Work and Energy
It is true that doing work makes people feel tired; yet feeling tired after standing for a long time in a train, or holding up a heavy weight, does not indicate that work has been done in a scientific sense. The opposing teams in a tug-of-war may fall exhausted to the ground after a 'drawn' match, neither side having moved from their starting positions, but they have done no scientific 'work'. Only when one side begins to pull the other over the line is mechanical work done, and this work is measured as the force applied multiplied by the distance moved: therefore, work = force x distance.
In some ways work, and its near relation, energy, is to physics what wealth and money is to economics. Wealth can exist in many forms. In the pocket it is usually found as coins or bills, but it can be spent in shops to buy goods, saved in a bank account, or invested in property or stock.
When a man lifts a heavy weight high into the air, he 'spends' an amount of work in the form of energy. But this work is really invested, for the weight he has lifted now has more energy than it had before. It is in the bank waiting to be released again. In scientific language, the weight is said to have increased its 'potential energy'. When the weight is allowed to drop towards the ground, the energy is withdrawn from the bank and converted into a new form. The weight gains what is called 'kinetic energy' - that is, energy of motion.
Just before hitting the ground the weight possesses the same amount of energy in this new form as was used by the man to lift it. Then the weight hits the ground and, one could say, fritters away its wealth in a number of new kinds of energy - heat and sound energy. The sound can clearly be heard, but the heat is not quite so evident in this case. Continually dropping the weight would, however, soon heat up the surroundings just as a large nail becomes quite hot as it is hammered into a block of wood.
All these transactions are based on the very important principle known as the law of conservation of energy. Energy can never be created or destroyed - it is simply transferred from one form to another.
Until the 20th century it was thought that man's only available source of energy was the sun. The sun's energy, transmitted as light and heat energy to the earth, was invested in prehistoric times inside the forests of the world and this energy only came to light again when it was dug up as fuel from the ground in the form of peat, coal or natural gas.
Man eventually learned to convert this energy into other forms and even temporarily to re-bank it as electrical energy inside storage cells and batteries for further use. He did not, until very recently, realize that there was another completely new source of energy available to him- atomic energy.
Today atomic energy plants are being built all over the world which are gradually replacing the conventional power stations run on coal or oil. These stations operate by converting matter directly into energy. Matter we now know is itself just another form of energy and this energy is not only the 'wealth' of the universe, it is also the 'stuff' from which the universe is made.