Download Newton`s Laws and Forces

NEWTON’S LAWS AND
FORCES
Chapter4
NEWTON’S FIRST LAW OF
MOTION
If
the net force exerted on an
object is zero, the object is zero ,
the object continues in its original
state of motion .If an object is at
rest it remains at rest .If it is
moving with constant velocity it
continues with that same velocity
in direction and magnitude.
NEWTON’S FIRST LAW
 When
an object does not interact with other
objects it can :
 Move in a straight line at constant speed
{i.e. constant velocity}
 Stay still {Newton’s first law of motion}
 Applications:
 Seat belt.
 Kids and the front seat
 Going out of a bus
FORCES
Are
interactions between objects
and other objects. An object may
change its speed or direction of
motion (i.e. accelerate) only by
interacting with other objects,
These interactions are called
forces.
CONTACT AND FIELD FORCES
MASS AND INERTIA
Inertia
is the tendency of an
object to continue in its
original motion
Mass is a measure of the
resistance of an object to
change in its motion due to
a force
NEWTON’S SECOND LAW

The acceleration of an object is directly proportional
to the net force acting on it and inversely
proportional to its mass
F
a
m
F  ma
UNITS OF FORCE AND MASS
 SI
system
 Units of force
 Newton
kg.m
1N  1 2
s
 Mass
unit is kg
 U.S.
customary
system
 Units of force
 Pound
1lb  1
slug  ft
s2
 Mass
unit is slug1
 N=0.225lb
THE GRAVITATIONAL FORCE
Definition
: is the mutual force
of attraction between any two
objects in the universe.
It is the weakest of the
fundamental forces.
NEWTON’S LAW OF UNIVERSAL
GRAVITATION

:every particle in the Universe attracts every other
particle with a force that is directly proportional to
the product of the masses of the particles and
inversely proportional to the square of the distance
between them.

G=6.67 X 10-11 N.m2/kg2
m1m2
F G
2
r
WEIGHT

Definition (w) :The magnitude of the gravitational
force acting on an object of mass m near Earth’s
surface
 w=mg
Variation of weight with altitudes.
ME
g G 2
r
NEWTON’S THIRD LAW
If two objects interact ,the force F12exerted by object
1 on object 2 is equal in magnitude but opposite in
direction to the force F21exerted by object 2 on
object 1.
 Forces in nature always exist in pairs.
 A single isolated force cannot exist.
 The normal force .
 Applications of Newton’s Third Law.

FORCES

A tennis ball moving to the right by a force to the
right has an acceleration to the right
Force
Velocity
acceleration

Speed increases.
FORCES

A tennis ball moving to the right by a force to the left
has an acceleration to the left
Force
Velocity
acceleration

Speed decreases.
IMPORTANT CONCLUSION
If
a single force acts on an object,
the force and the object’s
acceleration are in the same
direction!
If there are more than one force
the acceleration will be in the
direction of the net force(i.e.
Resultant)
THE MAIN FORCES DISCUSSED IN THIS
CLASSROOM
 Gravity
 Attractive
 Gravity:
force (pulls objects together)
an attractive force that all
objects exert on each other.
TENSION FORCE

Is the force exerted by a stretched cord on an
object attached by one end.
20N
20N
 
T F
string exerts
on the left scale
 
T exerts
F string
on the
right scale
As long as no forces act on interior of string tension is the same
throughout string.
NORMAL FORCE
 Which
is perpendicular to surface
exerting force.
Friction
 is
Force:
the force exerted parallel to the
surface exerting the force.
 Friction opposes sliding motion
between two objects with forces in
comtact.
FREE BODY DIAGRAM
Must
identify all the forces
acting on the object of interest
Choose an appropriate
coordinate system
If the free body diagram is
incorrect, the solution will likely
be incorrect
FREE BODY DIAGRAM, EXAMPLE
 The
force is the tension
acting on the box


The tension is the same at
all points along the rope


G & N are the forces
exerted by the earth and
the ground