Wheel It takes energy to move an object, which is the product of the force needed and the

6.1.1 Wheel It takes energy to move an object, which is the product of the force needed and the

distance traveled. It takes power to delivery energy quickly, which is the product of the force needed and the velocity of travel. On the land, the force required to move an object sliding on the ground and creating friction, is equal to the weight of the

200 CHAPTER 6 TRANSPORTATION

the object and the ground. The friction factor is small when the surfaces are smooth, such as steel on oiled steel, and is large when the surfaces are rough, such as lumber on a gravel road. Friction is also harmful as it creates heat, and wears out the objects. An engineer strives for fast delivery of heavy objects by using the least power.

Before the invention of the wheel, sleds and rollers with small diameters were used to move heavy objects, such as for the stones for building the pyramids. The pottery wheel was an earlier invention for a different purpose, and might have been

a source of inspiration for the transportation wheel. The transportation wheel came during the Bronze Age, and brought tremendous benefits, since a well-lubricated wheel can reduce the friction coefficient by a factor of 100 and also reduces wear and heat generation. The oldest wheels were simple wooden disks with holes for the axles, shown in many Sumerian pictures of clumsy chariots. A wheel has a sliding friction between the axle and the hub, but the axle and hub have smooth and hard surfaces that are kept clean and lubricated.

The wheel with spokes was invented later, which allowed for lighter and swifter vehicles. The Egyptian Pharaoh on a chariot was often depicted going to war, shooting arrows and trampling on the bodies of his enemies (see Fig. 6.1). This kind of light, two-wheeled cart with a single horse or a pair of horses is frequently seen on Greek vases and Chinese tomb paintings, and is suitable for speed and maneuver. Chariot races were described in the Iliad as part of the funeral games to commemorate the death of Patrocleus, and were also part of the racing events in the Roman Coliseum. But the heavy four-wheeled wagon is suitable for carrying heavy loads and people in everyday lives. Since wheels work best on smooth and hard roads, instead of paths with large rocks and quicksand, paved roads were built in

FIGURE 6.1 Pharaoh on chariot.

201 ancient Mesopotamia, Egypt, and Crete. The Romans built great road systems, suit-

6.1 LAND TRANSPORTATION

able for conquests and for bringing resources. The chariots and other wheeled cars were greatly improved with the inven- tions of the differential gear on the axle, and of ball bearings. When a vehicle with two wheels sharing a single solid axle enters into a turn, the two wheels must revolve at the same rate, but the outside wheel needs to travel a longer distance than the inside wheel, which creates sliding and friction. The differential gear allows the outside wheel to revolve faster than the inside wheel, and makes possible a smooth turn without sliding. Ball bearings were found in early Roman ships. The first patent for ball bearings was awarded in Paris in 1869. They change the sliding friction between two surfaces to the much smaller rolling friction of balls.

Railroads appeared in Greece as early as 600 BCE, in the form of 6 km of grooves in limestone to transport boats across the Corinth isthmus, using power from slaves. Wooden wagon ways were used in England by 1650 for transporting coal from mines to canal wharfs for boats. These roads were made of cast iron plates on top of wooden rails. The Surrey Iron Railway in London opened in 1803, and featured a horse-drawn public railway on cast iron, and later on wrought iron rails. When the steam engine arrived, the first steam trains were introduced in 1825 by George Stephenson. The connection of the North American continent from Atlantic to Pacific was celebrated in Promontory Point Utah in 1869, where the Union Pacific and Central Pacific railroads met and the Golden Spike was struck, to form the First Transcontinental Railroad.

The California Gold Rush began in 1848 when John Sutter discovered gold in

a creek. The “Forty-Niners” were some 300,000 people who went from the East Coast of America to the West, perhaps half by land and half by sea. The land travel- ers covered 5000 km by an assortment of vehicles, including the Conestoga wagon that was 7 m long and could carry a load of 5 tons. It was drawn by six to eight horses or oxen, and could have a top speed of 25 km/day. So this journey from New York to San Francisco would take something like 200 days. The Forty-Niners might also have sailed south to the Cape Horn, and then north to San Francisco, covering 30,000 km in 5–8 months. A combination sea–land route was to sail to Panama, cross the isthmus on land, then take another ship to San Francisco. This slow travel was dramatically changed on June 4, 1876, when an express train called the Trans- continental Express left New York City and arrived in San Francisco in only 83 h and 39 min! The railroad opened the American West, and accelerated the population movement and economic growth. Steam locomotives require numerous workers to load coal and remove ashes, and to maintain the boilers and pistons; locomotives are sooty and particularly troublesome in crowded metropolitan cities. Diesel trains used diesel engines to generate electricity from kerosene, and replaced the steam trains after World War II. Electric trains are much cleaner, and were adopted in the cities as soon as they became available.