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motion with the same speed and in the same direction unless acted upon
by an unbalanced force. It is the natural tendency of objects to keep
on doing what they're doing. All objects resist changes in their state
of motion. In the absence of an unbalanced force, an oject in motion
will maintain this state of motion. This is often called the law of
Consider for instance the unfortunate collision of a car with a wall.
Upon contact with the wall, an unbalanced force acts upon the car to
abruptly decelerate it to rest. Any passengers in the car will also be
deccelerated to rest if they are strapped to the car by seat belts.
Being strapped tightly to the car, the passengers share the same state
of motion as the car. As the car accelerates, the passengers
accelerate with it; as the car decelerates, the passengers decelerate
with it; and as the car maintains a constant speed, the passengers
maintain a constant speed as well.
If the car were to abruptly stop and the seat belts were not being
worn, then the passengers in motion would continue in motion. Assuming
a negligible amount of friction between the passengers and the seats,
the passengers would likely be propelled from the car and be hurled
into the air.
An object at rest tends to stay at rest and an object in motion tends
to stay in motion with the same speed and in the same direction unless
acted upon by an unbalanced force.
But what exactly is meant by the phrase unbalanced force? What is an
unbalanced force? In pursuit of an answer, we will first consider a
physics book at rest on a table top. There are two forces acting upon
the book. One force - the Earth's gravitational pull - exerts a
downward force. The other force - the push of the table on the book
(sometimes referred to as a normal force) - pushes upward on the book.
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directions, they balance each other. The book is said to be at
equilibrium. There is no unbalanced force acting upon the book and
thus the book maintains its state of motion. When all the forces
acting upon an object balance each other, the object will be at
equilibrium; it will not accelerated.
Consider another example of a balanced force - a person standing upon
the ground. There are two forces acting upon the person. The force of
gravity exerts a downward force. The floor of the floor exerts an
Since these two forces are of equal magnitude and in opposite
directions, they balance each other. The person is at equilibrium.
There is no unbalanced force acting upon the person and thus the
person maintains its state of motion.
Now consider a book sliding from left to right across a table top.
Sometime in the prior history of the book, it may have been given a
shove and set in motion from a rest position. Or perhaps it acquired
its motion by sliding down an incline from an elevated position.
Whatever the case, our focus is not upon the history of the book but
rather upon the current situation of a book sliding across a table
top. The book is in motion and at the moment there is no one pushing
it to the right.
The force of gravity pulling downward and the force of the table
pushing upwards on the book are of equal magnitude and opposite
directions. These two forces balance each other. Yet there is no force
present to balance the force of friction. As the book moves to the
right, friction acts to the left to slow the book down. There is an
unbalanced force; and as such, the book changes its state of motion.
The book is not at equilibrium and subsequently accelerates.
Unbalanced forces cause accelerations. In this case, since the
unbalanced force is directed opposite the object's motion, it will
cause a deceleration (a slowing down of the object).
If two individual forces are of equal magnitude and opposite
direction, then the forces are said to be balanced. An object is said
to be "acted upon by an unbalanced force" only when there is an
individual force which is not being balanced by a force of equal
magnitude and in the opposite direction .
Inertia and Mass
Inertia = the resistance an object has to a change in its state of
Galileo, the premier scientist of the seventeenth century, developed
the concept of inertia. Galileo reasoned that moving objects
eventually stop because of a force called friction. In experiments
using a pair of inclined planes facing each other, Galileo observed
that a ball will roll down one plane and up the opposite plane to
approximately the same height. If smoother planes were used, the ball
would roll up the opposite plane even closer to the original height.
Galileo reasoned that any difference between initial and final heights
was due to the presence of friction. Galileo postulated that if
friction could be entirely eliminated, then the ball would reach
exactly the same height.
Galileo further observed that regardless of the angle at which the
planes were oriented, the final height was almost always equal to the
initial height. If the slope of the opposite incline was reduced, then
the ball would roll a further distance in order to reach that original
Isaac Newton built on Galileo's thoughts about motion. Newton's first
law of motion declares that a force is not needed to keep an object in
motion. Slide a book across a table and watch it slide to a rest
position. The book in motion on the table top does not come to a rest
position because of the absence of a force; rather it is the presence
of a force - that force being the force of friction - which brings the
book to a rest position. In the absence of a force of friction, the
book would continue in motion with the same speed and direction -
forever! (Or at least to the end of the table top.) A force is not
required to keep a moving book in motion; in actuality, it is a force
which brings the book to rest.
All objects resist changes in their state of motion. All objects have
this tendency - they have inertia. But do some objects have more of a
tendency to resist changes than others? Absolutely yes! The tendency
of an object to resist changes in its state of motion is dependent
upon mass. Inertia is that quantity which is exclusively dependent
upon mass. The more mass which an object has, the more inertia it has
- the more tendency it has to resist changes in its state of motion.
Suppose that there are two seemingly identical bricks at rest on the
physics lecture table. Yet one brick consists of mortar and the other
brick consists of Styrofoam. Without lifting the bricks, how could you
tell which brick was the Styrofoam brick? You could give the bricks an
identical push in an effort to change their state of motion. The brick
which offers the least resistance is the brick with the least inertia
- and therefore the brick with the least mass (i.e., the Styrofoam