The Physics

  Terms, units, and abbreviations These metric units are used: force, effort or load in newtons (N) work in joules (j) power in watts (W) The equivalent customary units and conversion factors are as follows: 1 newton — 7.23 poundals 1 joule = 23.73 foot poundals 1,000 watts or 1 kW = 1.34 horsepower A machine is defined in physics as a device for applying or transmitting mechanical power. Its function may be to overcome resistance to motion or to change of shape or size at one point of an object by applying a force, often at some other point The multiplication of forces is possible in mechanical devices, enabling work to be performed that human strength alone could not do. Two terms commonly used in describing machines are work and power; both of which have been given precise, technical meanings. In a simple machine, the work done is equal to the product of the load (a force) and the distance it moves in the direction of the force, irrespective of the time taken; it

 Statics is concerned with bodies that are in equilibrium , which is the state of an object when it is not accelerated; a body that is at rest or moving at constant velocity is therefore in equilibrium . For an object to be in equilibrium it is necessary for all the forces acting on it to cancel each other out exactly. Center of gravity The concept of center of gravity is used in determining the stability of an object’s equilibrium : The center of gravity is the point where the entire weight of an object can be considered to be concentrated. A disk, for example, has its center of gravity at the center, whereas that of a rigid triangular sheet lies at the point of intersection of the lines that join the vertices of the triangle to the midpoints of the opposite sides. The center of gravity of an irregularly-shaped flat object can be found by suspending it from any two points on it and marking the point of intersection of a plumb line suspended from each point in turn.

  Terms, units, and abbreviations The following units are used: length in meters (m) mass in kilograms (kg) weight in newtons (N) density in kilograms per cubic meter (kg m-3) The equivalent customary units and conversion factors are as follows: 1 meter = 1.09 yards or 3.28 feet • kilogram = 2.20 pounds 1 newton = 7.23 poundals 1,000 kilograms per cubic meter = 62.43 pounds per cubic foot Time is measured in seconds (s) in both systems of units. Statics is the branch of physics that deals with the analysis of bodies that are held stationary under the influence of a system of forces. Using statics it is possible to predict what will occur when the forces acting on an object are changed, the size of the forces needed to keep an object stationary, and many other phenomena of interest to physicists and engineers. Forces The study of statics depends crucially on an understanding of the concept of force, which can be defined as an agent that is capable

  A train floats along, frictionless, on a magnetic cushion above the tracks. Electrical wires deliver power to homes and industries over long distances without any loss of energy. Supercomputers perform calculations with 50 times the speed of the fastest computers available today. These are a few of the advances scientists believe are coming closer to reality as advances in high-temperature superconductivity continue. Magnetic resonance imaging (MRI) , shown above, produces images of tissues inside the body. Doctors use these images to diagnose diseases and injuries. Reamde more Breaking the cold barrier: superconductivity

 Some substances exist in states that do not comply with the normal definitions of a gas, a liquid, or a solid. For example, jelly is neither a true solid nor a liquid, and smoke is neither a pure gas nor a solid. Matter in stars and in the tails of comets exists as a plasma, a mixture of charged particles that is outside the normal definition of a gas. In general, a plasma can exist only at extremely high temperatures. At extremely low temperatures, approaching absolute zero, some materials take on remarkable properties. Although they are strictly not different states of matter, their exceptional behavior is also described in this article. Mist is an unusual state of matter, despite being a familiar phenomenon. It is a colloid in which the dispersion medium is a gas (air) and the disperse phase is a liquid (water droplets). In physical terms, fog and cloud are similar types of colloids. Reamde more Unusual states of matter

 When two substances are mixed together, they may react to form a chemical compound, if they do not react, they form a mixture. The constituents of a chemical compound cannot be separated by physical or mechanical methods. The constituents of a mixture, on the other hand, can be separated. For example, a mixture of salt and sand can be separated by dissolving the salt in water. And a solution of salt in water can be separated by evaporating or distilling off the water. Alloys are made by melting together a metal and one or more other substances (usually also metals) to produce a new material with different physical properties. Alloying aluminum with copper, for example, produces an alloy with a greater hardness, although with less than 60 per cent copper the melting point is lowered. All alloys of zinc and copper, on the other hand, have melting points higher than that of pure zinc. Alloys An alloy consists of a metallic element and one or more other, usually metallic,

