Download Newtons Laws

NEWTON’S LAWS
CHAPTER 12
Newton's first law of motion
• Sometimes referred to as the law of inertia.
• Newton's first law of motion is often stated as
“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
external force.”
Applications of Newton's First Law
• Buckle up?
What is meant by inertia?
• Inertia is the tendency of an object to maintain it’s
current state of motion….or
• to resist changes in its velocity….or
• to resist accelerations.
• More mass = more inertia
–Ex.) Car vs. toy car
Newton's Second Law
of Motion
• Pertains to the behavior of objects for which all existing
forces are not balanced.
• The second law states that the acceleration of an object is
dependent upon two variables
–the net force acting upon the object
–and the mass of the object.
• Summed up in this equation:
F = ma
Newton’s Second Law
Can also be written two other ways:
a=F/m
OR
m=F/a
• The acceleration of an object depends directly upon
the net force acting upon the object, and inversely
upon the mass of the object.
Newton’s Second Law
• As the force acting upon an
object is increased, what
must happen to the
acceleration?
• the acceleration of the
object is increased.
a
=m*a
Newton’s Second Law
• As the mass of an object is
increased, what must
happen to the acceleration
of the object?
• The acceleration of the
object is decreased
=
m
m * a
.
Solve for the unknown value:
Newton's third law of motion
• For every action, there is an equal
and opposite reaction.
• Forces always come in pairs
–equal and opposite actionreaction force pairs
Check your understanding:
• While driving down the road, a firefly strikes the
windshield of a car and makes a quite obvious
mess in front of the face of the driver.
• This is a clear case of Newton's third law of
motion.
• The firefly hit the car and the car hits the firefly.
• Which of the two forces is greater: the force on
the firefly or the force on the car?
Check your understanding…
from the www.physicsclassroom.com
• Trick Question!
• Each force is the same size.
• For every action, there is an
equal ... (equal!).
• The fact that the firefly splatters
only means that with its smaller
mass, it is less able to withstand
the larger acceleration resulting
from the interaction.
Check your understanding…
from the www.physicsclassroom.com
• For years, space travel was
believed to be impossible
because there was nothing
which rockets could push
off of in space in order to
provide the propulsion
necessary to accelerate.
This inability of a rocket to
provide propulsion is
because ...
a. ... space is void of air so the
rockets have nothing to push off
of.
b. ... gravity is absent in space.
c. ... space is void of air and so there
is no air resistance in space.
d. ... nonsense! Rockets do accelerate
in space and have been able to do
so for a long time.
Check your understanding…
from the www.physicsclassroom.com
• For years, space travel was
believed to be impossible
because there was nothing
which rockets could push off of
in space in order to provide the
propulsion necessary to
accelerate. This inability of a
rocket to provide propulsion is
because ...
The answer is D.
Guns recoil when fired.
• result of action-reaction force pairs. a. greater than the
• A gunpowder explosion creates hot
acceleration of the bullet.
gases which expand outward
allowing the gun to push forward on b. smaller than the
the bullet.
acceleration of the bullet.
• Consistent with Newton's third law c. the same size as the
of motion, the bullet pushes
acceleration
of
the
bullet.
backwards upon the gun. The
acceleration of the recoiling rifle is
...
Guns recoil when fired.
c.
the same size as the acceleration of
the bullet.
• Consistent with
Newton's third law of • Yet, acceleration depends on both force
and mass.
motion, the bullet
• The bullet has a greater acceleration due
pushes backwards
to the fact that it has a smaller mass.
upon the gun. The
• Remember: acceleration and mass are
acceleration of the
inversely proportional.
recoiling rifle is ...
Challenge Question:
• If the action force is opposite and equal why do they not
cancel
….remember this….
• …have a net force of zero…
• …so no motion!
• Action and Reaction forces do not cancel because they
are acting on different objects!
Identifying Action and
Reaction Force Pairs 1.
• Action force:
Baseball pushes glove
leftwards.
• Reaction?
• The glove pushes the
baseball rightward.
Identifying Action and
Reaction Force Pairs 2.
• Action force:
• Bowling ball pushes pin
leftwards
• Reaction?
• Pin pushes bowling ball
rightward
Identifying Action and
Reaction Force Pairs 3.
• Action force:
Enclosed air particles
push balloon wall
outwards
• Reaction?
• Balloon wall pushes
enclosed air particles
inwards.
Review
Newton’s first law of motion- Objects in motion stay in motion
and objects at rest stay at rest, unless acted upon by an outside
force; aka Law of Inertia
Newton’s seconds law of motion- F=ma, force is proportional to
the mass and acceleration of an object
Newton’s third law of motion- For every action, there is an equal
and opposite reaction.