Transcript Inertia and Newtons laws of motion unit notes

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
1564 – 1642
Italy
Perhaps the first true
scientist.
Rolled and dropped
objects to discover
the true aspects of
motion
1642-1727
England
Developed laws for
motion and gravity
that explain why
objects move, and
worked with optics
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
stay in motion with the same speed and in the same
direction 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.
•
More mass, more inertia
The net force equals
mass times
acceleration.
Fnet = ma
or
a = Fnet/m
Explains the relationship between Net force, mass
and acceleration (direct vs. inverse relationships)
In the presence of a net force (greater than zero) an
object will experience acceleration
Forces act in pairs!
For every
action
force,
there is an
equal and
opposite
reaction
force.
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
3.3 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.
3.3 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.
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
Objects at rest tend to remain at rest.
3.4 Newton’s Law of Inertia
3.4 Newton’s Law of 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.