Transcript 3.4 Newton`s Law of Inertia - Fort Thomas Independent Schools

3.1 Galileo and Newton on Motion
1. Objects do not change motion without
unbalanced force.
2. Objects in motion do not always require a force
to keep them moving.
3. Objects have two “natural” states of motion, at
rest (static equilibrium) and moving at a
constant speed and direction (dynamic
equilibrium).
Simply put, things
tend to keep on
doing what they’re
already doing.
3.4 Newton’s Law of Inertia
Is a force required to keep an object moving?
Newton’s first law, usually called the law of inertia, is
a restatement of Galileo’s idea that a force is not needed
to keep an object moving.
 Galileo argued that only when friction is present is a force
needed to keep an object moving.
 Galileo stated that if friction were entirely absent, a ball
moving horizontally would move forever at the same
speed and in the same direction (at a constant velocity).
•Objects at rest stay at rest and objects in motion at
a constant velocity continue at a constant velocity
unless acted upon by an unbalanced force. (also
called the law of inertia).
•
Inertia: the tendency of an object to resist
acceleration
•
Inertia is not a force, it’s a property of matter.
•
Mass and inertia are proportional. More mass,
more inertia
3.5 Mass—A Measure of Inertia
Which has more mass, a feather pillow or a common
automobile battery? Which has more volume? Which has
a higher density? What has the most inertia?
The pillow has a
larger size (volume)
but a smaller mass
than the battery. The
battery is more dense.
The battery has more
inertia.
First Law of Motion
 The question is not why objects
keep moving, but why they
don’t keep moving.
3.4 Newton’s Law of Inertia
Objects at rest tend to remain at rest.
3.4 Newton’s Law of Inertia
3.4 Newton’s Law of Inertia
REFLECT ON THE MEANING OF THIS
CARTOON
3.3 Galileo on Motion
think!
A ball is rolled across a counter top and rolls slowly to a
stop. How would Aristotle interpret this behavior? How
would Galileo interpret it?
3.3 Galileo on Motion
think!
A ball is rolled across a counter top and rolls slowly to a
stop. How would Aristotle interpret this behavior? How
would Galileo and Newton interpret it?
Answer:
Aristotle would say that the ball stops because it seeks its
natural state of rest.
Galileo and Newton would say that the friction between the
ball and the table overcomes the ball’s natural tendency to
continue rolling—overcomes the ball’s inertia—and brings
it to a stop.
3.6 The Moving Earth Again
The law of inertia states that objects in
motion remain in motion and that
objects at rest remain at rest if no
unbalanced forces act on them.
Constant speed and same
direction!
3.6 The Moving Earth Again
•A person flips a coin
into the air while on a
jet that is traveling 500
mph. Where will the
coin land and why?
Flip a coin in an airplane,
and it behaves as if the
plane were at rest. The coin
keeps up with you—inertia
in action!
3.6 The Moving Earth Again
Objects Move With Vehicles
If we flip a coin in a high-speed car, bus, or plane, we
can catch the vertically moving coin as we would if the
vehicle were at rest.
We see evidence for the law of inertia when the
horizontal motion of the coin before, during, and after
the catch is the same.
The vertical force of gravity affects only the vertical
motion of the coin.
3.6 The Moving Earth Again
How does the law of inertia apply to
objects in motion?
•The law of inertia states
that objects in motion
remain in motion and that
objects at rest remain at
rest if no unbalanced
forces act on them.
3.3 Galileo on Motion
According to Galileo and Newton,
when is a force needed to keep an
object moving?
Only when friction (or some other oppositional force) is
present is a force needed to keep an object moving.
The net force equals
mass times
acceleration.
Fnet = ma
or
a = Fnet/m
Explains the relationship between Net
force, mass and acceleration.
Newton’s Second Law
 F represents the vector sum of all forces
acting on an object.
 F = Fnet
 Units for force: mass units (kg)  acceleration
units (m/s2)
 The units kg•m/s2 are also called newtons (N).
Newton’s 2nd law of motion shows
two general relationships in science
 The rate of acceleration is directly
related to net force
 Also called directly proportional
Newton’s 2nd law of motion shows
two general relationships in science
 The rate of acceleration is inversely
related to the object’s mass
 Also called inversely proportional
Forces act in pairs!
For every
action
force,
there is an
equal and
opposite
reaction
force.
Newton’s Third Law
 Forces always exist in pairs.
 You push down on the chair, the
chair pushes up on you
 Called the action force and reaction
force
 Occur simultaneously so either
force is the action force
Newton’s Third Law
 For every action force there is an equal and
opposite reaction force.
 The forces act on different objects.
 Therefore, they do not balance or cancel each other.
 The motion of each object depends on the net force
on that object.
7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply
B exerts force on A.
7.3 Identifying Action and Reaction
When action is A exerts force on B, the reaction is simply
B exerts force on A.
7.2 Newton’s Third Law
The dog wags the tail and the tail wags the dog.
http://www.nasa.gov/audience/forstudents/brainbites/nonflash/bb_home_bolt.html
7.4 Action and Reaction on Different Masses
The balloon recoils from the escaping
air and climbs upward.
A common misconception is that a
balloon is propelled by the impact of
exhaust gases against the
atmosphere.
Each molecule of exhaust gas acts like a
tiny molecular cannonball shot downward
from the balloon.
7.4 Action and Reaction on Different Masses
The rocket recoils from the
“molecular cannonballs” it
fires and climbs upward.
Gas pushes on
rocket
Rocket pushes
on gas
7.4 Action and Reaction on Different Masses
What can you say about the action and reaction forces
experienced by the cannon and the cannonball?
•The force the cannon exerts on the cannonball is
exactly equal and opposite to the force the
cannonball exerts on the cannon.
cannon
cannonball
7.4 Action and Reaction on Different Masses
F represents both the action and reaction forces;
m (large), the mass of the cannon; and m
(small), the mass of the cannonball.
Which has the greater
change in motion and why?
Same force
Different
masses
Different
accelerations
The cannonball! Because it has
less mass.
6.1 Force Causes Acceleration
What causes an object to accelerate?
Unbalanced forces acting on an object
cause the object to accelerate.
Net Force > 0
3.4 Newton’s Law of Inertia
Objects at Rest
• Objects in a state of rest tend to remain at
rest.
• Only a force will change that state.
3.4 Newton’s Law of Inertia
Objects in Motion
• In the absence of forces, a moving object
tends to move in a straight line indefinitely.
• Toss an object from a space station located in
the vacuum of outer space, and the object will
move forever due to inertia.
3.4 Newton’s Law of Inertia
think!
A force of gravity between the sun and its planets holds
the planets in orbit around the sun. If that force of
gravity suddenly disappeared, in what kind of path
would the planets move?
3.4 Newton’s Law of Inertia
think!
A force of gravity between the sun and its planets holds
the planets in orbit around the sun. If that force of
gravity suddenly disappeared, in what kind of path
would the planets move?
Answer: Each planet would move in a straight line at
constant speed.