Newton

1
Newtons Laws of Motion

• Chapter 4

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Background

• Sir Isaac Newton (1643-1727) an English scientist
  and mathematician used the observations of
  Galilleo, Kepler and others to develop a unified,
  universal explanation of the motion of all
  bodies. We now know this as his three laws of
  motion. He published them in his book
  Philosophiae Naturalis Principia Mathematica
  (mathematic principles of natural philosophy) in
  1687. His three laws describe the motion of all
  objects on the scale we experience in our
  everyday lives.

3

• If I have ever made any valuable discoveries,
  it has been owing more to patient attention, than
  to any other talent.
• -Sir Isaac Newton

4
A phorce????

• A push or pull.
• Contact forces
• Field forces act over distances (ex. Magnetic
  force, gravity, electrical force)
• Measured in newtons (N)
• 1 Newton is about equivalent to the weight of 1/5
  of a pound.

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How do we show forces?

• With free body diagrams!
• A free body diagram is a simple representation of
  any object as a simple rectangle or circle with
  forces shown as arrows, the length of which
  represent the magnitude of the force.
• See some examples on the board
• http//www.wisc-online.com/Objects/ViewObject.aspx
  ?IDtp1502

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Inertia

• The tendency of an objects motion to remain
  constant, or , put another way
• The tendency of an object to resist a change in
  its motion (either speed or direction)
• We may measure inertia as mass (kg)
• Can you think of some low inertia objects???
• Some very high inertia objects?

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Newtons First Law

• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion
  unchanged (straight line) unless acted upon by a
  net external force.

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What does this mean?

• Basically, an object will keep doing what it was
  doing unless acted on by an unbalanced force.
• If the object was sitting still, it will remain
  stationary. If it was moving at a constant
  velocity, it will keep moving.
• It takes net external force to change the motion
  of an object.

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What is meant by unbalanced force (or net
external force, NEF ?
If the forces on an object are equal and
opposite, they are said to be balanced, and the
object experiences no change in motion. If they
are not equal and opposite, then the forces are
unbalanced and the motion of the object changes.
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• Another statement of Newtons 1st law is that if
  there is a NEF on an object, it WILL accelerate.
  (It will change its speed and/or direction)
• If there is no NEF, it will continue doing
  whatever it was doing.

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Some Examples from Real Life
A soccer ball is sitting at rest. It takes an
unbalanced net external force of a kick to change
its motion.

• Two teams are playing tug of war. They are both
  exerting equal force on the rope in opposite
  directions. This balanced force results in no
  change of motion.

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Newtons First Law is also called the Law of
Inertia

• Inertia the tendency of an object to resist
  changes in its state of motion
• The First Law states that all objects have
  inertia. The more mass an object has, the more
  inertia it has (and the harder it is to change
  its motion).

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More Examples from Real Life
A powerful locomotive begins to pull a long line
of boxcars that were sitting at rest. Since the
boxcars are so massive, they have a great deal of
inertia and it takes a large force to change
their motion. Once they are moving, it takes a
large force to stop them.
On your way to school, a bug flies into your
windshield. Since the bug is so small, it has
very little inertia and exerts a very small force
on your car (so small that you dont even feel
it).
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If objects in motion tend to stay in motion, why
dont moving objects keep moving forever?
Things dont keep moving forever because theres
almost always an unbalanced force acting upon it.
A book sliding across a table slows down and
stops because of the force of friction.
If you throw a ball upwards it will eventually
slow down and fall because of the force of
gravity.
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In outer space, away from gravity and any sources
of friction, a rocket ship launched with a
certain speed and direction would keep going in
that same direction and at that same speed
forever.
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What does a Fnet/m mean?

• Acceleration is directly proportional to Force
• Acceleration is inversely proportional to mass

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Newtons Second Law
Acceleration is directly proportional to
Force Acceleration is inversely proportional to
mass

• Force equals mass times acceleration.
• Fnet ma

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What shape will it be?

• a
• (m/s2)
• Force (N)

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What shape will it be?

• a
• (m/s2)
• Mass (kg)

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Weight NOT equal to mass!!!!

• Fnet ma
• W
• So, you have a certain weight on the earth. This
  weight will be different on the moon etc but mass
  remains the same

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More about F ma

• If you double the mass, you double the force. If
  you double the acceleration, you double the
  force.
• What if you double the mass and the acceleration?
• (2m)(2a) 4F
• Doubling the mass and the acceleration quadruples
  the force.
• So . . . what if you decrease the mass by half?
  How much force would be needed to have the same
  acceleration now?

22
Friction

• A force that opposes motion between surfaces in
  contact.
• It is due to microscopic imperfections in the
  surface. Solid surfaces only make contact in a
  few specific points. Each point is under enormous
  pressure and sometimes actually bonds to the
  other surface (see diagram)

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Friction (contd.)

• Friction keeps your car on the road, allows the
  tires to grip.
• Friction wastes energy as kinetic energy is lost
  as heat.
• Friction force depends only on three things
• Surfaces moving or not.
• Force pushing surfaces together
• Roughness of surfaces.

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What does F ma say?
F ma basically means that
Something very massive (high mass) thats
changing speed slowly (low acceleration), like a
glacier, can still exert (push with) a
____________ force.
Something very small (low mass) thats changing
speed very quickly (high acceleration), like a
bullet, can still exert a _______ force.
Something very low mass changing speed very
slowly will exert a very ________ force.
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Newtons Third Law

• If object A pushes or pulls on object B,
• Object B pushes or pulls on object A with the
  SAME force in the OPPOSITE DIRECTION.

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What does this mean?
For every force acting on an object, there is an
equal force acting in the opposite direction.
Right now, gravity is pulling you down in your
seat, but Newtons Third Law says your seat is
pushing up against you with equal force. This is
why you are not moving. There is a balanced
force acting on you gravity pulling down, your
seat pushing up.
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Think about it . . .
What happens if you are standing on a skateboard
or a slippery floor and push against a wall? You
slide in the opposite direction (away from the
wall), because you pushed on the wall but the
wall pushed back on you with equal and opposite
force.
Why does it hurt so much when you stub your toe?
When your toe exerts a force on a rock, the rock
exerts an equal force back on your toe. The
harder you hit your toe against it, the more
force the rock exerts back on your toe (and the
more your toe hurts).
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Examples

• A jet engine pushes _____ backward. The ______
  pushes the ______ _______ .
• A rocket engine pushes ______ out and backwards,
  the _______ pushes the ________ __________.
• A fly hits a windshield with 0.1N of force and
  pushes it _______ , the windshield pushes on the
  ________ with ___ N and pushes the fly
  ________.
• The bat hits the baseball forward with 1200N of
  force, the ____hits the bat ______ with _____ N
  of force.

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Review
Newtons First Law
Objects in motion tend to stay in motion and
objects at rest tend to stay at rest unless acted
upon by an unbalanced force.
Newtons Second Law
Force equals mass times acceleration (F ma).
Newtons Third Law
For every action there is an equal and opposite
reaction.
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Vocabulary
Inertia
the tendency of an object to resist changes in
its state of motion
Acceleration
a measurement of how quickly an object is
changing speed