Transcript Physics and Beyond PowerPoint

.
Or …
Things don’t just
happen all by
themselves,
there’s a reason.
350 BC - Aristotle
• Aristotle divided motion into 2 types :
• Natural Motion
•
Objects seek their
natural resting positions
• Violent or imposed motion
Natural Motion
• Mostly considered straight up
•
Or
Straight down … like a boulder falling
or smoke drifting up in the air like clouds
or the circular motions of the planets
• These motions were considered
“natural” and not caused by forces.
VIOLENT MOTION
• Imposed Motion
• Motion with an external cause
• The Result of forces that push or pull.
• EXAMPLES :
• A cart moving
a rope tugged
a ship pushed by the force of the wind
Natural and Violent Motion
• For almost 2000 years it was considered that if
an object was moving “against its nature”, then
a force of some kind was involved.
• If there were no force then there would be no
motion (except vertically : falling rocks and
rising smoke).
• Therefore:
• The proper state of objects is at rest.
Nikolai Kopernik (Copernicus)
• Nicolaus Copernicus – 1473 – 1543
• The Theory of the Moving Earth
• The Earth was not the center of the Universe
• The Earth and planets revolved around the Sun
(contrary to religious orthodoxy)
THE UNIVERSE
IS HELIOCENTRIC,
NOT GEOCENTRIC !
• Working in secret, Copernicus’ work was not
published until the day of his death.
• Copernicus needed a champion for his ideas and
he found Galileo.
Heliocentric vs. Geocentric
• Heliocentrism is the theory that the
Sun is at the center of the
Universe and/or the Solar System.
• Geocentric model (in Greek: geo =
Earth and centron = center) of the
Universe is a disproven model which
places the Earth at the center of the
universe.
Galileo Galileii
Supported the ideas of
Copernicus
•Worked for the Medici family
•Suffered house arrest the last
20 years of his life because of
his support of the heliocentric
universe theory
•Galileo’s work showed that
forces were not necessary to
keep an object moving.
•
Galileo on Motion
• A Force = ANY push or pull
• Friction = The force that acts between
materials that touch as they move past
each other.
• Caused by surface irregularities that touch
• Galileo argued that when friction is
present, a force is required to keep an
object moving !
Testing Galileo’s Theories
• If the slope is downward, speed increases
• The ball is rolling partly in the direction of Earth’s
gravity
Testing Galileo’s Theories
• If the slope is upward, speed decreases
• The ball is rolling in a direction opposed to
Earth’s gravity
Testing Galileo’s Theories
• If there is no slope, speed or velocity is constant
!?
• The ball is not rolling with or against Earth’s
gravity
A Second test of Galileo
• 2 inclined planes facing each other
• A ball released to roll down one plane will
roll up the other to nearly the same height
A Second test of Galileo
• 2 inclined planes facing each other
• A ball released to roll down one plane will roll up
the other to nearly the same height no matter
what the angle of the second plane.
A Second test of Galileo
• 2 inclined planes facing each other, one reduced
to zero or a flat plane
• A ball released to roll down one plane will roll
forever with only friction to slow it.
Galileo’s Conclusions
• It is not the “nature” of the ball to come to
rest as Aristotle had claimed.
• In the absence of friction, the moving ball
would naturally keep moving.
• The tendency of a moving body to keep
moving is called INERTIA
• (NEWTON’S 1st LAW!!!!!)
A New Vision of the Universe
• On Christmas Day 1642, (The
same year Galileo died) Sir
Isaac Newton (1642 – 1727)
was born.
• By age 24 (1666),
Newton had developed
the Laws of Motion.
Newton’s First Law of Motion
•
THE LAW OF INERTIA
• An object in motion tends to remain in
motion at the same speed and in the
same direction until acted upon by an
external force. And an object at rest
tends to remain at rest until acted upon
by an external force.
Newton’s First Law of Motion
The Law of Inertia
• Examples:
• Objects at rest – the ol’ tablecloth trick ….
• Objects in motion – a hockey puck
or a table hockey disk
• Anything chucked out of the space station
will move forever by virtue of its own
inertia
INERTIA- amount of resistance
to a change in velocity
NEWTON’S SECOND
LAW OF MOTION
Mass – A Measure of Inertia
• Mass = The measure of how much matter
is present
The more mass an object has, the greater the
inertia
The greater the inertia, the more force it takes to
change an object’s state of inertia
Mass is NOT Volume
• Volume is a measure of the amount of
space an object takes up.
• Volume is measured in
cubic centimeters, cubic meters or liters
• Mass is measured in grams or kilograms
• Objects with large but equal volumes may
have very different masses (and density!)
• EX : A bag of cloth and a bag of rocks
Which has more mass ?
Which is harder to set in motion ?
