Describing Motion With Words - Intro
Lesson 2: 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 types of forces in which the two
interacting objects are physically contacting each other. Examples of
contact forces include frictional forces, tensional forces, normal
forces, air resistance forces, and applied forces.
Action-at-a-distance forces are types of forces in which the
two interacting objects are not in physical contact with each other,
yet are able to exert a push or pull despite a physical separation.
Examples of action-at-a-distance forces include gravitational forces
(e.g., the sun and planets exert a gravitational pull on each other
despite their large spacial separation; even when your feet leave the
earth and you are no longer in contact with the earth, there is a
gravitational pull between you and the Earth), electric forces (e.g.,
the protons in the nucleus of an atom and the electrons outside the
nucleus experience an electrical pull towards each other despite
their small spacial separation), and magnetic forces (e.g., two
magnets can exert a magnetic pull on each other even when separated
by a distance of a few centimeters).
Force is a quantity which is measured using the standard
metric unit known as the Newton. One Newton is the amount of
force required to give a 1-kg mass an acceleration of 1 m/s/s. A
Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of
force. Thus, the following unit equivalency can be stated:
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. 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. Furthermore, because forces are vectors, the influence of an individual force upon an object is often cancelled by the influence of another force. For example, the influence of a 20-Newton upward force acting upon a book is cancelled by the influence 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.
Lesson 2: Force and Its Representation