Some notions about motion

Which falls faster, a blade of grass or a stone? Anyone can see that a stone falls faster. And that is what the ancient Greeks believed. Based on everyday experience Aristotle determined that heavy objects fall faster than light objects and that objects fall with a speed proportional to their weight.

Aristotle also studied horizontal motion. He observed that whenever he pushed a rock or other object, it always rolled for a while and then came to rest. He hypothesized that the natural state of an object is to be at rest and a force is necessary to keep an object in motion. Aristotle’s hypotheses were accepted for two thousand years because they were consistent with logic and informal observation.

It was not until the early 1600s that these long-established beliefs were challenged. Galileo was not content to accept ideals without verifying them with experiments. He dropped various weights from a height and recorded the results. Disproving Aristotle’s hypotheses, he determined that all bodies fall at equal rates, if you discount the air resistance. A blade of grass will fall more slowly than a stone only because it meets with more resistance from the air.

Galileo also disproved Aristotle’s hypotheses about horizontal motion. He demonstrated that a body pushed on a smooth surface could go much further than one pushed on a rough surface. When a lubricant such as oil was used almost no force was required to keep the object in motion. He concluded that if an object does not meet with resistance (friction), it will continue to move at a constant speed even if no force is applied.

Half a century later, Newton extended Galileo’s ideals and formulated a theory that a body at rest will remain at rest and a body in motion will remain in motion unless some outside force acts on it. This theory is now a universally accepted it is referred to as Newton’s first law of motion.

My speciality

(technology of woodworking)

I am a student of the mechanical woodworking department. My future speciality in technology of woodworking.

To become good specialists we must be well grounded in many theore­tical and special subjects such as mathematics, physics, theoretical mechanics, strength of materials, organic and analytical chemistry, technology of woodworking, colloid chemistry and so on. We must be able to make various experiments with chemicals, plastics and synthetic gums.

Wood is an ancient material. It is used in almost all branches of the national economy, dwelling houses, ships, railway trains, sport equipment; musical instruments make good use of wood. Wood is cheap, light, durable, easily processed and finished. The use of wood as a fibrous material for making pulp began slightly more than a century ago. The manufacture of pulp is nowadays one of the world's largest industries.

The technological engineer of woodwork works at timber plants, furniture factories, shipbuilding plants, construction bureaus. The engineer deals with the manufacture of furniture, sport equipment, plywood, wooden slabs, saw timber, constructional parts and so on.

The main tasks of technological engineers of woodwork are:

• all-round usage of raw materials;
• manufacture of того beautiful kinds of furniture;
• effective usage of wastes;
• introduction of advance methods of work and new technological processes.

Woodwork industry is one of the main branches of the national eco­nomy in the Far East.

The properties of woods as a material

It is not enough to explore the structure of a wood. Its properties must be learned. A few examples of properties follow, with certain of their advantages.

Workability. Sharp tools skillfully used can do wonders with almost any wood. White pine has long been a favourite among carpenters for this reason, and yellow poplar took its place in the hardwood regions for the same reason.

Bending Strength. Hickory and ash find their way into sporting equipment and tool handles because of toughness and the ease with which they may be bent without breaking or weakening.

Resistance to decay. Certain oils and other materials in cedars and the redwoods enable them to resist the attack of organisms causing decay. Both these woods are rather soft, but they are used in contact with soil and moisture as railroad ties, posts, telephone poles, and as outside covering for building because of their decay-resisting properties.

Hardness. Where abrasion from the walking of people or animals or the moving of heavy objects is likely to wear away floors, pillars, or steps, hardness is an advantage. Sugar or hard maple, white oak, and eastern hop horn-bean (ironwood) possess this property.

Combined Strength and Light Weight. Boats and certain sporting goods are more practicable if light in weight but cannot sacrifice strength to this feature. Several of the spruces exhibit the happy combination of these needed characteristics.

The manufacture of particle boards

To produce a particle board the raw material must be shredded into shavings. The size and geometry of shavings, the specific gravity of the board define the quality of the particle board. Wood used in the production of boards is cut into certain required length (33 cm). Then the cut to size material is fed to special machines for reducing to shavings of definite form and size.

The machine is called a shaving machine. From the shaving machine wood particles flow to a hammer mill for further grinding. Then the particles are dried. The drum driers, belt driers and jet driers may be used for this purpose. To separate coarse particles and dust from standard-sized shavings the fractions are conveyed to the vibrating screen.

The dry standard-sized shavings are directed to a mixer. In the mixer the fractions are mixed with an adhesive. The adhesive must answer certain requirements: it must supply high strength to the particle boards, must not change the colour of wood, must be resistant to fungi and insects, must be easily glued, must be cheap and available. To reduce swelling and water adhesion of particle boards hydrofobic additives are to be induced. The main additive is wax. Accurate proportioning of adhesives and waxes to wood weight is of considerable importance.

Date: 2015-12-11; view: 982