Download Inertia and Newtons laws of motion

Aristotle, Galileo and Newton
and Newton’s Laws of Motion
Chapter 3.1-3.6
Chapter 6.1-6.3
Chapter 7.1-7.4
384 BC – 322 BC
Ancient Greece
One of the first to try to
explain the natural world
Geocentric view of the
universe
Ideas based on
observations that seemed
to be true
Thought that objects in
motion must have a force
keeping them moving
1564 – 1642
Italy
Perhaps the first true
scientist.
Rolled and dropped
objects to discover
the true aspects of
motion—that
objects in motion do
not need a force to
keep them moving
1642-1727
England
Developed laws for
motion and gravity
that explain why
objects move, and
worked with optics
Simply put, things
tend to keep on
doing what they’re
already doing.
3.1 Aristotle on Motion
1. Objects do not move without a force.
2. Objects in motion always require a force to keep
them moving.
3. Objects seek their natural state, which is at rest.
4. Mechanical equilibrium can only be static.
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).
3.4 Newton’s Law of Inertia
The law of inertia provides a
completely different way of viewing
motion from the ancients.
• Objects continue to move by
themselves.
• Forces are needed to overcome
any friction that may be present
and to set objects in motion
initially.
• Once the object is moving in a
force-free environment, it will
move in a straight line
indefinitely.
•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.
3.5 Mass—A Measure of Inertia
The stone’s inertia, or mass, is a property of the stone
and not its location.
The same force would be required to shake the stone
with the same rhythm whether the stone was on Earth,
on the moon, or in a force-free region of outer space.
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!
If the Earth is rotating at 800
mph at our latitude, why
can’t we sense this?
3.6 The Moving Earth Again
Objects Move With Earth
You can refute this argument using the idea of inertia.
Earth moves at 30 km/s, but so do the tree, the worm
below, and even the air in between.
Objects on Earth move with Earth as Earth moves
around the sun.
3.6 The Moving Earth Again
Earth does not need to
be at rest for the bird
to catch the worm.
3.6 The Moving Earth Again
Copernicus announced the idea of a moving Earth in the
sixteenth century. One of the arguments against a moving
Earth was:
• Consider a bird sitting at rest in the top of a tall tree.
• The bird sees a worm, drops down vertically, and
catches it.
• It was argued that this would not be possible if Earth
moved as Copernicus suggested.
• The fact that birds do catch worms from high tree
branches seemed to be clear evidence that Earth
must be at rest.
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
3 x 106 kg mass damper
•http://www.boreme.com/boreme/funny-2008/taipei-101damper-p1.php
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).
Classroom Practice Problem
 Space-shuttle astronauts experience
accelerations of about 35 m/s2 during
takeoff. What force does a 75 kg
astronaut experience during an
acceleration of this magnitude?
 Answer: 2600 kg•m/s2 or 2600 N
Newton’s 2nd law of motion
 Increasing the force will increase the
acceleration.
 Which produces a greater acceleration on a 3-kg
model airplane, a force of 5 N or a force of 7 N?
 Answer: the 7 N force
 Increasing the mass will decrease the
acceleration.
 A force of 5 N is exerted on two model airplanes,
one with a mass of 3 kg and one with a mass of
4 kg. Which has a greater acceleration?
 Answer: the 3 kg airplane
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.
What do you think?
 Two football players, Alex and Jason,
collide head-on. They have the same
mass and the same speed before the
collision. How does the force on Alex
compare to the force on Jason? Why
do you think so?
What do you think?
 Suppose Alex has twice the mass of
Jason. How would the forces
compare?
 Why do you think so?
 Sketch as before.
 Suppose Alex has twice the mass and
Jason is at rest. How would the forces
compare?
 Why do you think so?
 Sketch as before.
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.
Hammer Striking a Nail
 What are the action/reaction
pairs for a hammer striking a nail
into wood?
 Force of hammer on nail = force
of nail on hammer
 Force of wood on nail = force of
nail on wood
 Which of the action/reaction
forces above act on the nail?
 Force of hammer on nail
(downward)
 Force of wood on nail (upward)
 Does the nail move? If so, how?
 Fhammer-on-nail > Fwood-on-nail so the
nail
accelerates downward
Hammer Striking a Nail
 What forces act on the hammer?
 Force of nail on hammer (upward)
 Force of hand on hammer (downward)
 Does the hammer move? If so, how?
 Fnail-on-hammer > Fhand-on-hammer so the
hammer accelerates upward or slows
down
 The hammer and nail accelerate in
opposite directions.
Action-Reaction: A Book on a
Desk
Action Force
Reaction Force
 The desk pushes up •
on the book.
•
 Earth pulls down
on the book
(force of gravity).
The book pushes
down on the desk.
The book pulls up
on Earth.
Action-Reaction: A Falling Book
Action
 Earth pulls down on
the book (force of
gravity).
Reaction
• The book pulls up
on Earth.
•
 What is the result
of the action force
(if this is the only
force on the book)?
 Unbalanced force
produces an
acceleration of 9.81 m/s2.
What is the result of
the reaction force?
•
Unbalanced force
produces a very
small upward
acceleration
(because the mass
of Earth is so large).
Chapter 7, Page 118
9.
The cannon and cannonball have
very different accelerations because
equal forces on unequal masses
produce unequal accelerations. The
more massive cannon has more
inertia and is harder to accelerate
than the much less massive
cannonball.
Chapter 7, Page 118
10. The force that propels a rocket is
the force of the exhaust gases
pushing the rocket in reaction to the
rocket pushing the exhaust gases.
Rocket pushes
on exhaust gas
Exhaust gas
pushes on
rocket
RECOIL
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.