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 Hands-On-Physics
Introductory Mechanics
Concepts
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 centrury. 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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