Changes in Both Speed and Direction

Acceleration often involves both a change in speed and a change in direction. Changing both components of velocity results in a curved path of motion. In these cases, the acceleration vector is the sum of two parts (components). One part, the tangential acceleration, acts along the direction of motion, parallel to the velocity, resulting in a change of speed. The other part, the radial acceleration, acts perpendicular to the direction of motion, resulting in a change of direction. In order to change the speed of an object moving in a circle, for example, one needs some acceleration along the direction of motion, in addition to the component of acceleration in the radial direction (pointing to the center) that keeps the object moving in a circle. In the case of a space shuttle in orbit, the radial acceleration is the force of gravity pulling the shuttle toward Earth, while a tangential acceleration is achieved by firing rockets along the direction of motion.

 

Changes in Direction

Acceleration can also involve a change in the direction an object is moving. A ball on the end of a string being whirled overhead at a constant speed is an example of this type of acceleration. Since velocity is a vector quantity like acceleration, velocity has a speed component (magnitude) and a direction component. At every instant in its motion overhead, the ball’s velocity is changing because the velocity’s direction is different at every point on the circular path. Changing velocity is acceleration. The acceleration of the object is directed toward the center of the circle, and is of constant magnitude a=v2/r, where r is the radius of the circle and v is the speed of the object (with mass m). This type of acceleration is called radial or centripetal acceleration. Radial acceleration results from the action of the force generated by the string that pulls the ball toward the center of the circle. In the case of a satellite in orbit, the force causing the radial acceleration is Earth’s gravity pulling the satellite toward the center of the planet.

 

Changes in Speed

A car that starts at a standstill and then increases its speed along a straight road is subject to acceleration. That acceleration is due to the application of a force originating in its engine. A car that reduces its speed, by application of a force generated by its brakes for example, is also subject to acceleration. In such situations, where acceleration is in a direction opposite to velocity, the acceleration is often called deceleration.

constant acceleration (a) over a given time interval (Δt), results in a change in velocity (Δv) that can be calculated using the equation Δv = aΔt m/s (the Δ symbol is often used in physics equations to indicate a change in the quantity that follows it.)

The force of gravity near Earth’s surface results in a very familiar form of straight-line acceleration. The strength of Earth’s gravitational field near the surface (g) is acceleration equal to 9.8 m/s2. So every second that an object falls, its speed increases by 9.8 m/s. A ball dropped from a rooftop, for example, would start with 0 velocity. It would have a velocity of 9.8 m/s one second after it was dropped. After two seconds, it would be moving 2(9.8) = 19.6 m/s.

 

Aircraft Carrier Design and Aircraft

An aircraft carrier comprises several different sections, as do all warships. The lower decks of a carrier are similar to those on other ships. They house the engine rooms and compartments for either oil-fired boilers or nuclear reactors, depending on the type of propulsion being used. (Newer carriers are all nuclear powered. The United States has not built an oil-powered carrier since 1968.) A carrier is driven by four massive propellers, and the ship’s engines can generate over 280,000 horsepower to accelerate the carrier to a speed of just over 56 km/h (35 mph).

The upper areas of an aircraft carrier differ from those of conventional warships. An enormous hangar bay runs the length and width of the ship and is referred to as the first deck, or hangar deck. Planes can be serviced on the hangar deck, safe from the effects of wind and weather. Massive elevators, each the size of a typical home, move aircraft between the hangar deck and the flight deck on top, where the planes take off and land. In the area between the hangar deck and flight deck, called the gallery, the ship’s pilots live, work, and prepare for flights.

The flight deck may look quite large, but it is actually small for all the activities that take place on it. Because the runway is so short, planes must be flung into the air by steam catapults that are built into the flight deck. These catapults are 90 m (300 ft) long and draw their power directly from the ship’s engines; they can accelerate a plane from 0 to 240 km/h (150 mph) in three seconds. Each carrier has four catapults: two on the forward area, or bow, and two in the center area, referred to as amidships. The bow area is only used for launching planes because it is too short for landings. The amidships area and the deck behind it can be used for launching and landing planes. It is angled slightly from the centerline of the ship so that landing planes do not interfere with planes taking off from the bow. All four catapults can be used to launch airplanes in a hurry, but during continuous exercises, planes need to land in order to refuel and reload ammunition. When landings are taking place, planes are launched only off the bow catapults so that other jets can land on the angled area. The bow area can also serve as a parking area, out of the way of landings, when launches are not taking place.

