is that its direction is perpendicular to vectors F and OM (or OQ). Its module depends on the sine of the angle between F and OM. Its strength is thus maximal for an angle of 90° and zero when the direction of the force passes along the axis.

The term couple is associated with a system of two forces where the resultant is zero (one pushes while the other pulls) but where the moment is not zero. In this example, one can speak of a couple if one force F’ of the same amplitude but opposite in direction, was applied to the other pedal.