Transcript Chapter 12 powerpoint

Chapter 12
 Force



– push or a pull that acts on an object
Balanced forces – two forces that are equal in size and
opposite in direction
Unbalanced forces – NOT equal and opposite
Net force – changes the velocity of the object
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 Friction
– a force that opposes motion
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1.
2.
3.
4.
Static Friction – acts on objects that are
not moving
Sliding friction – when two solid surfaces
slide over each other
Rolling friction – acts on rolling objects
Fluid friction – when an object moves
through liquid or gas
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 Force
that acts
between any two
masses
 Gravity pulls down,
air resistance pushes
up
 Terminal Velocity–


constant velocity of a
falling object
Air resistance equals
force of gravity
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An
object at rest will
stay at rest, and an object
in motion will stay in
motion at constant
velocity, unless acted
upon by an unbalanced
force.
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1st Law Continued…

Inertia is the
tendency of an
object to resist
changes in its
velocity:
whether in
motion or
motionless.
These pumpkins will not move
unless acted on by an unbalanced
force.
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1st Law Continued…

Unless acted
upon by an
unbalanced
force, this
golf ball
would sit on
the tee
forever.
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1st Law Continued…

Once
airborne,
unless acted
on by an
unbalanced
force (gravity
and air –
fluid friction),
it would
never stop!
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Newton’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including
you) resist changes in their motion. When
the car going 80 km/hour is stopped by
the brick wall, your body keeps moving at
80 m/hour.
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Why then, do we observe
every day objects in motion
slowing down and becoming
motionless seemingly without
an outside force?
It’s a force we sometimes
cannot see – friction.
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The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.
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

Newton’s 2nd Law
proves that different
masses accelerate to
the earth at the same
rate, (the rate of
gravity) but land with
different forces.
So why is it that
different objects
appear to fall slower or
faster?
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 “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
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
Example: If you punch someone in the face,
your hand applies a force to that person’s face
and their face applies a force to your hand
(don’t try this at home!)
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 Momentum
is the product of mass and
velocity
 Abbreviated with a little
“p”
 Law of Conservation of
Momentum: In a closed system, the loss
of momentum of one object equals the
gain in momentum of another;
momentum is conserved
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There are four universal forces that we currently
know of. Scientists continue to study these and
are searching for a unified theory that relates
them better!
 Universal forces act over a distance between
particles of matter.
• The particles need not be in contact.
• Force is affected by the distance between
particles.

1.
2.
3.
4.
Electromagnetic
Strong Nuclear
Weak Nuclear
Gravitational
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 Associated
with charged particles
 Electric and magnetic force are two
different aspects of the
electromagnetic force.
 Electric and magnetic force are the
only forces that both attract and
repel
•
•
Objects with opposite charges attract one
another.
Objects with like charges repel one another.
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1.
Strong Nuclear Force: powerful attraction
acting on neutrons and protons in the
nucleus, holding them together. Overcomes
the electric force of repulsion that acts
among the protons in the nucleus. More
than 100 times stronger than the
electromagnetic force. Very short range.
1.
Weak Nuclear Force: weaker than the
strong force. Involved in certain types of
radioactive processes.
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Gravity is the weakest universal force, but it is
the most effective force over long distances.
 Newton’s law of universal gravitation states
that every object in the universe attracts
every other object.
 Gravitational force is an attractive force that
acts between any two masses.
 Gravitational force depends upon mass and
distance.

F=Gm1m2
r2
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The
moon’s inertia
acts to move it away
from Earth.
Earth’s gravitational
attraction keeps the
moon in a nearly
circular orbit around
Earth.
A centripetal force is
a center-directed force
that continuously
changes the direction
of an object to make it
move in a circle.
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