Automobiles are a form of wheeled land transport that have a motor for propulsion. They typically have four or more wheels, and are constructed principally for the transport of passengers. The term can be applied to a car, truck, motorcycle, or van. In a broad sense, it may also include sport utility vehicles (SUVs), pickup trucks, and sports cars. The automotive industry is a large and growing sector of the economy. It employs millions of people and is the single largest consumer of steel, iron, petroleum, and other raw materials. The automotive industry has a significant impact on the world’s environment, and its production creates substantial quantities of pollutants and other hazardous wastes.

The automobile has been one of the most important forces shaping the modern world. It has transformed personal mobility and enlarged economic opportunities for families, communities, and nations. In the United States it is the primary mode of transportation for most urban dwellers, and more than three trillion miles (5 billion kilometres) are driven each year in the country. In the first decades of the twentieth century, automobiles were the dominant force in a new consumer-oriented society and the backbone of a new industrial economy.

Many different types of engines are used to power automobiles. The most popular type of engine is the internal combustion engine (or simply the engine). It uses chemical energy from gasoline or other fuel to turn the crankshaft of the car, which in turns drives the wheels. The speed of the wheels and the amount of torque generated is proportional to the engine’s power, which is measured in kilowatts or horsepower.

Another method of propelling an automobile is by the use of a mechanical transmission system. Mechanical transmission systems usually have two shafts with gears that can be shifted to increase or decrease the speed of the drive wheels. In most cases, the gear ratio is a function of the number of teeth on the gears.

A major problem with automobiles is the risk of injury or death from collisions. Drivers make mistakes, and the force of impacts can be high. Many accidents can be caused by road conditions or by bad weather. Automobiles can be a source of pollution, especially when they are operated in metropolitan areas, where air quality is often poor.

Automobiles have also been associated with crime and other negative social effects. Traffic congestion can be a serious problem, as can parking problems and the risk of theft. In some areas, public transit systems have been used to reduce the dependence on automobiles. These alternatives can move more people more quickly than a car, and do not contribute to the problem of traffic congestion. They can also reduce the need for people to take risks by driving under the influence of alcohol or drugs, which is often a contributing factor in crashes. In addition, these forms of public transportation can help people stay healthy by avoiding the potential for exposure to smoke and germs in crowded vehicles.