Propulsion as a means of transportation is very common in the world around us. In the natural world, one can point to graceful flight of birds, the rhythmic swimming strokes of fish, and the jetting of squids and octopi. In the mechanical world, aircraft, spacecraft, ships, and submarines have been developed for transporting humans and cargo. The need for transportation can be related to location of resources. Quite often food or material resources are not located where they are needed or can be used. In these cases transportation of the resources and/or the end-user is necessary. The ability to transport materials or individuals provides a great deal of flexibility to adapt to changing circumstances. Several examples are given below.

Canadian Goose. Photographed by Alan Wilson, www.naturespicsonline.com Click on the photo for the high resolution image.
Migration: Many species migrate to different locations on the globe according to seasonal variations. Bird migration is a common example. During the winter many bird species in the Northern Hemisphere migrate south to take advantage of the more temperate climate and ready availability of food resources. During the spring and summer, they return to their northerly home when food is once again plentiful. The migration often takes place over days or weeks, so speed is not crucial during this process, but an ability to keep moving over long distances is important because the birds may need to move hundreds or thousands of miles to reach the appropriate climate.
Tiger Shark. Photographed by Terry Goss at Shark Reef Marine Preserve, Beqa Lagoon, Fiji. Click on the photo for the high resolution image.	Bald Eagle. Photographed by Dave Menke, U.S. Fish and Wildlife Service. Click on the photo for the high resolution image.
Hunting: Food is not always located where it is needed, but it is also not always necessary to go thousands of miles to find it (as in the case of migration). Sometimes the next meal is just around the corner, but you have to go out and look for it. Sharks and hawks face this dilemma daily. In order to find food they have to go out searching or hunting until they find (and catch!) something edible. Speed is not necessarily important during the searching/prowling phase, but they must be able to keep moving long enough to find something.
Shipping: As humans we require a great deal of resources to go about our daily lives. In addition to food and water, we use clothing, paper, gasoline, etc. on a regular basis. These resources rarely exist where we are at, so they are brought to us in a variety of ways including transportation through air and water. We may also wish to ship ourselves to a different location just for a vacation or to visit friends. In either case it is often necessary to cover large distances (hundreds or thousands of miles), and speed can be a critical factor because you may want the shipment to arrive as soon as possible.
FedEx Cargo Jet. Airbus A310-200. Photographed by Adrian Pingstone, July 2006. Click on the photo for the high resolution image.	American Airlines Boeing 777-200ER. Photographed by Adrian Pingstone, September 2005. Click on the photo for the high resolution image.
An additional example is moving military equipment and resources into the battle field. The United States military engages in operations throughout the world. In order to accomplish this, military equipment and supporting resources (fuel, spare parts, ammunition, etc.) must be transported to the appropriate location. Typically this involves transportation through air and water. Transportation over long distances is critical for global access and speed is typically important as well since the resources are often time-sensitive. Example Problem: Shipping Time

C-5 Galaxy, military transport aircraft.
Photo courtesy of Lockheed Martin.
Click on the photo for the high resolution image.