FORCE

The Meaning of Force

A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces onlyexist as a result of an interaction.

For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:

·                     contact forces, and

·                     forces resulting from action-at-a-distance

Contact forces are those types of forces that result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces. These specific forces will be discussed in more detail later in Lesson 2 as well as in other lessons.

Action-at-a-distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation. Examples of action-at-a-distance forces include gravitational forces. For example, the sun and planets exert a gravitational pull on each other despite their large spatial separation. Even when your feet leave the earth and you are no longer in physical contact with the earth, there is a gravitational pull between you and the Earth. Electric forces are action-at-a-distance forces. For example, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation. And magnetic forces are action-at-a-distance forces. For example, two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters. These specific forces will be discussed in more detail later in Lesson 2 as well as in other lessons.

Examples of contact and action-at-distance forces are listed in the table below.

Contact Forces	Action-at-a-Distance Forces
Frictional Force	Gravitational Force
Tension Force	Electrical Force
Normal Force	Magnetic Force
Air Resistance Force
Applied Force
Spring Force

Force is a quantity that is measured using the standard metric unit known as the Newton. A Newton is abbreviated by an "N." To say "10.0 N" means 10.0 Newton of force. One Newton is the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s. Thus, the following unit equivalency can be stated:

A force is a vector quantity. As learned in an earlier unit, a vector quantity is a quantity that has both magnitude and direction. To fully describe the force acting upon an object, you must describe both the magnitude (size or numerical value) and the direction. Thus, 10 Newton is not a full description of the force acting upon an object. In contrast, 10 Newton, downward is a complete description of the force acting upon an object; both the magnitude (10 Newton) and the direction (downward) are given.

Because a force is a vector that has a direction, it is common to represent forces using diagrams in which a force is represented by an arrow. Such vector diagrams were introduced in an earlier unit and are used throughout the study of physics. The size of the arrow is reflective of the magnitude of the force and the direction of the arrow reveals the direction that the force is acting. (Such diagrams are known as free-body diagrams and are discussed later in this lesson.) Furthermore, because forces are vectors, the effect of an individual force upon an object is often canceled by the effect of another force. For example, the effect of a 20-Newton upward force acting upon a book is canceled by the effect of a 20-Newton downward force acting upon the book. In such instances, it is said that the two individual forces balance each other; there would be no unbalanced force acting upon the book.

Other situations could be imagined in which two of the individual vector forces cancel each other ("balance"), yet a third individual force exists that is not balanced by another force. For example, imagine a book sliding across the rough surface of a table from left to right. The downward force of gravity and the upward force of the table supporting the book act in opposite directions and thus balance each other. However, the force of friction acts leftwards, and there is no rightward force to balance it. In this case, anunbalanced force acts upon the book to change its state of motion.

The exact details of drawing free-body diagrams are discussed later. For now, the emphasis is upon the fact that a force is a vector quantity that has a direction. The importance of this fact will become clear as we analyze the individual forces acting upon an object later in this lesson.

THE TYPES OF FORCE

Different Kinds Of Forces

Gravitational Force

Gravitation is the agent which gives weight to objects with mass and causes them to fall on the ground when dropped. Generally speaking, gravitation causes the dispersed matter in the universe to come together. It is the gravitational force that helps the earth and other planets to stay in their respective orbits around the sun. Gravitational force is also responsible for keeping the moon revolving around the earth, for tidal formations, for natural form of convection and innumerable other minor and major phenomena observed in our day-to-day living. While Isaac Newton defined gravity as a force that attracts all objects to each other, Albert Einstein used the general theory of relativity to explain the concept of gravity. He described that gravitational force was the consequence of the curvature of space-time, which, in turn, governs the motion of inertial objects.

Electromagnetic Force

An electromagnetic force is a particular force or influence that affects charged particles. The photons are messenger particles of an electromagnetic force and are responsible for holding the electrons and protons together in an atom as well as holding the atoms together in a molecule. These particles may be positively or negatively charged. The photons have no individual mass and possess the ability to travel at the speed of light. So, they are easily able to carry the force and attract the electrons and protons together. Electromagnetic force forms the core of the theory of electromagnetism, which explains the relationship between electricity and magnetism. The electromagnetic force plays a role in friction and is a central tenet in Einstein’s Theory of Relativity. Although this force is present practically everywhere, its presence can be proved by the presence of electrically charged bodies, which are either positive or negative. Neutron particles which are neutrally charged are completely immune to it. The electromagnetic force follows the inverse square law, which signifies that the strength of the force is inversely proportional to the square of the distance from the source of the force.

Weak Nuclear Force

The weak nuclear force is one of the four fundamental forces of nature and is also known as weak interaction. This force underlies few forms of radioactivity, governs the decay of unstable subatomic particles such as mesons, and initiates the nuclear fusion reaction that fuels the sun. The weak nuclear force acts upon fermions, which are elementary particles with half-integer values of intrinsic angular momentum or spin. These particles interact with the help of weak nuclear force by exchanging the force-carrier particles known as W and Z particles. The particles are heavy, with a mass content of 100 times the mass of a proton. It is this heaviness, which defines the extremely short range nature of this particular force.  

Strong Nuclear Force

It is one of the four basic forces in nature and as the name implies, it is the strongest of all. Since it has the shortest range, the particles should be extremely close to each other for the effect to be felt. What it actually does is holds the subatomic particles of the nucleus together. We all know that opposites attract. So, two positively charged protons present in an atom should repel each other. This is where the working principle of nuclear force comes in. The strong nuclear force is created between nucleons (protons and neutrons) with the help of exchange of particles called mesons. The nucleons should be closely placed for this exchange to take place.  

Other Forces

• Applied force is a type of force which is applied to any object by a person or any other object. For instance, if you pull a chair in order to sit on it, there is applied force acting upon the chair. The force applied by you on the chair is nothing but applied force.
• Frictional force is the force exerted by a surface on the object moving across it or trying to move across it. The two basic types of frictional force are sliding and static friction. Air resistance force is a special category of frictional force, which acts upon objects traveling through air. It actually opposes the motion of the object in air.
• The tension force is a force transmitted through a string, rope, cable or wire, while it is pulled by opposite forces acting on the other end.
• The spring force is a force which is exerted by a compressed or stretched spring upon a specific object attached to its end. Usually, the magnitude of the force is directly proportional to the amount of stretch or compression of spring.