Isaac Newton defined
three laws concerning the behavior of moving objects in
the 1687. These scientific statements help to explain
the nature of matter and space. Newton's first law of
motion is often called the Law of Inertia. His second
law concerns forces and acceleration. His third law is
often called the Action-Reaction Law of Motion. It is
amazing that he was able to formulate these laws of
motion through his observations so many years ago.
Some questions you may have are:
What is the Law of Inertia?
What is Newton's Second Law?
What is action-reaction about?
This lesson will answer those questions. There is a
mini-quiz near the end of the lesson.
Newton's First Law
Newton's First Law was actually formulated by Galileo
many years previous. It is called the Law of Inertia and
Every object in a state of uniform motion tends to
remain in that state of motion unless an external force
is applied to it.
Another way of stating this law in more detail is:
If an object is motionless, it will stay motionless
unless acted upon by some force.
If an object is moving at a constant speed or velocity,
it will continue at that speed unless acted upon by some
force along the line of motion.
If an object is moving, it will move in a straight line
unless acted upon at an angle by some force.
The Law of Inertia assumes there is no friction or other
resistive force that can slow down an object. Inertia
can be best demonstrated in outer space.
Objects that are not moving will remain motionless,
unless you apply a force such as a push or pull. This
law of nature makes sure things will stay where you put
A trick using inertia
There is the parlor trick of quickly pulling a
tablecloth from under a setting of heavy dishes or some
other objects, leaving them on the table. This trick
works because the inertia of the heavy objects tends to
keep them in place. By quickly pulling the tablecloth,
the force of friction is easily overcome. If the
tablecloth was pulled slowly, the friction would be
greater than the inertia, and the dishes would follow
(Note: The tablecloth must be pulled down at the edge,
otherwise the dishes may fly upward.)
Once you start an object moving, it will keep moving
unless you apply a force in the opposite direction to
slow it down. Typically, the force of friction will slow
things down. But in order space, where friction is
almost zero, an object will move at its given velocity
forever unless acted upon by some external force.
If you push a moving object in the direction of motion,
it will accelerate to a new velocity. Once you stop
pushing, the object will continue at the velocity it had
once you stopped pushing.
The Law of Inertia states that moving objects go in a
straight line. You must apply a force on an object to
make it go in a circular motion. For example, when you
spin an object around on a string, you are applying a
force on that object from the string to make it go
around. Once you let the string (or the force) go, the
object will fly off in a straight line.
Likewise, the Moon is attracted to the Earth by the
force of gravity. That force is just enough to make the
Moon spin around the Earth. If gravity would stop, the
Moon would fly off in a straight line into outer space.
Newton's Second Law
The second law is sometimes called the Law of Dynamics,
because it concerns forces and what causes objects to
move. It can be stated as:
The acceleration of an object of constant mass is
proportional to the force acting upon it.
Acceleration is the changing of the velocity of the
object. Usually, we are talking about the object
speeding up. The word "deceleration" is usually used
when the object is slowing down, but it also is
acceleration or changing of the velocity.
A force is a push or pull on the object. It may pushing
in direct contact or pulling at a distance in the case
This law determines the relationship between force, mass
F = m*a
a = F/m
Note that the force F and acceleration a are in the same
direction. Since they have a direction, they are called
What this law says is that while you are applying a
force on an object, it will continue to accelerate or
change its velocity. It also states that the greater the
force on an object, the greater the acceleration.
Newton's Third Law
Newton's Third Law is sometimes called the Law of
Reciprocal Actions or the Action-Reaction Law:
Whenever one body exerts force upon a second body, the
second body exerts an equal and opposite force upon the
This is often stated as: "For every action there is an
equal and opposite reaction," which can be confusing and
even incorrect in some situations. Although it is easier
to remember, it probably should be avoided.
One example of this law is if you push against a door
with a certain force, the door is also pushing with the
same force against you.
Adding Newton's Second Law to this, if you and a friend
are on roller skates or ice skates and facing each
other, and then you push on your friend with a certain
force, your friend will be accelerated backwards
according to F = m*a. But because of Newton's Third Law,
your push causes an equal opposite push on you. So you
will also accelerate backwards.
The force you apply on your friend is F = m*a. So, the
acceleration (a) of your friend's motion is dependent on
the force you push (F) and his or her mass (m).
But also, that same force is applied on you. Suppose we
call your mass (M) and your acceleration (A). Then since
the forces are the same M*A = m*a. If your weight (or
mass) is twice that of your friend, then your friend
would move back twice as fast as you.
2*m*A = m*a
a = 2*A
Note: If you weigh twice as much as your friend, then
You should go on a diet,
You should get bigger friends, or
You should not push little people around
The action-reaction law also applies to the force of
gravity, especially combined with Newton's Law of
Dynamics. If you jump off a ladder, the force of gravity
will pull you to the Earth according to F = m*g, where m
is your mass and g is the acceleration due to gravity.
But that same force is working in an opposite direction
on the Earth, pulling it toward you according to F =
M*G, where M is the mass of the Earth and G is its
acceleration. Since the mass of the Earth is so much
greater than your mass, its movement is extremely small.
Isaac Newton defined his three laws of motion, which are
the Law of Inertia, the Law of Dynamics and the Law of
Reciprocal Actions. These laws can be verified in many
common experiments, and they explain how and why objects
move when forces are applied to them.