Independent University, Bangladesh

PHY 101

Introduction:
In Newton’s first law we have discussed the equilibrium condition for a particle and
seen that when the resultant force acting on the particle is zero, it is in equilibrium and,
of course, the acceleration of the body is also zero. The next logical step is to ask how
a body behaves when the resultant force on it is not zero. The answer to this question
is contained in second law, which shows that when the resultant force is not zero the
body moves with accelerated motion, and that with a given force, the acceleration
depends on a property of the body known as mass.
Newton’s first law of motion:
According to Newton's first law “An object at rest will remain at rest and an object in
motion continues in motion with the same speed and in the same direction unless it is
acted upon by an unbalanced force.”
This law is often called “the law of inertia". This means that there is a 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 object in motion will maintain this
state of motion.
The equilibrium condition of a body:
If an object is not accelerating, it is in equilibrium. Newton's First Law says that" if the
net force on an object is zero, it will be in equilibrium - it won't accelerate". The
converse of this statement is also true - "If an object is in equilibrium (is not
accelerating), then the net force on it must be zero."
Suppose there is a book lying (at rest) on a table. The book is not accelerating - in
other words, it is in equilibrium. Since it is in equilibrium, the net force on the book is
zero. The Earth is pulling downward on the book with a force we call the book's weight.
Suppose the book's weight is 10 N. If this were the only force on the book, there would
be a net force on the book, and the book would not be in equilibrium. The table must
be exerting an upward force on the book of exactly 10 N in order to balance (or cancel)
the weight. This force, exerted by the table on the book, is called a support force or
normal force.
Newton’s second law of motion:
We know, from experience, that an object at rest never starts to move by itself; a push
or pull must be exerted on it by some other body. Similarly, a force is required to slow
down or to stop a body already in motion, and to make a moving body deviate from
straight line motion requires a sideway force. All these processes (speeding up,
slowing down, or changing direction) involve a change in either the magnitude or
direction of the velocity. Thus in each case the body accelerates, and an external force
must act on it to produce the acceleration. The second law discusses about these
factors and states that “The rate of change of momentum of a body is directly
proportional to the applied force and acts in the direction of the force.”
Mathematically this law can be represented as
dt
dp
F



where p  momentum  mv  massvelocity of the body