Newton’s Laws
Force and Motion
 The relationship between a force and
the acceleration it causes was first
described by Isaac Newton (16421727).
Newtonian
Mechanics
 Although Newtonian mechanics are
able to describe most interactions,
they do not apply to all situations.
 For very large speeds (an
appreciable fraction of the speed
of light), we must use Einstein’s
Special Theory of Relativity.
 On an atomic scale, we must use
quantum mechanics.
If there is no net force on an
object, it will not accelerate (or
its velocity will not change).
Newton’s
1st Law
In more familiar words:
An object at rest will stay at
rest unless acted on by a net
force; likewise an object in
motion will stay in motion
unless acted on by a net
force.
What is a
force?
Forces are generally
defined as something
that causes a change in
an objects motion
(acceleration).
Categorized as either
contact forces or field
forces
The net force on an
object is equal to the
product of the object’s
mass and its
acceleration.
Newton’s
2nd Law
𝐹 = ma
Only consider forces
acting on a single object
when using this
equation.
For every force, there is an
equal and opposite force.
Newton’s
3rd Law
When two objects interact,
the forces on the objects from
each other are always equal in
magnitude but opposite in
direction.
Common
Forces
Gravitational
Normal
Tension
Friction
Any two objects with mass
exert a force on each other.
For now, gravitational force on
an object is caused by the
Gravitational
interaction between the object
Force
and the earth.
𝐹𝑔 = 𝑚𝑎𝑔
The force of gravity on an
object is called its weight.
Mass is the measure of
how much matter
makes up an object.
Mass and
Inertia
An object’s mass
determines its ability to
resist a change in its
state of motion (an
acceleration) -- Inertia
The normal force on an object
is the reaction force caused by
contact between the object
and another surface.
Normal
Force
The normal force is always
perpendicular to the contact
surface.
When a cord is attached to an object
and pulled taut, a force is exerted on
(and away) from the object, because
the cord is said to be under tension.
Tension
The cord is often defined as massless
and unstretchable.
Friction results from the two
surfaces being pressed together
closely, causing intermolecular
attractive forces between
molecules of different surfaces.
Friction
As such, friction depends upon the
nature of the two surfaces and
upon the degree to which they are
pressed together.
𝑭𝒇 = 𝝁𝑭𝑵
Frictional Force
Lab