Newtons First Law

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Newtons First Law

• Dr. Don Franceschetti
• January 24, 2005

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Aristotle (384-322 B. C.)

• Kept 4 elements, added quintessence
• No atoms
• Terrestrial motions were natural or forced
  (violent)
• Heavier objects fall faster
• Celestial motions were circular
• Heavens were perfect with no change

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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)

• De Revolutionibus Orbum Coelestum 1543 (note
  date)
• 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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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.
• Forces are measured in Newtons (to be defined
  later) or pounds (British)

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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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The equilibrium rule.

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

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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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Newtons First Law of Motion

• Every object continues in a state of rest, or of
  uniform motion in a straight line, inless 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,