Sir Isaac Newton’s famous principle of action and reaction explains equally what drives the flight of an inflated balloon around the room when you let go of its neck as well as that of a rocket into space. The force that is exerted by gases escaping from the base of each pushes them forward in an equal and opposite reaction to the escape of the gases.

exerted means:

The rotating hour and minute hands of a clock indicate the time of day. rotating means:

If you have ever used a library, you ought to be thankful that all of its materials are classified into specific categories. If there were no system for the arrangement of all the books, maps, and other materials, you might have to spend many hours looking for the item you wanted.

classified means:

A popular type of glove for doing heavy-duty work is one that is made of leather and lined with cotton. Leather gives good protection to the hands from cuts and burns, while cotton keeps the hands cool and comfortable.

lined means:

Iron is the most used of all metals, making up 95% of all the metal produced worldwide. When iron is mixed with certain small amounts of carbon (a non-metal), steel is formed. Steel is said to be an alloy of iron and carbon. Alloys are manufactured to improve on the properties of their components. Steel is, in fact, stronger than iron, and it therefore has many uses.

alloy means:

When the moving parts of a machine come into contact with one another, there is friction. To reduce this wear as much as possible, it is important to make sure that oil or grease is applied to all the moving parts. Regular lubrication of the machine will ensure a longer life.

lubrication means:

Jafar buys Turkish coffee from a specialty shop. He selects the coffee beans he wants and then has them ground into powder.

powder means:

People who repair watches, cameras, medical equipment, or other precision instruments need to have a high level of skill and attention to detail. Such work has to be done with great exactness and according to professional standards.

precision means:

The average service life of sidewalks in Canada is 20–40 years and mainly depends on the geographic location and quality of the construction. Especially when and where climate and soil conditions are severe, sidewalks become more easily deformed. They may show signs of sinking, rising, and cracking, all of which make a sidewalk less practical and safe.

deformed means:

Citrus fruits (including oranges and grapefruit) are a $9 billion industry in the American state of Florida. Sometimes, however, freezing temperatures or hurricanes hit the industry hard, ruining large amounts of the crop and causing great economic loss.

ruining means

Preferences for different designs of pants change over time. In the 1960s and 1970s, “bell-bottoms,” whose legs became wider from the knees down, were commonly worn. In the 1980s and 1990s, tapered legs, which fit close to the ankle, were popular. Nowadays, straight legs are widely preferred.

tapered means:

Have you ever wondered why automobile engines, electric motors, or computer hard drives operate as smoothly and quietly as they do? The answer lies with a device called a bearing. In fact, bearings make possible our everyday use of numerous machines that make life easier and more comfortable.

A bearing is a part of a machine that holds or guides moving parts. Bearings support a load exerted on them and reduce friction and wear by enabling rotating or sliding parts to move smoothly.

There are many kinds of bearings, but they may be classified into two major types: plain bearings and roller-contact bearings. They differ primarily in the way they reduce friction.

Plain bearings operate on the principle of sliding friction. They are sometimes called “fluid film” or sliding bearings because they support moving parts on a thin film of oil. Plain bearings usually consist of a split cylindrical shell of hard, strong metal and an inner cylindrical part, which holds the rotating shaft. This inner part of the bearing is lined with a soft alloy and is separated from the shaft it is holding only by a film of oil.

Some plain bearings used for low speeds and light loads need no lubrication because they are made of plastic and have non-metallic linings. There are also self-lubricating bearings made of compressed and heated metal powder that are used in the electric motors of vacuum cleaners and other home appliances.

Roller-contact bearings, the other major type of bearings, operate on the principle of rolling friction. These bearings make use of the rolling motion between parts, which reduces friction better than plain bearings do.Other advantages these bearings have over the plain type are that they can operate at high speeds and are easy to lubricate.

Roller-contact bearings are of two kinds—ball bearings and roller bearings. Ball bearings are probably the most common kind of bearings. They are usually found in light precision machinery where high speeds are maintained.

A ball bearing consists of two rings with a number of balls fitted between them. The balls are held in place by metal rings called “separators.” The space between the inner ring and the outer ring is called the “cage.” The opening of the inner ring is called the “bore.” Ball bearings are made in many different sizes to suit the shaft diameters of different machines. The shaft must fit exactly into the bore of the bearing.

In a ball bearing, the load is transmitted from the outer ring to the ball and from the ball to the inner ring. Since the ball is a sphere, it only comes into contact with the inner and outer ring at a very small point, which helps it spin very smoothly. This also means, however, that there isn’t much contact area holding that load. If the bearing is overloaded, the balls can become deformed, in which case the bearing is ruined.

Roller bearings are quite similar to ball bearings. The main difference is that, instead of balls, they have cylindrical or tapered rollers. The contact point between the inner and outer rings is therefore a line rather than a point. This spreads the load over a larger area, allowing the bearing handle much greater loads than a ball bearing.

What is the main idea of the article?

Plain bearings operate on the principle of sliding friction. They are sometimes called “fluid film” or sliding bearings because they support moving parts on a thin film of oil.

Roller-contact bearings operate on the principle of rolling friction. These bearings make use of the rolling motion between parts, which reduces friction better than plain bearings do.

Plain bearings and roller-contact bearings mainly differ in the _____.

Plain bearings operate on the principle of sliding friction. They are sometimes called “fluid film” or sliding bearings because they support moving parts on a thin film of oil.

Plain bearings use _____ to reduce friction and wear.

Some plain bearings used for low speeds and light loads need no lubrication because they are made of plastic and have non-metallic linings. There are also self-lubricating bearings made of compressed and heated metal powder that are used in the electric motors of vacuum cleaners and other home appliances.

If a plain bearing is made of plastic or has a non-metallic lining, it is likely to _____.

Roller-contact bearings, the other major type of bearings, operate on the principle of rolling friction. These bearings make use of the rolling motion between parts, which reduces friction better than plain bearings do.Other advantages these bearings have over the plain type are that they can operate at high speeds and are easy to lubricate.

Among the four choices below, the one that is NOT mentioned in the article as an advantage of roller-contact bearings over plain bearings is that they _____.

Roller-contact bearings, the other major type of bearings, operate on the principle of rolling friction. These bearings make use of the rolling motion between parts, which reduces friction better than plain bearings do.Other advantages these bearings have over the plain type are that they can operate at high speeds and are easy to lubricate.

Roller-contact bearings are more efficient at reducing friction than plain bearings because of the _____.

Roller-contact bearings are of two kinds—ball bearings and roller bearings. Ball bearings are probably the most common kind of bearings. They are usually found in light precision machinery where high speeds are maintained.

Ball bearings are a kind of _____ bearing.

A ball bearing consists of two rings with a number of balls fitted between them. The balls are held in place by metal rings called “separators.” The space between the 40 inner ring and the outer ring is called the “cage.” The opening of the inner ring is called the “bore.” Ball bearings are made in many different sizes to suit the shaft diameters of different machines. The shaft must fit exactly into the bore of the bearing.

The shaft of a machine is held by the inner ring of a ball bearing and fits into the _____.

A ball bearing consists of two rings with a number of balls fitted between them. The balls are held in place by metal rings called “separators.” The space between the inner ring and the outer ring is called the “cage.” The opening of the inner ring is called the “bore.” Ball bearings are made in many different sizes to suit the shaft diameters of different machines. The shaft must fit exactly into the bore of the bearing.

In separator outer ring cage bore a ball bearing, the cage is the _____.

A bearing is a part of a machine that holds or guides moving parts. Bearings support a load exerted on them and reduce friction and wear by enabling rotating or sliding parts to move smoothly.

Bearings would not likely be found in _____.