Why study uniform motion?

The first successful application of the physical laws of motion was in understanding how the solar system works. The time between successive full moons is always the same: the moon moves without slowing down. According to the law of everyday motion, this would require having something push it. But actually the moon isn't being dragged across the sky; it's just coasting at constant speed. (In this example we ignore the gravitational attraction of earth and sun; without them, the moon would travel in a straight line, and soon be out of sight).

Oceans, lakes, rivers, and canals were very important to transportation before the railroad and the highway. Water provides a horizontal surface, and it isn't very hard to push it out of the way. A barge is much more isolated from the rest of the world than a sledge or even a wagon. A donkey or two was enough to keep tons of coal moving.

The situations in everyday life where we become aware of the law of inertia tend to involve massive moving objects, such as locomotives, heavily loaded trucks, or large linebackers. Left to themselves, these objects would move in a straight line at constant speed. Mass becomes important because this determines how hard it is to make the object stop or change direction.

The Law of Inertia is the secret to riding a bicycle. When the bicycle is not moving, it will fall over to one side or the other, and once this starts, there is nothing you can do to stop it (except to put one foot to the ground). But it is easy to "stay up" when the bicycle is moving. When you sense that you are falling to the right, you turn the handle bar so that the bicycle moves to the right. Meanwhile, you move in a nearly straight line at constant speed. Soon the bicyle is back under you. For example, the diagram at right shows the path of the bicycle wheel and the rider, seen from directly above. At point A, the rider realizes that she is too far to the left, and is falling over to that side. So she steers the bicycle to the left, with the result that it is back under her at point B. With practice, we learn to do this so that the rider ends up right over the bicycle. But in the example shown, the bicycle and the rider are going in different directions at B, and soon the rider is falling over to the right, at point C. By again steering to the right, the bicycle moves back under the rider.

Things all around us are moving at many different speeds. Speed and motion are so common that we usually just accept them as a normal part of life and don't think specifically about measuring them. When we actually measure some speeds, even the simple question "how fast does it go?" can become interesting, as the speeds of some everyday motions show. Putting them in a list together highlights the tremendous range of possibilities. To determine the speeds that are listed, someone had to measure a distance moved, and a time span during which the motion occurred. the earth rotates as it goes around the sun

Sometimes speed is (nearly) constant, and sometimes it distinctly varies. A bicycle can coast at nearly constant speed on a level road, but it slows as it goes uphill and then speeds up on its way down. A hockey puck glides across the ice until it encounters a stick that causes a change in speed and a change in direction. Some kinds of birds fly or glide in a straight line, while others dart back and forth, changing speed and direction unpredictably. When a clock pendulum swings back and forth, it goes fast at the low point of its arc and slows to a stop at the high points. A screen door on a spring hangs open momentarily and then speeds up to slam shut.

The unit on uniform motion