Overlooking the flight deck area on the right, or starboard, side is the carrier’s command and control tower, referred to as the island. The island is small, so as not to encroach on valuable flight deck space. The seven-story structure is covered with antennas and radar scanners. The top level of the island is like the control tower at an airport. Here, the ship’s air boss, or controller, controls the movement of planes on the flight deck and those flying near the ship. The next level below the controller is the carrier’s pilothouse, also referred to as the bridge. On the bridge, the captain and other members of the crew control the ship’s speed and direction.

Unlike other warships such as cruisers and destroyers, a carrier has very few weapons built directly onto the ship itself. The aircraft on board the ship serve as its primary weapon. American carriers have the widest variety of aircraft. The typical 86-plane air wing of an American carrier is composed of several types of offensive aircraft. Fighter aircraft include F-14 Tomcat fighters (to shoot down enemy planes that may try to attack the carrier) and F/A-18 Hornet strike fighters (a type of plane that can both drop bombs and shoot down enemy planes). Support aircraft on board include the E-2C Hawkeye airborne early warning aircraft (the only plane on the ship still flown by propellers). It carries a large radar and transmits information back to the carrier, so the ship knows what is going on up to 480 km (300 mi) away. The EA-6B Prowler electronic warfare airplane jams enemy radar, and the S-3B Viking antisubmarine airplane hunts for enemy subs and sinks them if they get too close to the carrier. A carrier also carries about a half dozen SH-60F Ocean Hawk helicopters, which can also hunt submarines, as well as rescue any pilots who suffer an accident. See also Military Aviation.

Landing a plane on a carrier is a complicated procedure that requires tremendous skill on the part of the pilot. When planes are cleared to land, pilots come in behind the ship, lower the landing gear and tailhook, and line up with a series of lights and lenses on the carrier commonly called the meatball. The meatball tells pilots if they are too high or too low as they are coming in to land. Their goal is to keep the light centered in the middle of the set of lenses. Navy pilots refer to this procedure as “calling the ball.” As the plane crosses over the carrier deck, the tailhook snags one of four heavy steel cables stretched across the deck, bringing the plane to a stop in about 90 m (300 ft) of deck space.

When carrier pilots land (or trap, as Navy pilots like to call it), they apply full power to their engines so that if the cables break (which happens rarely) they will have enough power to fly off and try to land again. Landings are made both day and night in clear and bad weather. Pilots prefer to land on large carriers, not because of the larger landing area, but because a large carrier is steadier since it weighs more, and therefore does not pitch as much in heavy seas.

Alessandro Volta

Alessandro Volta (1745-1827) was an Italian physicist, known for his pioneering work in electricity. Volta was born in Como and educated in the public schools there. In 1774 he became professor of physics at the Royal School in Como, and in the following year he devised the electrophorus, an instrument that produced charges of static electricity. In 1776-77 he applied himself to chemistry, studying atmospheric electricity and devising experiments such as the ignition of gases by an electric spark in a closed vessel. In 1779 he became professor of physics at the University of Pavia, a chair he occupied for 25 years. By 1800 he had developed the so-called voltaic pile, a forerunner of the electric battery, which produced a steady stream of electricity. In honor of his work in the field of electricity, Napoleon made him a count in 1801. The electrical unit known as the volt was named in his honor.

Airway

Airway is any of the routes that airplanes follow in flying from one air terminal to another. The term was originally defined as a straight line connecting navigational-radio transmitting sites, or the intersection of straight lines connecting two sites. By the 1980s the Federal Aviation Authority defined and maintained more than 560,000 km (350,000 mi) of airways in airspace over the continental United States. The maximum ceiling in an airway is 75,000 ft (22,860 m); the minimum allowable height follows the contours of the ground and, at airports, touches the ground. Virtually all airspace over the continental United States that lies above 18,000 ft (5486 m) altitude is defined as Positive Control Area (PCA). All airways in the PCA are called jet routes, and all flights in this area are under positive radar control. Flights in airways below the PCA are not, except for those operating under instrument flight rules.

