Download Newton`s Laws

Newton’s Laws
Compare/Contrast these situations
Does the length of rope matter?
Newton’s First Law (of Inertia)
• DON’T COPY THIS:
“An object at rest stays at rest and an object in motion stays in
motion with the same speed and in the same direction unless
acted upon by a force”
• If the forces acting upon an object are balanced the object
will continue to do what it is doing.
• Inertia: the resistance to change in the state of a motion
• Objects maintain a constant velocity unless acted upon by
another force, therefore acceleration requires unbalanced
forces.
Inertia & Mass
• Remember inertia is the resistance an object has to
a change in it’s state of motion.
• Forces aren’t need to keep an object in motion,
rather forces are needed to bring an object to rest.
• The tendency of an object to resist a change in
motion depends on it’s mass; the more mass
something has the more inertia it has.
State of Motion
• Redefined inertia is the tendency of an object to
resist changes in it’s velocity
• According to Newton’s First Law:
o An object at rest will remain at rest with a velocity of zero unless acted
upon by an unbalanced force
o An object traveling at a speed will remain at that speed unless acted
upon by an unbalanced force
• Therefore inertia can be redefined as the tendency
of an object to resist acceleration
Remember a force is a push or a pull on an object
Balanced Forces
Unbalanced Forces
• In order for two forces
to balance they must
be equal in magnitude.
• What does a force
diagram look like for a
book at rest on my
table?
• There is no equivalent
force opposing
• Cause acceleration
• What would the force
diagram look like for a
book on the table
pushed leftwards?
Newton’s First Law
“An object at rest stays at rest and an object in
motion stays in motion with the same speed and in the
same direction unless acted upon by a force”
• Define Newton’s First Law in your own words.
Forces
Contact Force
• When two interacting
forces are perceived as
physically touching.
• Ex: friction, tension,
normal, air resistance,
applied, & spring force.
Field Force
• A force which is able to
exert a push or pull
without physical
contact.
• “action at a distance
force”
• Ex: gravity, electric, &
magnetic force.
• Forces are vector quantities (magnitude & direction)
• Forces are measured in Newtons
• 1 N= the force required to five a 1kg object an
acceleration of 1m/s2
Forces
Force Research
• Create a power point slide (emailed to me) or
poster to “present” a force
• Make sure to:
o Describe/define the force
o Identify any specific equations associated with it
o Show an example of the force in a force diagram
• You will briefly share your findings tomorrow in class
Resources
Happy Homecoming 10/7
In preparation for tonight we will copy down the school fight song for our opener:
Go Vikes Go!
Fight Vikes Fight!
Go Go Go Go Vikings!
Fight You Mighty Vikings
Fight Vikings Bold
Our (Wo)Men are Fighting
For the Purple and Gold
Cheer For Dear Old NK
(Wo)Men Without Fear
Fight Vikings Fight
While We Give Our Cheer
V-I-K-I-N-G-S!
Vikings! Vikings! Wooooooooooh!
Applied Forces
An applied force is a force that is applied to
an object by a person or another object. If a
person is pushing a desk across the room,
then there is an applied force acting upon
the object. The applied force is the force
exerted on the desk by the person1.
1http://www.physicsclassroom.com/Class/newtlaws/U2L
2b.cfm#applied
Fapp
Gravity: Fundamental force of the universe
• Gravity is dependent on mass
o directly proportional to the mass of the object
• the force of gravity acting between the earth
and any other object is directly proportional to
the mass of the earth
• inversely proportional to the square of the
distance that separates the centers of the earth
and the object
o (The farther away from the object, the lesser the
pull of gravity. )
• more mass = more gravitational force and
attract smaller object
toward it.
o Everything emits a gravitational pull.
Earth: makes objects fall
Solar System: keeps solar system together
• the force exerted by the surfaces of two objects in contact with each other
when they attempt to move
• Attraction results from the intermolecular forces between molecules of the
different surfaces
• Two types:
• Sliding – when an object moves across a surface
• Static – when the objects are at rest and a force attempts to set one of
them in motion
Ffrict = µ • Fnorm
µ: measure of the amount of adhesion or attraction
between the surfaces
Ffrict-sliding = μfrict-sliding • Fnorm
Ffrict-static ≤ μfrict-static• Fnorm
Magnetic Force
● F=qv x B or F=BILsin(Θ)
○ Force is equivalent to a fixed
amount of charge(v) moving
at a fixed velocity (q) in a
uniform magnetic field (B).
○ Force is equivalent to a
current (I) moving through a
length of wire (L) in a uniform
magnetic field (B) with a
given vector (Θ).
Magnetic fields are classified as
vector fields, as they exhibit
both a direction and
magnitude. Two objects with
opposite charges will attract
and two objects with similar
charges will repel each other.
ELECTRICAL FORCE
Electrical forces are an attractive or
repulsive force between electrically
charged objects. If the objects have
opposite charges, they are attracted
to each other. If they are the same,
they are repulsive . Electrical forces
are very similar to magnetic forces,
but at still fundamentally different.
The difference between a
magnetic field and an electrical
field is that a electrical field may
move a charged particle were as
a magnetic field may not.
Also in contrast a electrical field
force acts parallel to the field,
were as a magnetic force acts
orthogonal.
Another key difference is that a
magnetic force does not produce
net work were as a electrical force
does.
Equation to express work done by an
electrical Force
Mass vs. Weight
• Mass: the amount of “stuff” making up an object
• Weight: the force of gravity interacting with the
amount of “stuff” making up an object
• On earth weight is equivalent to 9.8Newtons per a
kilogram of “stuff” or 9.8N/kg
• On the moon your weight is equivalent to 1.7N/kg
• If there are .45kg in 1 pound what is your weight?
• What is your mass?
• What is your mass on the moon?
• What is your weight on the moon?
Free Body Diagrams
• Free body diagrams show the relationship between
relative magnitude and direction of all forces
acting upon an object.
• The size of the arrow represents the magnitude of
the force
• Steps for drawing a diagram
o Identify what forces are acting upon the object
o Identify the direction and magnitude of these forces
o Draw the diagram representing:
• The object (or a box if you are not artistic)
• Arrows representing forces
Practice Free Body Diagrams
• A book falls of the desk and is free-falling to the
ground
• A painter hangs on a rope secured to the ceiling.
• A force is applied to drag a sled with a rightwards
acceleration across snow (ignore air resistance)
Net Force
• Unbalanced force: when vertical forces (up/down) do not
cancel each other out and/or horizontal forces (right/left)
do not cancel each other out.
• Unbalanced forces have a net force ≠ 0
• Net force is a vector sum of all forces
• Observe in the following examples that downward vectors
provide a partial or full cancellation of an upward vector.
What are the net forces for each example?
• If balanced forces result in no acceleration, unbalanced
forces, or the presence of a net forces causes an
acceleration.
Newton’s Second Law
• The acceleration of an object directly
depends upon the net force acting upon the
object and is inversely related to the mass of
the object
Fnet=ma or a=Fnet/m
1 Newton= 1kg * 1m/s2
• Acceleration is in the same direction as the net
force.
Practice Calculations
5 m/s/s
10 m/s/s
5 m/s/s
20 N
1 kg