Force has a specific meaning in physics, and – unlike in movies – it doesn't have anything to do with the underlying harmony of the universe. In physics, a force is a push or pull resulting from an interaction between two objects. A force can result from direct contact, such as a child pushing a wagon, or from action at a distance, such as the gravitational attraction the Earth exerts on the moon. Within these two broad categories, it's possible to identify at least 10 different forces that help shape the universe and condition our experience in it.
When he formulated his laws of motion, Sir Isaac Newton no doubt imagined contact forces as his primary examples. These are the forces that result from direct physical interaction between two objects. According to Newton's Second Law:
a force of magnitude F produces an acceleration "a" when applied to an object with mass "m."
Applied Force – This is the easiest type of force to understand. Push on an object and the object pushes back, says Newton's First Law, until the magnitude of the force overcomes the object's inertia. At that point, the object begins moving and, in the absence of other forces, accelerates by an amount proportionate to the magnitudes of its mass and the applied force.
Normal Force – Force is a vector quantity, which means its magnitude depends on direction. In any interaction between two objects, the normal force is the force perpendicular to the interface between the interacting objects. Normal force doesn't always produce movement. For example, a table exerts a normal force on a book to overcome the force of gravity and keep the book from falling.
Frictional Force – Frictional force usually resists movement. It's a result of the fact that surfaces in the real world aren't perfectly smooth. The magnitude of the frictional force exerted by a surface depends on the coefficient of friction of the material from which the surface is made as well as that of the object moving along it. The force of friction on a resting object, called static friction, is different from that on a moving object, called sliding friction.
Air Resistance – Objects moving through Earth's atmosphere encounter a resistive force created by the friction generated by air molecules. This force becomes stronger with increasing speed and increasing surface area perpendicular to the direction of motion. It's an important quantity in the aviation and aerospace industries.
Tension Force – Tie a string to a fixed object, pull on the other end, and the string pulls back until it breaks. The force the string exerts is the tension force, which is applied along its length. It's a property of the material from which the string is made as well as the diameter.
Spring Force – The amount of force necessary to compress a spring depends on the material from which the spring is made, the diameter of the wire that forms the coils, and the number of coils. These properties are quantified in a number characteristic of the spring called the spring constant "k." The force needed to compress the spring a distance "x" is given by Hooke's Law:
Action at a Distance Forces
The fundamental forces of nature that keep the planets spinning and the sun and stars burning all act at a distance. Without them, the universe we know probably would not exist or, if it did, it would be a very different place.
Gravitational Force – The reason for the existence of this force is something of a mystery, but if it didn't exist, planets and stars wouldn't be able to form. The magnitude of the gravitational force objects exert on each other depends on the masses of the objects and the inverse of the square of the distance between them. The more massive the objects and/or the shorter the distance between them, the stronger the force.
Electromagnetic Force – Although they don't seem to be the same, electricity and magnetism are related. Flowing electrons produce magnetism, and a moving magnet produces electricity. The relationship between these phenomena was explained by Scottish physicist James Clerk Maxwell in the 19th century and is quantified in his equations. Electricity exerts a force via the attraction or repulsion of charged particles, whereas the magnetic force is due to the attraction or repulsion caused by magnetic poles.
The Strong Force – Because all protons are positively charged, they repel one another, and they wouldn't be able to form an atomic nucleus if the strong force didn't exist to hold them together. The strong force is the most powerful force in nature. It's also the one that binds quarks together to form protons and neutrons.
The Weak Force – The weak force is another fundamental nuclear force. It's stronger than gravity, but it only works at infinitesimally short distances. Carried by subatomic bundles of energy called bosons, the weak force causes protons to change into neutrons and vice versa during nuclear decay. Without this force, nuclear fusion would be impossible, and stars, such as the sun, wouldn't exist.