Conservation Laws
Work Energy and Power
Conservation of Momentum
The human invented concept of energy helps to simplify the calculations
necessary when an applied force causes changes in elevation or
velocity. This brings up one of the most famous of scientific laws, The
Law of Conservation of Energy. In words, it says that energy is not
created or destroyed. This is in conflict with the society idea of
conservation of energy - don't waste energy, use it wisely. The
Law of Conservation of Energy is also known as the First Law of
Thermodynamics. It does not take into account the other laws that
describe different types of energy and the fact that energy generally
becomes so spread out over the universe that it is unusable to us. This
spread out energy is called entropy and while it is still energy it is
no longer useful to society so in society we try to minimize the energy
that goes into increasing the entropy.
In classical physics there are three types of "energy".
Work (W=Fs) is equal to force times displacement. Because both force
and displacement are vectors they must be in the same direction to
calculate the correct amount of work.
Potential energy (U=mgh) is weight (the force of gravity or mg) times
height. You should notice that this is just a special kind of work of
lifting an object straight up. This is only gravitational potential
energy. In Physics B we will consider other forms of potential energy
of electricity and magnetism.
Kinetic energy (K=1/2 mv2) is one half mass times velocity
squared. You need to remember that mass is NOT WEIGHT. This is an easy
way to find the velocity of an object starting from either (or both)
work or potential energy. This kinetic energy in Physics A will only be
considered on a "large object" level. It is interesting to note that on
a molecular level this same equation gives kinetic energy on average of
molecules that we often call heat.
Work and energy have units of Joules (the same as a Nm). One calorie
about 4.18 Joules and energy conversion that is often used in measuring
food energy. Just a reminder, a food calorie is 1000 calories (or
actually a kilocalorie).
Applying the law of conservation of energy to these three ideas results
in Wi + U1 + K1 = Wo + U2
+ K2. Using these concepts greatly simplifies the math
necessary compared to Newton's Second Law. It is especially useful in
calculating operations of simple machines.
Power is defined as the rate of work (or energy), how fast work can be
done. P=w/t Power is measured in watts where one watt is
equal to one joule per second. Watts are familiar to many Americans in
dealing with electrical power. Watt ratings of light bulbs or electric
motors and purchasing energy from electric companies in kilowatt hours.
Conservation of Momentum
Conservation of Momentum
Newton's Third Law is sometimes known as The Law of Conservation of
Momentum. One form says that for every force there is an equal
but opposite force. Many people misunderstand this concept because it
involves forces between two bodies. An easier form for most people says
that momentum must be conserved (not be created or destroyed). Momentum
(p=mv) is equal to mass times velocity. Mass is scalar but both
momentum and velocity are vectors which means that momentum must be
conserved in all directions. This is very useful in situations
where two or more bodies are interacting like in a car crash, an
explosion or in discussing the propulsion of objects either in land,
through air or through fluids (most often water).
Useful Links Work & Energy
The Physics Classroom, our usual great link source
The
University of Winnepeg Physics Site
Homework
Help Site
JC Physics
Momentum
The Physics Classroom
From Jim Tucek
The
University of Winnepeg
New York State High School Regents Preparation