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Newton's Laws of Motion
1.Newton
1stlaw of motion
2.Newton 3rdlaw of motion
3.Newton 2ndlaw of motion
Newton 1stlaw of motion
Newton's
first law of motion is often stated
as:
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
there are two parts to this statement:
1-prediction of the behavior of stationary object
2-prediction of the behavior of moving object
The two parts are summarized in the following diagram
Balanced and Unbalanced Forces

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 downward
Force the other force the push of the table on the book
(normal force) pushes upward on the book
Since the tow forces are of equal magnitude and in opposite
Direction they balance each other the book is said to be in
Equilibrium there is o unbalanced force act upon on the book
And thus the book maintain its state of motion
Newtons 2nd law of
motion
For every action there is an equal and opposite
reaction.
In other words, when one object exerts a force on .
another object, the second object exerts a force of
equal strength in the opposite direction on the first
object

Consider the flying motion of birds. A bird
flies by use of its wings. The wings of a
bird push air downwards the air must also
be pushing the bird upwards Actionreaction force pairs make it possible for
birds to fly.
Newton 2ndlaw of motion
The acceleration of an object is directly
proportional to the net force acting on the
object, is in the direction of the net force, and is
inversely proportional to the mass of the object.
Example: what force is necessary to cause a 1000
kg car to accelerate at 2 m/s2?
F=m a = 1000 kg x 2 m/s2 = 2000 kg-m/s2 = 2N


1.
2.
3.
From our daily life, we can can observe that:
When a force F is applied on an object, V, the
change of the velocity of the object, increases
with the length of time delta t increases;
The greater the force F, the greater V; and
The larger the body (object) is, the less easily
accelerated by forces.
Newton's second law
The acceleration is directly proportional to
the
net force
applied.
•The net force also
determines
the direction
of the

acceleration
•The direction of the acceleration is in the same direction as
the net force.
•The acceleration is inversely proportional to the mass of the
object.
Newtons second law state that:
"If the net force on an object is not zero, the
object will accelerate. The direction of the
acceleration is the same as the direction of
the net force. The magnitude of the
acceleration is directly proportional to the
net force applied, and inversely proportional
to the mass of the object."


F  ma
Free Body Diagrams

Some examples will have more than one force
acting on an object. It is advisable to draw a
diagram of the situation showing the direction of
all forces present acting through one point.
These are known as free body diagrams.
Examples
A woman is standing on a set of bathroom
scales in a stationary lift everyday .The
reading on the scales is 500 N. When she
presses the ground floor button, the lift
accelerates downwards and the reading on
the scales at this moment is 450 N. Find
the acceleration of the lift

A ski tow pulls two skiers who are connected by a thin
nylon rope along a frictionless surface. The tow uses a
force of 70 N and the skiers have masses of 60 kg and
80 kg.
Find
a) acceleration of the system
b) tension in the rope.
a) Total mass, m = 140 kg, a = F/m = 70/140 = 0.5 m s-2
b) Consider the 60 kg skier alone.
Tension, T = ma = 60 x 0.5 = 30 N
Force Acting Down a Plane

If an object is placed on a slope then its
weight acts vertically downwards. A
certain component of this force will act
down the slope. The weight can be split
into two components at right angles to
each other.