Download 1. Newton`s Laws

University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
1. Newton's Laws
Isaac Newton (a 17th century scientist) put these three laws have become known as Newton's
three laws of motion.
Newton's first law of motion An object at rest stays at rest and an object in motion stays in motion with the same speed
and in the same direction unless acted upon by an unbalanced force.
There are two parts to this statement - one that predicts the behavior of stationary objects and the
other that predicts the behavior of moving objects. The two parts are summarized in the following
diagram.
For Newton's first law of motion predicts the behavior of objects for which all existing
forces are balanced. The first law - sometimes referred to as the law of inertia - states that if the
forces acting upon an object are balanced, then the acceleration of that object will be 0 m/s/s.
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
Objects at equilibrium (the condition in which all forces balance) will not accelerate. According
to Newton, an object will only accelerate if there is a net or unbalanced force acting upon it. The
presence of an unbalanced force will accelerate an object - changing its speed, its direction, or both
its speed and direction.
Newton's second law of motion
The behavior of objects for which all existing forces are not balanced. The second law
states that the acceleration of an object is dependent upon two variables
The net force acting upon the object
The mass of the object.
The acceleration of an object depends
Directly upon the net force acting upon the object.
Inversely upon the mass of the object.
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
As the force acting upon an object is increased, the acceleration of the object is increased.
As the mass of an object is increased, the acceleration of the object is decreased.
Second Law of Motion. It states, “The force acting on an object is equal to the mass of that
object times its acceleration.” mathematical form as:
F = ma
F is force,
m is mass
a is acceleration
. The math behind this is quite simple. If you double the force, you double the acceleration,
but if you double the mass, you cut the acceleration in half.
Consistent with the above equation, a unit of force is equal to a unit of mass times a unit
of acceleration.
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
1 Newton = 1 kg • m/s2
The definition of the standard metric unit of force is stated by the above equation. One
Newton is defined as the amount of force required to give a 1-kg mass an acceleration of 1 m/s/s
Newton's Third Law
. Formally stated, Newton's third law is:
(For every action, there is an equal and opposite reaction).
The statement means that in every interaction, there is a pair of forces acting on the two
interacting objects. The size of the forces on the first object equals the size of the force on the
second object. The direction of the force on the first object is opposite to the direction of the force
on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.
Energy
Defined as the capacity of a physical system to perform work. Object must have energy to
accomplish work - it is like the "currency" for performing work. In SI units, energy is measured in
joules then to do 100 joules of work, need expend 100 joules of energy. Energy transferred to
other objects or converted into different forms, but cannot be created or destroyed.one joule is
defined "mechanically", being the energy transferred to an object by the mechanical work of
moving it a distance of 1 meter against a force of 1 newton.
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
There are two main types of energy
 Kinetic energy
 Potential energy
Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or
horizontal motion - has kinetic energy Kinetic energy is a form of energy that represents the energy
of motion. It is a scalar quantity, which means it has a magnitude but not a direction. It is, therefore,
always positive (as will be evident when we see the equation that defines it).
There are many forms of kinetic energy - vibrational (the energy due to vibrational motion),
rotational (the energy due to rotational motion), and translational (the energy due to motion from
one location to another). To keep matters simple, we will focus upon translational kinetic energy.
The amount of translational kinetic energy (from here on, the phrase kinetic energy will refer to
translational kinetic energy) that an object has depends upon two variables: the mass (m) of the
object and the speed (v) of the object. The following equation is used to represent the kinetic
energy (Ek) of an object.
Ek=0.5m*v2
m is mass
v is velocity
This equation reveals that the kinetic energy of an object is directly proportional to the
square of its speed. That means that for a twofold increase in speed, the kinetic energy will increase
by a factor of four. For a threefold increase in speed, the kinetic energy will increase by a factor
physics 1st stage
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
petroleum dept.
of nine. And for a fourfold increase in speed, the kinetic energy will increase by a factor of sixteen.
The kinetic energy is dependent upon the square of the speed. As it is often said, an equation is not
merely a recipe for algebraic problem solving, but also a guide to thinking about the relationship
between quantities.
Kinetic energy is a scalar quantity; it does not have a direction. Unlike velocity,
acceleration, force, and momentum, the kinetic energy of an object is completely described by
magnitude alone. Like work and potential energy, the standard metric unit of measurement for
kinetic energy is the Joule. As might be implied by the above equation, 1 Joule is equivalent to 1
kg*(m/s) ^2.
1 Joule = 1 kg • m2/s2
Question 1: A body of mass 75 kg has a velocity of 20m/s. Calculate its kinetic energy?
Solution:
Where m = Mass = 75 kg,
v = 20m/s.
Kinetic energy, K.E. =0.5 mv2
=0.5 × 75kg × 400 m2/s2
= 15000 Joules.
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
physics 1st stage
petroleum dept.
Potential energy: it is the stored energy is the ability of a system to do work due to its position or
internal structure. For example, gravitational potential energy is a stored energy determined by an
object's position in a gravitational field while elastic potential energy is the energy stored in a
spring.
Forms of potential energy
Gravitational Potential Energy
Elastic Potential Energy
Electromagnetic Potential Energy
Nuclear Potential Energy
u=mgh
Solved Examples
Question 1: Find the potential energy of a body of mass 5Kg and is 25m above from the ground?
Solution:
Given: Mass m = 5Kg,
Height h = 25 m,
The Potential energy is given by U = mgh
= 5 Kg × 9.8 m/s2 × 25 m
= 1225 J.
Question 2: If we throw a body upwards with velocity of 4ms-1, then at what height its kinetic
energy reduces to half of the initial value? Take g = 10 ms-2.
Solution:
University of Thi-Qar
College of Engineering
Dr. Tuqa Abdulrazzaq
Initial energy = 0.5 m(4)2 = 8m m/s2.
Let kinetic energy at a height h be 82 = 4m m/s2.
It will also be equal to the potential energy.
Hence K.E. = P.E
0.5 mv2 = mgh.
4m m/s2 = m × 10 m/s2 × h
h = 0.4m.
physics 1st stage
petroleum dept.