Forces are felt by us in everyday life and in every physical activity we pursue, forming the very basis of the material universe we live in and of life itself. Forces pull things together and often push them apart, they make things move and stop things from moving.
The idea of a physical force has been around since the fourth century BC when Aristotle considered the existence of 'forces' created by living objects (gods, people, animals) and 'tendencies' which were natural or fundamental. His contemporary, Democritus, introduced the idea of a fundamental indivisible particle, the atom. Since that time, natural philosophers and physicists have been trying to understand the fundamentals of nature and today national and international collaborations often spend millions and occasionally billions of dollars to these ends, which at times change the way we live our lives profoundly.
In physics, forces conform to the principle of the preservation of energy or the 1st Law of Thermodynamics, which states that energy cannot be created or destroyed. To date four fundamental natural forces have been identified upon which our perception of the physical world around us relies. These forces are the Gravitational Force, the Electromagnetic Force, the Weak Force and the Strong Force. A fifth may also exist.
Isaac Newton is probably best known for his work on gravity. His Law of Universal Gravitation was the first true attempt at describing a fundamental force of nature in a rigorous and quantifiable manner. He determined that gravity was linked to the mass of an object and that it was weakened by distance. It is understood to be nature's weakest force and different to all the other forces since it is purely attractive. Although Newton's work described the effects of gravity well, it failed to explain why it existed. Einstein's theory of General Relativity was a monumental breakthrough in this respect. It led to the concept of time as a fourth dimension, that a body with a mass would bend light and that this effectively distorts space and time, bringing objects together.
Despite the undeniable success of General Relativity, the theory of Quantum Mechanics was quick to follow. Initiated by Einstein's work on how light interacts with matter, Ernest Rutherford's discovery of radioactivity and the endeavours of physicists Niels Bohr and Richard Feynman, the existence of this bizarre sub-atomic world was firmly established. However, to this day the two theories sit somewhat at odds with one another.
Quantum mechanics inferred that the fundamental forces occur due to the interactions of sub-atomic particles which behave in a manner unlike anything we ever experience, randomly moving from one point to another. They perform a statistical dance with their movements becoming less likely the further away they try to move. They can even affect particles on the other side of the Universe through a phenomenon known as entanglement. The Graviton is such a particle and it is thought to carry the gravitational force. Although the work of Bohr and Rutherford initially led to an understanding of the make-up of atoms through their discovery of protons, neutrons and electrons, it was soon eclipsed by the revelation that these were not necessarily fundamental in nature. In fact, their work culminated in identifying entire families of such fundamental particles, which are responsible for all physical properties such as charge, mass and force.
Throughout history people have been fascinated by electricity and magnetism. Significant progress in understanding these forces truly began in the 1800s when Andre-Marie Ampere quantified the relationship between an electric current and the magnetic force it generated. This was followed by Michael Faraday's groundbreaking work at the Royal Institute and culminated in the development of the field theory of electromagnetism by James Clerk Maxwell during his time at King's College in the 1860s. His work unified the forces of electricity and magnetism, showing that they were one and the same thing, forming electromagnetic waves which could move through the vacuum of space. It defined the nature of light and all other electromagnetic forms of energy, including the radiation of heat. However, the success of this advance led to some unexpected observations. For some inexplicable reason energy was 'reflected' by a body discontinuously, leading to Einstein's discovery of the Photoelectric Effect in 1905 which showed how energy was released in discrete packets or quanta. It established the existence of the Photon, the fundamental particle responsible for the electromagnetic force and the energy which electromagnetic waves carry. It also gave science the first real glimpse of the world of Quantum Mechanics.
The early 1900s were filled with discoveries in physics. Feynman’s work on radioactive beta-decay led to the discovery of one of nature's less obvious fundamental forces, the Weak force. He showed how this force effectively ‘kicked out’ protons and neutrons from the nucleus of an atom, its strength diminishing with sub-atomic distance. He identified a family of fundamental particles known as Fermions which were responsible for this.
It is perhaps surprising that nature's most powerful fundamental force was only discovered 40 years ago. The Strong force was also nature's best hidden force as it could not be detected until the development of high energy particle colliders, which would allow physicists to study sub-atomic particles by literally smashing them apart. Its discovery led to an understanding of how atoms remained intact despite the powerful repulsion experienced by their constituent positively charged protons, and that it is the result of interactions between a family of sub-atomic particles known as quarks which create a force that does not diminish with distance, and lives up to its name.
Modern physics relies on entire families of sub-atomic particles that interact to create the physical world we live in, conveying properties such as charge, mass and force. As such they form the basis of the Standard Model and the current search for a Grand Unified Theory in which all forces might come together to form a single fundamental force. So in time we might be looking at a single force connecting everything in the Universe, much like the fabled Force of Star Wars fame. Science and fiction collide. "May the force be with you!”
Little Einstein’s Corner - Measuring the Force
This experiment shows how mass and gravity produce a downwards force. You will need:
1. A plastic cup
2. Two rulers
3. A heavy book
4. Two rubber bands
5. Some sticky tape
6. A pencil
7. A sheet of paper
8. Some water
9. A pair of scissors
Place one of the rubber bands across the bottom and top of the cups and stick it in place with some tape. Next tie the second rubber band to the middle of the first above the cup so that you can suspend the cup from the ruler with it. Use some more tape to stick this to the ruler. Place the ruler with the cup over the edge of a table and the ruler under your heavy book to hold it in place.
Note the length of the rubber band on the sheet of paper, and then using the measuring jug pour out 100 ml of water into the cup. Do this a number of times measuring by how much the rubber band has stretched, and noting this down. The mass of the water increases each time and so does the force on the rubber band making it longer. Don't wet the kitchen too much!