Newtons First Law

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Newtons First Law

• Dr. Don Franceschetti
• August 30, 2007

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Aristotle (384-322 B. C.)

• A student of Plato.
• Tried to systematize knowledge of the natural
  world like Euclid systematized geometry.
• Tried to classify everything. A sort of
  biologist.
• Wrote first physics (nature) book. Objects had
  goals and strove to achieve those goals.
  Teleology.

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Aristotle (384-322 B. C.)

• We have his writings on nearly all subjects
  preserved in caves near his home and later sold
  to the library at Alexandria, Egypt.
• Preserved by Byzantines and Arabs.
• First prohibited and then embraced by church.

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Aristotles Theory of Motion

• Celestial motions circular because perfect
• Terrestrial motion
• Natural, depending on amount of each element
  contained, no force required
• Violent or forced motion pushing a cart
• Projectile motion was a problem
• Vacuum not possible (nothing isnt something), no
  point in talking about it

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Copernicus (1453-1543)

• A cleric and medical doctor.
• Book reached him on his deathbed.
• Heliocentric Universe, but still orbits are
  circles

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Galileo Galilei (1564-1642)

• Discredited idea that heavy bodies fall faster.
  (Pisa demo)
• Principle of inertia, end of distinction between
  natural and violent motion.
• Showed that heavens are imperfect.

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Galileo

• Three children and no wife.
• Cultivated powerful patrons the Medici.
• Recanted when threatened with torture.
• Eppur si muove

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Galileos argument

• Galileoa argument that objects would keep moving
  in the absence of friction is one of the first
  examples of theoretical physics.

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Sir Isaac Newton (1642-1727)

• Born December 25, 1642, a few months after
  Galileo died.
• Worked his way through school.
• Discoveries at age 23
• Hair turned gray at 30---did not wear wigs.

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Newtons first law of Motion

• a.k.a. principle of inertia
• Every object continues in its state of rest or of
  uniform motion in a straight line unless it is
  compelled to change that state by forces
  impressed upon it
• Key word is continues

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Force (intuitive)

• A force is a push or pull.
• Forces (applied to the same object)
• add like vectors to give the net force (on the
  object).
• The net force on an object is the vector sum of
  all the forces acting on it.
• An object will move in a straight line with
  constant speed only if the net force on it is
  zero. (no forces acting or forces acting in
  balance).
• Forces are measured in Newtons (to be defined
  later) or pounds (British)

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The equilibrium rule.

• If an object is at rest or moving in a straight
  line with unchanging speed, the net force on it
  must be zero
• ?F0
• Analyze various situations. Introduce spring
  scale. Spring scale can be used to exert known
  force.

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Your first bunch of forces

• Weight, due to gravitational pull of Earth. A
  non-contact or field force.
• Support force (solid surface), a contact force,
  due to stretching of chemical bonds
• Tension in a string or rope, a contact force
• Friction and fluid resistance, also contact
  forces.
• Spring force. Stretch of spring proportional to
  force it exerts.

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Moving Earth

• Inertia explains why falling objects fall at
  point below release, why you can drop an object
  on an airplane without punching a hole in the
  plane.
• Newton talked about sailing ships
• Einstein about railroads
• Us about the space shuttle and jet planes.

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Note to bomber pilots.

• Neglecting air resistance, that bomb will go off
  right underneath your plane unless you change
  course!

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Newtons First Law of Motion

• Every object continues in a state of rest, or of
  uniform motion in a straight line, unless it is
  compelled to change that motion by forces
  impressed on it (i. e. a non-zero net force.)
• This is an answer.

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So what was the question?

• Why is it dumb to drive fast on an icy road?
• Why do we have to wear seat-belts when we drive?
• Why do jumbo jets require such long runways?
• Why is it dumb to dash across a RR crossing when
  the train is approaching?

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Have some more questions.

• Why does my car slow down when I step on the
  brake pedal?
• Why do the planets not move in straight lines
  past the sun?
• How can you sit there while being pulled towards
  the Earth?
• Why is it OK to jump out of a plane with a
  parachute but not without one?

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Linear Motion

• Dr. Don Franceschetti
• August 30, 2007?

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Guide Questions

• 1. Is there any way to tell if we are in motion?
• 2. Is there a difference between speed and
  velocity?
• 3. Is there a difference between velocity and
  acceleration?
• 4. What is meant by free fall?
• 5. If you jumped out of your dorm room window,
  how long would it take you to hit the ground?

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We are moving!

• 107,000 kilometers per hour with respect to the
  sun
• Faster with respect to center of milky way galaxy
  (How do we know--Youll find out.)
• Motion is relative. No meaning to absolute rest.
• Usually we mean relative to the Earths surface.

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Units of Measurement

• The meter
• Originally one ten-millionth of the distance from
  the North pole to the equator on the meridian
  passing through Paris, France.
• Then the distance between two marks on a Pt-Ir
  rod.
• Then a fixed number of wavelengths of a certain
  wavelength of light
• Now distance traveled by light in a 1/299,792,458
  sec.

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Units of Measurement

• The second
• Short for the second minute part of an hour
• Originally 1/3600 of 1/24th of a mean solar day.
• Now time for a fixed number of vibrations of a Cs
  atom. (atomic clock). 9,192,631,770 periods on
  the hyperfine transition in Cs-133.

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Speed

• Speed distance/time
• So units are meters/second (m/s.) or other
  distance/time
• E. g. 100 km/hr
• Instantaneous speed
• Average speed
• Total Distance Covered
• Time interval

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Velocity

• Velocity is speed and direction, taken together.
  E. g. 100 km/h due north.
• Instantaneous velocity is usually meant.
• Average Velocity is
• Net displacement
• time interval

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Acceleration

• Acceleration
• change in velocity
• time interval
• Measured in meters per second per second
• or m/s2. Think of this as m/s each second
• Has both magnitude and direction
• Or in mixed units, e. g. 5 km/hrsec
• This is the one you feel

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You have a (nonzero) acceleration if.

• Your speed changes as you move in a straight
  line.
• Your direction changes while your speed does not.
• Your speed and direction are both changing.

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Galileo again

• Found that balls rolling down an inclined plane
  did so with constant acceleration
• Velocity acceleration x time
• Measured from rest.

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How Far constant acceleration

• Distance traveled is
• (average velocity) x time
• (v0vf)/2 x time
• d(1/2)gt2 starting from rest

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Free fall

• A body responding only to the gravitational pull
  of other bodies is said to be in free-fall.
• This means all other forces are negligible.

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Little g

• Galileo showed that all bodies in free-fall near
  the earths surface accelerate downward at 9.8
  m/s2.
• Call this g. Its best to remember as 10
  meters per second every second.
• Falling from rest vgt.

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Freely Falling bodies

• How far
• D (1/2)gt2 from rest
• Or D v(average) x time, always true for
  constant acceleration.

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Study problems

• Calculate the speed and distance covered at 1, 2,
  3, 4, and 5, seconds by.
• A lead brick dropped from rest.
• A rock tossed upward at 20 m/s.
• c) A rock thrown downward at 20 m/s.