In a gas , the atoms or molecules are free to move about rapidly. Collisions with the walls of the container give rise to gas pressure. Because the atoms or molecules of a gas are far apart and move independently of each other, a gas can be expanded or compressed to a much greater degree than can a solid or a liquid. Changes in volume are accompanied by changes in pressure and/or temperature. These changes can be explained in terms of the kinetic theory of gases and the gas laws.

In a liquid , the atoms or molecules are free to move randomly within the constraints of the container and the liquid’s surface. As in a solid, the atoms or molecules of a liquid are held together by attractive forces. But these forces are not great enough to hold the atoms or molecules in a fixed pattern; instead, they move about at random. As a result, a liquid can flow and it cannot be stretched or distorted. Like a solid, however, it can be compressed slightly and shows the same sort of elasticity when subjected to compressive stress. Unlike a solid, but like a gas, a liquid exerts pressure, which at any point depends on the depth and the density of the liquid.

In a pure solid , the atoms are held by electrostatic forces in a regular array in a crystal lattice. This arrangement gives a solid its characteristic properties of strength and hardness. The forces that hold atoms and molecules in place give solids their strength. Interatomic forces are generally stronger than intermolec-ular forces, so that solids composed of collections of single atoms, such as most metals and diamond, are the strongest. Molecular solids, such as plastics, iodine, dry ice (frozen carbon dioxide), and some metals and alloys, are softer and melt more easily.

Many substances occur naturally as crystals . Here the yellow crystals of sulfur contrast with the shiny white aggregations of cal-cite (calcium carbonate). Most simple chemical compounds consist of crystals. These cannot always be seen clearly because they are often grouped together in a mass that has no particular shape. But if a lump of crystalline material is examined closely, tiny individual crystals can be seen. All crystals have a definite geometric shape, determined by the way in which the atoms of the substance are linked together. For example, in a crystal of common salt (sodium chloride) the atoms of sodium and chlorine are arranged so that they lie at the corners of a series of (imaginary) cubes; the result is a Cubic crystal.

  Atoms in a solid  maintain their regular positions in a crystal lattice and vibrate because of their thermal energy; on being heated, they vibrate more vigorously, causing the solid to expand. In a liquid, the atoms move about randomly; a liquid has no regular form and takes on the shape of its container. In a gas the atoms move rapidly, collisions with the walls of the vessel resulting in gas pressure; a gas totally fills its container. All single substances can, under the right conditions, exist as a solid, liquid, or gas; these are the three common physical states (or phases) of matter.

A simple model of the atom visualizes it as having a central nucleus surrounded by orbiting electrons. Near the beginning of the 20th century, the New Zealand scientist Ernest Rutherford carried out one of the most significant experiments in physics. He directed a beam of alpha particles (helium nuclei) at a thin sheet of gold foil. Most of the particles passed straight through, some were deflected slightly from a straight-line path, and a few were scattered through quite large angles. Rutherford deduced that the positively-charged alpha particles were scattered by near collisions with the positively-charged nuclei at the center of the gold atoms. Such scattering techniques are .now central to the study of atomic and subatomic structures. It is now known that every atom contains a nucleus, whose size is about 10,000 times less than that of the atom itself. Most of an atom is empty space. With the exception of hydrogen, every nucleus contains neutrons and protons (colle

  Physicists study the properties of matter in the universe and how matter interacts through forces. One way to make these studies is to consider the individual components of matter itself. By studying the microscopic and sub-microscopic constituents of matter, physicists can often deduce how large bits of matter will interact. There can, however, be conceptual difficulties in explaining physics at this sub-microscopic level. The minute particles that make up matter can be “seen” only by the effects they have on visible things, or by the gross phenomena that result when millions of them act together.