Transcript Forces and Newton`s 3 Laws of Motion

Forces and Newton’s 3 Laws of
Motion
Robert Strawn
Compiled 10/16/11
A force is any push or pull that one object exerts
on another that causes a change in motion,
direction, or shape.
Confusion of Mass and Weight
• Many students of physics confuse
weight with mass.
• The force of gravity acting upon an
object is sometimes referred to as
the weight of the object.
• The mass of an object refers to the
amount of matter that is contained
by the object.
• The weight of an object is the force
of gravity acting upon that object.
Mass is related to how much stuff
is there and weight is related to the
pull of the Earth (or any other
planet) upon that stuff.
About Force
• Whenever there is an interaction between two
objects, there is a force upon each of the objects.
When the interaction ceases, the two objects no
longer experience the force.
• A force is a vector quantity.
• Forces only exist as a result of an interaction.
• Force is measured using the standard metric
unit known as the Newton (N).
• For simplicity sake, all forces (interactions)
between objects can be placed into two
broad categories: contact forces, and forces
resulting from action-at-a-distance
Contact Forces
• Those types of forces which result when the
two interacting objects are perceived to be
physically contacting each other. Examples of
contact forces include frictional forces,
tensional forces, normal forces, air resistance
forces, and applied forces.
• Contact Forces
– Frictional Force
– Tension Force
– Normal Force
– Air Resistance Force
– Applied Force
– Spring Force
Actions at Distance
• Those types of forces which result even when the two
interacting objects are not in physical contact with each
other, yet are able to exert a push or pull despite their
physical separation. Examples of action-at-a-distance
forces include : gravitational forces, Electric forces, and
magnetic forces
Drawing Free-Body Diagrams
Free-body diagrams are diagrams used to show
the relative magnitude and direction of all forces
acting upon an object in a given situation. A
free-body diagram is a special example of the
vector diagrams.

Forces are Vectors so Directions are
Important
Total Force
Force #2
Force #1
Force #1
Total Force = 0
Force #2
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Net Force
Net Force is an unbalanced force when all of the
forces acting on an object are combined. It is
necessary to find the net force when there are two or
more forces acting on an object. If all of the forces
acting on an object cancel each other out there is no
net force and the forces are balanced.
Balanced Forces
• The book is said to be at equilibrium.
Balanced Forces
• Balanced forces do not cause change in
motion
• The Total Forces applied to an object
produce a net force of zero
• They are equal in size and opposite in
direction
Unbalanced Forces
• In this case, an unbalanced force acts upon
the book to change its state of motion.
Unbalanced Forces
3 N, right – 6 N, left = 3N, left
Unbalanced Forces
• An unbalanced force always causes a change in
motion
• When unbalanced forces act in opposite directions
you can find the net force
– Net force
• Magnitude
– The difference between the two forces
• Direction
– Direction of the largest force
Normal Force
• Normal Force is a force
exerted by one object on
another in a direction
perpendicular to the
surface of contact.
• The normal force is always
perpendicular to the
surface but is not always
opposite the force of
gravity.
Newton's First Law of Motion
• 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.
Isaac Newton
• Newton’s 3 laws
– 1st : Law of Inertia An object at rest
will stay at rest, an object in motion will
stay in motion, unless acted upon by an
outside force.
– 2nd : F = ma An object will accelerate
in the presence of an unbalanced force.
The larger the mass the smaller the
acceleration.
– 3rd : For every action there is an equal
and opposite reaction.