The sun

The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers (865,000 mi), about 109 times that of Earth, . About three quarters of the Sun's mass consists of hydrogen, while the rest is mostly helium. Less than 2% consists of heavier elements, including oxygen, carbon, neon, iron, and others.

The Sun's color is white, although from the surface of the Earth it may appear yellow because of atmospheric scattering. In its core, the Sun fuses 430–600 million tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now presumed to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs. The Sun's hot corona continuously expands in space creating the solar wind, a hypersonic stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System.

The Sun is currently traveling through the Local Interstellar Cloud in the Local Bubble zone, within the inner rim of the Orion Arm of the Milky Way galaxy. Of the 50 nearest stellar systems within 17 light-years from Earth (the closest being a red dwarf named Proxima Centauri at approximately 4.2 light years away), the Sun ranks 4th in mass. The Sun orbits the center of the Milky Way at a distance of approximately 24,000–26,000 light years from the galactic center, completing one clockwise orbit, as viewed from the galactic north pole, in about 225–250 million years.

The mean distance of the Sun from the Earth is approximately 149.6 million kilometers , though this varies as the Earth moves from perihelion in January to aphelion in July. At this average distance, light travels from the Sun to Earth in about 8 minutes and 19 seconds. The energy of this sunlight supports almost all life on Earth by photosynthesis, and drives Earth's climate and weather. The enormous effect of the Sun on the Earth has been recognized since prehistoric times, and the Sun has been regarded by some cultures as a deity. An accurate scientific understanding of the Sun developed slowly, and as recently as the 19th century prominent scientists had little knowledge of the Sun's physical composition and source of energy.

Water is the most precious resource on our planet, a key ingredient together with air for development of all life on Earth, in fact even our bodies are mostly water.If we go way back in the past, around 4.5 billion years ago, we could see the Earth was bequeathed with sufficient water for oceans to form and for life to find favorable areas in the seas .

The most accepted theory suggests that oceans were formed from vapors emitted during intense volcanism that happened at the time when our planet appeared in endless space. According to this theory both the ocean as well as the atmosphere were formed from volcanic gases that originated in Earth's interior. According to this theory water was key part of Earth's initial inventory, but not all scientists believe in this theory. There are some scientists like Francis Albarède of the Laboratoire des Sciences de la Terre that believe how water in fact came much later on our planet from the turbulence caused in the outer Solar System by giant planets. According to his theory the ice-covered asteroids thus reached the Earth one hundred million years after the birth of the planets.

This would of course mean that the origin of water on our planet is extraterrestrial,and could have arrived on our planet much later than dominant theories believe it had. The main thesis of his theory is that during the formation of the Solar System, the temperature between the Sun and the orbit of Jupiter never dropped enough for volatile elements to be able to condense with planetary material. The arrival of water on Earth therefore has to correspond to a later episode in Earth's history.

The water on our planet could have its origin in a space phenomenon that happened some tens of millions of years after the lunar impact, namely the big clean up of the outer Solar System initiated by many giant planets. These planets had very strong gravity, and so were able to sent the final ice-rich planetary bodies in all directions, including the direction of our planet.

These ice-rich planetary bodies then penetrated into the mantle through the surface and so the plate tectonics began creating continents surrounded by vast oceans. This theory would even explained why there is no life on Mars because Mars probably Mars dried out before water managed to penetrate in depth.

human digestive systems

Digestion is the mechanical and chemical breaking down of food into smaller components that can be absorbed into a blood stream. Digestion is a form of catabolism: a break-down of larger food molecules to smaller ones.

Food enters the mouth, being chewed by teeth, with chemical processing beginning with chemicals in the saliva from the salivary glands. Then it travels down the oesophagus into the stomach, where hydrochloric acid kills most contaminating microorganisms and begins mechanical break down of some food, and chemical alteration of some. The hydrochloric acid also has a low pH, which is great for enzymes. After some time (typically an hour or two in human) the results go through the small intestine, through the large intestine, and are excreted during defecation.

Friction

Friction is all around us. Like gravity, friction is a force. Whenever two things rub together, some of the energy they have is lost to friction. Basically, in things like car engines, the wheels of your skates, and the chain of your bike, friction is a force that slows you down. In order to overcome friction, you have to work harder. However, the energy doesn't just disappear but it's turned into heat!

As it turns out, the rougher two surfaces are, the more friction there is between them when they rub together. By making things really smooth, or by adding lubricants like grease and oil, engineers work hard to reduce friction and in the long run, save energy.

Friction isn't always bad though. If you've ever tried to start running on a wet floor, you know that too little friction can just make you slip and slide. You need a certain amount of friction to get a grip in order to get yourself going.

That's why winter car tires usually have deeper treads than regular tires. The extra grooves make the surface rougher and help them get a grip on ice and snow.

.