Aircraft Products: Aircraft for General Aviation

Aerospace manufacturers produce more than 30 types of general aviation aircraft, a category that encompasses corporate aircraft, recreational airplanes, planes used to spray agricultural crops, and helicopters for police, ambulance, and patrol service. Corporate aircraft are usually powered by jet engines and carry up to 40 passengers. Major manufacturers in the corporate jet market include the Cessna Aircraft Company, Gulfstream Aerospace Corporation, and Raytheon in the United States, Bombardier in Canada, and Dassault Aviation in France. Recreational pilots commonly fly single-seat or twin-seat planes designed and manufactured by several companies, including Cessna and The New Piper (formerly Piper Aircraft Corporation).

 

Aircraft Products: Commercial Aircraft

Aerospace products in the commercial aircraft category include jet airplanes used by commercial airlines. Jet airliners generally fall under one of two classifications, depending on the number of aisles in the main passenger cabin. In narrow-body jets, a single aisle divides the cabin into two banks of seats. In wide-body jets, twin aisles separate the cabin into three banks of seats. The first of the wide-body jets, the Boeing 747, entered service in 1970. This massive jetliner is capable of transporting more than 400 passengers. Today, a variety of wide-body jets are produced by Boeing and Airbus. Airbus has launched production of a ‘superjumbo’ jet, the A380, with seating for 555 passengers on two decks. It is scheduled to begin service in 2006.

Narrow-body jets seat fewer passengers. Boeing and Airbus build large narrow-body jets that carry between 100 and 200 passengers. For commuter flights, airlines use smaller jets, called regional jets, some seating as few as six passengers. The majority of these planes are built by Canadian airplane manufacturer Bombardier and Brazilian manufacturer Empresa Brasileira de Aeronautica (Embraer).

 

Air Quality

Air Quality is an indication of the healthfulness of the air based on the quantity of polluting gasses and particulates (liquid droplets or tiny solid particles suspended in air) it contains. Air is considered safe when it contains no harmful chemicals and only low levels of other chemicals that become harmful in higher concentrations to humans, other animals, plants, or their ecosystems.

Air is commonly monitored by the United States Environmental Protection Agency (EPA) and state and local environmental agencies for concentrations of six pollutants: carbon monoxide, lead, nitrogen dioxide, sulfur dioxide, ozone, and particulates. Air samples are collected and analyzed several times daily in cities and other industrial areas. The samples are graded on a scale of 0 to 500, indicating how many parts per million (ppm) contain these pollutants. A sample of 0 to 50 ppm indicates good air quality; 50 to 100 ppm, moderate air quality; 100 to 200 ppm, unhealthy; 200 to 300 ppm, very unhealthy; and 300 to 500 ppm, hazardous. If the concentration of one or more pollutants reaches either the very unhealthy or hazardous categories, people with heart or respiratory problems are warned to stay indoors.

EPA data show an increase in air quality in the United States between 1985 and 1994. During this period, concentrations of carbon monoxide decreased 28 percent; lead, 86 percent; nitrogen dioxide, 9 percent; sulfur dioxide, 25 percent; ozone, 12 percent; and particulates, 20 percent.

 

Air Compressor

Air Compressor, also air pump is a machine that decreases the volume and increases the pressure of a quantity of air by mechanical means. Air thus compressed possesses great potential energy, because when the external pressure is removed, the air expands rapidly. The controlled expansive force of compressed air is used in many ways and provides the motive force for air motors and tools, including pneumatic hammers, air drills, sandblasting machines, and paint sprayers.
Air compressors are of two general types: reciprocating and rotating. In a reciprocating, or displacement, compressor (Fig. 1), which is used to produce high pressures, the air is compressed by the action of a piston in a cylinder. When the piston moves to the right, air flows into the cylinder through the intake valve; when the piston moves to the left, the air is compressed and forced through an output-control valve into a reservoir or storage tank.

A reciprocating, or displacement compressor

A reciprocating, or displacement compressor

 

A rotating air compressor (Fig. 2), used for low and medium pressures, usually consists of a bladed wheel or impeller that spins inside a closed circular housing. Air is drawn in at the center of the wheel and accelerated by the centrifugal force of the spinning blades. The energy of the moving air is then converted into pressure in the diffuser, and the compressed air is forced out through a narrow passage to the storage tank.

A rotating air compressor

A rotating air compressor

 

As air is compressed it is also heated. Air molecules tend to collide more often with each other in a smaller space, and the energy produced by these collisions is evident as heat. This heat is undesirable in the compression process, so the air may be cooled on the way to the reservoir by circulating air or water. For high-pressure compressed air, several stages of compression may be employed, with the air being further compressed in each cylinder and cooled before each stage.