

In physics, a law is the result of experimental observation. The laws of nature
can not be proved by mathematical manipulation of mathematical expressions,
they are only supported by experimental observations.
The first scientist to recognize the law of inertia was Galileo
Galilei. Galileo's conclusion was deduced after observing the motion of an
object moving over two incline planes set as shown in Figure 2. The figure
below shows Galileo's experiment in three different parts. In all parts, two
inclined planes are placed opposite to each other, A and B. At the top of the
inclined plane A, a ball is let roll freely toward the bottom of the inclined.
The inclines A and B are placed in such a way that when the rolling object
reaches the bottom of the first inclined, the object rolls smoothly over the
second plane, B. In this experiment, Galileo's observes that no matter what is
the slope of the incline B, the tendency of the rolling ball is to reach the
height (level) from where the ball was originally released (dashed red level).
Part I and Part II of Figure 2 illustrate this experiment for to different inclinations
for plane B. As Galileo theorized, Part III shows the extreme case when the
incline B is horizontal. In this case, the ball should roll forever because the
ball can not reach the original height (red level) when no external force is
applied to the rolling ball.
Newton's Law  Figure 2  Galileo's Inertia Experiment


Nevertheless, erroneously, Galileo Galilei
excluded from the requirement of satisfying the law of inertia objects moving
in circles (such as celestial objects). When circular motion is presented in
xxx, it will be understood that circular motion also follows the same rules,
in accordance with the law of inertia. The way in which the law of inertia is
presently understood was firstly recognized by Rene Descartes.
As the result of these observations, the law of inertia can be stated:
If no net force acts on an object, the
object maintains the same state of motion.
For an object, to maintain the same state of motion means that the vector
velocity of the object is unchanged. Thus, the magnitude of the velocity is
constant and the direction of motion is unchanged. An object at rest correspond
to the special case of zero velocity.
The law of inertia implies that if no external force act on an object,
when at rest, the object will stay at rest; and, when in motion the object
will move in straight line with constant velocity.
In order to understand the law of inertia in practical situations,
remember that friction is a force, Thus, imagine a wooden block sliding over
a surface such as the floor of a classroom as compared to the same block
sliding over the icy surface in an ice skating ring. Where does the block
slide further? Of course, the block will slide the further on the skating
ring.
Definition of mass: Inertia is the tendency of an object to maintain the same
state of motion. The mass is the quantity used to measure the inertia of an
object. In fact, the relation is equivalent to the relation between height
and tall.
How tall is an object? The answer to
this question is the height of the object.
Similarly,
How much inertia an object has? It
depends on how much mass the object has.
An object with a large inertia will have a large mass. If
there were an object for which no force is required to change the state of
motion, that object will not have inertia and no mass. Therefore, the minimal
mass for an object is zero. Additionally, there is not such thing as
objects with negative masses (minimal inertia correspond to zero mass). Experimentally,
negative masses have never been observed. Thus
all masses are positives.
The mass of an object is measured in Kilograms (kg) in the
International System (SI) of measurements.
The following are the approximate masses of different
objects:

Mass (kg) 
Universe 
1 × 10^{52} 
Milky Way Galaxy 
7 × 10^{41} 
Earth 
2 × 10^{30} 
Human 
7 × 10^{1} 
Mosquito 
1 × 10^{5} 
Hydrogen Atom 
1.67 × 10^{27} 
Electron 
9.11 × 10^{31} 
Suppose that you are inside a bus that it is being driven at
constant velocity. In the center of the isle there is a soccer ball which is
free to move. As long as the bus moves at constant velocity, the soccer ball
stays at the same place at the center of the isle.
However, if the driver suddenly applies the break, the
soccer ball will roll to the front of the bus without the application of a
force. An observer inside the bus, observer A, will conclude that the
ball does not obey the law of inertia. Is the observer right?
On the other side, for an observer on the street, observer
B, the ball rolls because it obey the law of inertia. When the brakes
are applied to the bus, the brake pads exert the necessary force to slow down
the wheels and the bus. At the same time, there is not a force to slow down
the ball; therefore, the ball will maintain the same state of motion without
changing the velocity and moving faster than the bus reaching the front of
the bus. In this example there are two observer, the one inside the bus for
which the law of inertia is not valid, and the one on the street for
which the law of inertia is valid. The last one is called an Inertial
Frame of Reference.
Frames of reference where the
law of inertia is valid are called inertial frames of reference.
The frame of reference associated to the observer inside the bus is not an
inertial frame of reference. In conclusion, accelerating
frames of reference ARE NOT inertial frames of reference.
If you are carrying a heavy bag
of groceries and bang your hand against the wall, the concept that most
explains why you are hurt is
N 
a) 
inertia 


b) 
acceleration 

c) 
resistance. 

d) 
gravity 

e) 
None of the above. 
A truck is moving at constant velocity. Inside the storage
compartment, a rock is dropped from the midpoint of the ceiling and strikes the
floor below. The rock hits the floor

a) 
ahead of the midpoint of the ceiling 


b) 
behind the midpoint of the ceiling 

c) 
more information is need to solve this problem 
N

d) 
exactly below the midpoint of the ceiling 

e) 
None of the above. 
The law of inertia fails when
N 
a) 
The frame of reference is accelerating 


b) 
The frame of reference is moving with unknown
constant velocity. 

c) 
The frame of reference is not moving. 

d) 
The mass of the object is unknown 

e) 
None of the above. 
