Transcript No Slide Title

Forces
and
Newton’s
Laws of Motion
Causes of Motion
Aristotle (384-322 BC) believed that all objects had a
“natural place” and that the tendency of an object
was to reside in its “natural place.”
All objects were classified into
categories of earth, water, air, or fire.
“Natural motion” occurred when an object
sought to return to its “natural place” after being
moved from it by some type of “violent motion.”
The natural state of an object was
to be “at rest” in its “natural place.”
To keep an object moving would require a force.
These views remained widely
supported until the 1500s
when Galileo Galilei (1564-1642)
popularized experimentation.
Isaac Newton (1642–1727)
proposed that the tendency of
an object was to maintain its
current state of motion.
INERTIA
the tendency of an object
to resist any change in its motion
Inertia is a property of matter and does not
depend on the position or location of the object.
MASS
a quantitative measure of inertia
FORCE
“a push or pull”
“Law of Inertia”
An object at rest remains at rest,
and an object in motion continues in motion
at a constant speed in a straight line,
unless acted on by a nonzero net force.
* OR *
The velocity of an object remains constant
unless acted on by an unbalanced force.
The net force acting on an object is the
vector sum of all the forces acting on it.
Examples:
9 lb
8 lb
8 lb
4 lb
7 lb
12 lb
6 lb
8 lb
?
5 lb
4 lb
If an object is remaining at rest, it
is incorrect to assume that there
are no forces acting on the object.
We can only conclude that the
net force on the object is zero.
4 lb
3 lb
7 lb
EQUILIBRIUM
the condition of an object
such that it is not accelerating
The first condition for equilibrium is that
there must be no net force acting on
the object. All forces must be balanced.
SF = 0
SFx = 0 and SFy = 0
SFleft = SFright and SFup = SFdown
“Law of Acceleration”
A net force causes an object to accelerate
in the direction of the net force.
The acceleration is directly proportional
to the net force and inversely
proportional to the object’s mass.
Fnet
a=
m
Fnet = m a
The SI unit of force is the Newton, named
in honor of Isaac Newton.
One Newton of force
is the amount of force
needed to cause a one
kilogram mass to accelerate
at a rate of 1 m/s2.
WEIGHT
a measure of the gravitational force
that a massive object,
such as a star or planet,
puts on another mass
F = ma
weight = mass x acceleration of gravity
W = mg
An object’s weight on planet Earth in Newtons
is equal to its mass in kilograms times 9.8 m/s2.
Why do all objects with negligible
air resistance accelerate toward
the Earth at the same rate?
How can Newton’s 2nd Law be used
to explain how air resistance
affects the acceleration of an
object in free fall?
How does Newton’s 2nd Law relate to
what a weight scale would read as you
move up and down on an elevator?
“Law of Interaction”
Short Version
“For every action there is
an equal and opposite reaction.”
Longer Version
When one object exerts a force on a
second object, the second exerts a force
on the first that is equal in magnitude,
but opposite in direction.
Newton’s Third Law deals with two forces
acting on two different objects.
Newton’s Third Law pairs of forces
always, sometimes,
never
cancel each other out.
The only way for two forces to cancel
each other out is for them to be equal
and opposite and act on the same object!
Learn more about
Newton’s Laws of Motion
at these links:
link1, link2, link3, link4
How do Newton’s Laws of motion
apply to these situations?
• an object rests in your hand
• a ball is tossed upward
• a car windshield hits a bug
• a person sits on a table
• a person jumps up from the floor
• a baseball bat hits a baseball
• a truck and car hit head-on
• Others?
FRICTION
the force that opposes the motion between
two surfaces that are in contact
Characteristics
1. Friction acts parallel to the surfaces in contact
and in the direction opposite to the motion of
the object or to the force tending to produce
such motion.
2. Friction depends on the nature of the materials
in contact and the smoothness of their surfaces.
3. Sliding friction is less than or equal to
starting friction.
4. Friction is virtually independent of surface area.
5. The friction force is directly proportional
to the force pressing the two surfaces
together (the normal force).
The ratio of the friction force, Ff,
to the normal force, Fn, is called
the coefficient of friction, m.
Ff
m =
Fn
m has
no units.
Explore forces and friction
using this free simulation.