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Forces & Newton’s Laws
Chapter 11 Notes
Forces
A
force is a push or pull exerted on
an object
Forces tend to change the motion of
an object
A force has two components
Magnitude (how much)
Direction (which way)
Mass vs Weight
Many
confuse the difference
between mass and weight
Mass
= How much matter an object has
Measured
Weight
typically in grams (or kg)
= Gravitation pull on an object
Weight
= mass * gravitational acceleration
Mass vs Weight
Take
an object of mass 100 kg
On earth the mass = 100 kg
On the moon the mass - 100 kg
In outer space the mass = 100 kg
Weight
of same object
On Earth the weight = 980 N
On the moon the weight = 160 N
In outer space the weight = 0 N
Unit of Forces
There
are several units that can be
used to measure force
Many
of you are most familiar with the
unit of pound (lb)
The metric unit for force (the one we
will use) is the Newton (N)
Unit of Force
What
is a Newton?
It
is a derived unit
It
is an abbreviation for:
(Kg * m ) / s2
Net Force
Net
forces are more important than
individual forces
A net force of the sum total of all
the individual forces acting on an
object
Net force is written as:
∑F
Net Force
If
the net force is zero the forces
are considered to be BALANCED
If
the net force is NOT zero the
forces are considered to be
UNBALANCED
Balanced vs Unbalanced
When
forces are BALANCED the
motion of an object will remain
constant
When forces are UNBALANCED the
motion of an object will change…….it
will ACCELERATE
Net Force
Example:
Find
the net force if a 300 N force acts
to the right and a 120 N force acts to
the left
Net Force
Example:
Find
the net force if a 400 N force acts
to the right, a 120 N force acts to the
left, and a 75 N forces acts to the left
Friction
Friction
motion
is a force that opposes
It
occurs when two surfaces are in
contact
It depends on:
The
weight of the object
The type of surfaces
Friction
always slows down an object
Types of Friction
Static
friction
Occurs
when the surfaces in contact do
NOT slide past one another
Kinetic
friction
Occurs
when the surfaces in contact
slide past one another
Kinetic Friction
Kinetic
friction is often referred to
as sliding friction
If
a round object rolls over a flat
surface it is often referred to as
rolling friction
Types of Friction
Static
friction is always greater than
kinetic/rolling friction
Rolling
friction is typically less than
sliding friction
Static > Sliding > Rolling
Drag
Any
object that moves through a
fluid (like air) experiences a frictional
force called air resistance
Another
DRAG
term for air resistance is
Friction
Can
be harmful
Can
be helpful
Engine
parts
Air resistance
Warming
your hands
Starting/Stopping/Turning
Brillo pads
Newton’s Laws
Sir
Isaac Newton
(1643-1727)
Developed
motion
Laws of
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Newton’s Laws
Newton
was a student on break when
he started to form his thoughts on
motion
He challenged the belief system in
regard to motion that had been
establish over 2000 years ago
Newton’s Laws
He
summarized his thoughts into
three Laws of Motion
Law
of Inertia
F = ma
Action/Reaction Pairs
Newton’s 1st Law
An
object at rest will stay at rest
OR an object in motion will stay in
motion UNLESS a NET force acts on
the object
Newton’s 1st Law
Often
referred to as the Law of
Inertia
What is Inertia?
property
of matter
tendency of an object to resist changes
in motion
more mass = more inertia
Newton’s 2nd Law
A
net force acting on an object will
cause the object to change its motion
- it will accelerate
The magnitude of the force and the
mass of the object will both influence
the acceleration
Newton’s 2nd Law
Written
as an equation
∑F=ma
Newton’s 3rd Law
For
every force there is an EQUAL
but OPPOSITE reaction force
Examples
Baseball
- Bat
Bug - Windshield