Which has the greater inertia ?
Wait !
Mass is NOT Weight
• We often determine the amount of matter in an
object (it’s mass) by measuring its gravitational
attraction to the earth.
• Mass is more fundamental and doesn’t depend on
gravity to be measurable.
• A bowling ball is just as difficult to shake in space
as it is on the ground.
Force Causes Acceleration
• If a force is applied to an object, whether it
is at rest or moving, the motion will
change. IT ACCELERATES.
• If the force is removed, the object will
continue moving at a constant velocity.
Newton’s Second Law of Motion
• The acceleration produced by a net
force on an object is directly
proportional to the magnitude of and in
the same direction as the net force, and
is inversely proportional to the mass of
the object.
net force
• acceleration =
mass
F
or a = m
FRICTION
• Friction is a force that acts to oppose motion
between materials that are in contact with each
other.
• Mainly due to irregularities between the two
surfaces.
• When an object slides against another, it must rise
over irregular bumps or scrape them off.
• Either way requires force.
FRICTION
• The force of friction depends on the types
of materials that are in contact with each
other and the amount of surface in
contact.
• The shape of concrete dividers is such
that the rubber of the tire against the
concrete is more effective at slowing a car
than steel barriers against the steel side of
a car. And less damaging.
FREE FALL EXPLAINED
• Galileo showed that falling objects accelerate equally…
• When air resistance is neglected… or
• When the air resistance is very small compared
with the mass of the falling object.
(but he didn’t know why)
• Galileo is said to have demonstrated this from
the Leaning Tower of Pisa with a 10kg cannon
ball and 1 kg stone.
NEWTON FIGURED IT OUT
• Galileo knew that an object falls
because of the force due to
gravity pulling on it.
• Sir Isaac Newton’s second law
of motion showed the mass of
a falling object must be considered
as well.
OUCH !
• 10 times the force acting on 10 times the mass
produces the same acceleration as a smaller
force acting on a smaller mass.
F=
•
.
ma
Force = mass x acceleration
Styrofoam golf
ball
ball
Book and paper
Flying
squirrel
• Air resistance keeps things from falling at the same
rate.
Newton’s Second Law
•
F = m.a
NEWTON’S SECOND
LAW OF MOTION
Be careful bending down !
• When you bend down,
if you keep in contact
with a wall you will
be less likely to fall.
• This is because as you push on the
wall for balance, the wall pushes back
on you !
Forces and Interactions
• In simplest terms, a FORCE is a push or a pull.
• Sir Isaac Newton realized that a force is not a
thing in itself but a part of a mutual action, an
interaction between things.
• For example, The interaction that
drives the nail is the same one
that halts the hammer
Newton’s Third Law of Motion
• For every action there is an equal and
opposite reaction.
• Whenever one object exerts a
force on a second object, the
second object exerts an equal
and opposite force on the first
object.
Newton’s Third Law of Motion
• For every interaction, the forces always
come in pairs (twos).
• The ACTION FORCE and
• The REACTION FORCE
• They are equal in strength and opposite in
direction.
A simple recipe for treating action and
reaction forces
• Identify the interaction:
• ACTION : Object A exerts a force on object B
• REACTION : Object B exerts a force on object A
• For Example :
• ACTION : Tire pushes road
• REACTION : Road pushes tire
• ACTION : Rocket pushes gas
• REACTION : Gas pushes rocket
Questions
1. Does a stick of dynamite contain force?
No, force is not something an object has, like mass and
volume. An object may posses the capability of exerting
force on another object but it does not possess force.
2. A car accelerates along a road. Strictly speaking,
what is the force that moves the car ?
It really is the road that pushes the car along ! The
rotating tires push back on the road (action) and the road
pushes forward on the tires (reaction) at the same time !
ACTION AND REACTION
Action and Reaction on Different
Masses
• When a cannon is fired, there is an interaction between
the cannon and the cannon ball.
• The forces the cannon ball and cannon exert on each
other are equal and opposite.
• The cannonball moves fast while the cannon only Kicks
a bit because of the difference in their masses.
OTHER ACTION – REACTION
INTERACTIONS
• A Balloon
• A Rocket
• Or a jet
• Air foils : a bird’s wing,
•
a helicopter blade
•
an airplane wing
LIFT
WHAT’S IT MEAN ?
• If the action and reaction forces are
internal to a system, they cancel each
other and no acceleration occurs.
• For example : Blowing on a sail when you’re IN the
boat doesn’t work very well.
• If either the action or the reaction force is
external to the system, then acceleration
occurs.
• However, the WIND blowing on the sail can work
very well !
• FOR EVERY ACTION THERE IS AN
EQUAL AND OPPOSITE REACTION
A fly gets smashed onto the windshield of a speeding bus.
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