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


The Physics Classroom

From Jim Tucek

The University of Winnepeg

New York State High School Regents Preparation