Force and Its Representation
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 only exist 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 which 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 which 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.
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Force is a quantity which is measured using the standard metric unit known as the Newton. A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons 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 which 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 Newtons is not a full description of the force acting upon an object. In contrast, 10 Newtons, downwards is a complete description of the force acting upon an object; both the magnitude (10 Newtons) and the direction (downwards) are given.
Because
a force is a vector which 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 of physics. The size of
the arrow is reflective of the magnitude of the force and
the direction of the arrow reveals the direction which 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 affect of an individual force upon an object is often
canceled by the affect of another force. For example, the
affect of a 20-Newton upward force acting upon a book is
canceled by the affect 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, an unbalanced
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 which 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.