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Newton’s Laws of Motion - 1
Classical Mechanics
Describes the relationship between the motion
of objects in our everyday world and the forces
acting on them.
Conditions when Classical Mechanics does not
apply:
- Very small objects (< atomic sizes)
- Objects moving near the speed of light
Forces
Simple definition: a force is a push or pull on
some object.
Force is a vector quantity.
Distunguish:
contact forces
and
remote (“action at a distance” or “field”
forces.
Contact forces result from physical
contact between two objects.
Field forces act between disconnected
objects.
Examples of Contact and Field Forces
The Four Fundamental Forces
In order of decreasing strength:
Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravity
All are field forces.
Only gravity and electromagnetic forces
are treated in classical mechanics.
Lex 1: Corpus omne perseverare
in statu suo quiescendi vel
movendi uniformiter in directum,
nisi quatenus a viribus impressis
cogitur statum illum mutare.
Law 1: Every body continues in its state of
rest, or of uniform motion in a straight line,
unless it is compelled to change that state by
forces impressed upon it.
Law 1: Every body continues in its state of
rest, or of uniform motion in a straight line,
unless it is compelled to change that state by
forces impressed upon it.
This is sometimes called the law of inertia.
Lex II: Mutationem motus proportionalem esse
vi motrici impressae, et fieri secundum lineam
rectam qua vis illa imprimitur.
The acceleration produced by a particular
force acting on a body is directly proportional
to the magnitude of the force and inversely
proportional to the mass of the body.
Lex II: Mutationem motus proportionalem esse
vi motrici impressae, et fieri secundum lineam
rectam qua vis illa imprimitur.
The acceleration produced by a particular
force acting on a body is directly proportional
to the magnitude of the force and inversely
proportional to the mass of the body.
F

a
m
Second Law:
F

a
m
1. The acceleration vector is in the direction of
the net force.
1.
a is in the direction of
a 
F
1
a 
m
F
Second Law:
F

a
m
2. The magnitude of the acceleration vector is
proportional to the magnitude of the net force.
a is in the direction of
2.
a 
F
1
a 
m
F
Second Law:
F

a
m
3. The magnitude of the acceleration vector is
inversely proportional to the mass.
a is in the direction of
a 
3.
F
1
a 
m
F
The SI unit for force:
The force that gives an object of 1 kg mass an
acceleration of 1 m/s2 is defined as
1 newton (N).
1 N = 1 kg.m/s2