The Concept of Force

A force is a push or pull. Typically, we think of forces arising from contact between objects, but forces can act over distances without contact. Think of a magnet, for example pulling a nail across the table. Or think of the Sun pulling the planets in their orbits.

What force (if any) causes a distant star to drift freely through space? What causes your socks to stick to your shirt when pulled from a dryer? What pulls a baseball back to earth after it is hit? Scientists have identified four categories of forces. Note, they are arranged in order of strength.

Type of Force	Relative Strength	Description
Strong Nuclear	1	A force of attraction between the nucleons (protons and neutrons) inside the nucleus of atoms.  This force holds the nucleus together, but acts over very small distances (10-15m) only.
Electromagnetic	1/100	A force of attraction and repulsion between charges (electrons and protons mainly).  Opposite charges attract while like charges repel.  This force acts over large and small distances, and follows an inverse square law, such that , where Q is charge and r is the separation between the charges.
Weak Nuclear	1/1014	A short-range nuclear force that tends to cause instability in certain nuclei.
Gravitational	1/1032	A force of attraction between any two masses.  This force acts over large distances and follows an inverse square law, such that, , where M is mass and r is the separation between the masses.

The two forces that govern most of our everyday experiences are the gravitational and the electromagnetic forces. Gravity is responsible for giving things weight, for holding us to the earth and making things fall. Contact forces, like kicking a ball or walking up stairs are electromagnetic forces, arising from the interactions between the negatively charged electrons on the outside of the atoms that make up all matter.

Force is a vector—it has both magnitude and direction. The SI unit of force is the Newton, N.

Inertial Mass

If you attempt to change the state of motion of any body, the body will resist this change. Inertia is the property of matter that relates to the tendency of an object to remain at rest or in uniform motion. For instance consider two large solid cylinders of equal size, one being made of balsa wood and the other steel. If you were to push the cylinders to make them roll, it would certainly take more effort to get the steel cylinder rolling. Likewise, once they are in motion, it would require more effort to bring the steel cylinder to rest. Therefore we say that the steel cylinder has more inertia than the balsa-wood cylinder.

Mass is the term used to measure inertia and the SI unit of mass is the kilogram. The greater the mass of a body, the less it will accelerate (change its state of motion) under the action of an applied force.

Mass is independent of location. If you have a mass of 65kg on earth and you take a vacation on the moon, you will still have a mass of 65kg.