Hands-On-Physics
MECHANICS

Physics
Concepts

- Introduction -

Motion is fundamental in nature and the understanding and control of motion is essential for human activity. Physics often traces its origins to Galileo's study of motion in the 17 th century. He saw the essential roles of time and distance in the description of motion, and established a basis for the A few decades later Newton established the model of motion used today, his familiar three laws of motion. He showed that objects which have no forces acting on them do not change their speed, and that objects which do have unbalanced forces do change their speed. When the speed of an object changes, we say that its kinetic energy increases. The object gets its increase in energy from a force which pushes on it and does work. These basic ideas are not complicated.

Introductory Ideas
The energy of an object increases when work is done on it. Work is the product of force and distance moved. As the energy of an object increases, some measurable aspect of that object increases; like its temperature, its height, or its speed. If the speed of an object nears the speed of light (not a problem with cardboard aircarts) it mass increases. The rate at which work is done and energy increased is called power. When electricity is used in a power system, the role of electricity is evaluated in terms of electrical charge, its rate of flow (current), and its energy ( voltage).

Energy
Energy is a major consideration in much of what we do. Whether heating, cooling, lighting, lifting, speeding up, or stopping, energy is important. In this Introductory Mechanics Unit, energy for transportation is investigated. Sources of energy, energy storage, and energy transformations are all factors which must be considered the design of a transportation system. An understanding of energy is a starting point as well as a reference to return to.

Speed

Speed itself is not measured directly, by calculated from a combination of time and distance measurements. Speed is simply the ratio of position change to time needed for that change, or the distance traveled each second. Vehicles in a transportation system do a lot of changing speed. They start, they speed up, they travel at constant speeds, they slow down, and they stop. Vehicle speeds in general can be very complicated. However, complicated motion can often be approximated with a combination of constant speeds and steady speed changes. Constant speeds are quite easy to deal with. Constant speed is determined by measuring the time required to travel some predetermined distance or by measuring the distance traveled in some predetermined time. If time intervals are predetermined, time is called the independent variable with distance the dependent variable (it depends on time). If the distance intervals are predetermined by the experimenter then the role switches, with time becoming the dependent variable. Which ever technique is used, the speed is still the ratio of distance traveled to time elapsed. Changing speed may be approximated with successive "constant"speeds. (This is a standard trick physicists use.) While the speed of an object is changing, we sense that the speed is never really constant, so we think of each "constant" speed as an average over some short time interval. When speed is increasing, the actual speed will be less than average at the beginning of the time interval and more than the average at the end of the time interval. When the change is speed is steady, we say the acceleration is constant. When the speed change is steady, the "instantaneous" speed in middle of the time interval turns out to be the same as the average speed over the whole interval. By determining a bunch of average speeds, the pattern of changing speed may be seen.

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