Newtons Second Law

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Newtons Second Law

• Physics 1010
• Dr. Don Franceschetti
• September 4, 2007

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Study problems from Linear Motion

• 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.

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Guide Questions

• How does force relate to acceleration?
• Can an object have a velocity, a force, an
  acceleration.
• What is mass?
• What is meant by weightlessness?
• Is antigravity possible?
• How do objects respond when more than one force
  acts on them?

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Newtons First Law

• 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.)

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Newtons First

• Q Consider a book that weighs15 N at rest on a
  flat table. How many newtons of support force
  does the table provide? What is the net force on
  the book in this case?
• Q If you push on a crate with a force of 100 N
  and it slides with a constant velocity, how much
  is the friction acting on the crate?

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Linear Motion

• Q What kind of speed is registered by an
  automobile speedometer, average speed or
  instantaneous speed?
• Q If a car moves with a constant velocity, does
  it also move with constant speed?

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Newtons Second Law

• On any body
• Net force mass x acceleration
• Mass measured in kilograms (Kg)
• 1 newton 1 kg m / s2

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Algebra

• Fma
• aF/m, explains why bodies fall with same
  acceleration in a vacuum
• mF/a explains how astronauts can measure masses

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A metric moment

• The kilogram is defined as the mass of a standard
  platinum-iridium cylinder kept at the
  International Bureau of Weights and Measures in
  Paris, France
• Someday the kilogram will be defined as the mass
  of a specific number of atoms of a specific
  isotope.

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Weight

• The weight of an object is the strength of the
  force the Earth (or other planet) exerts on the
  object.
• In free fall, gravity is the only force at work,
  and it produces an acceleration of g9.8m/s2
• Thus the weight of a kilogram is its mass times
  that acceleration
• 9.8 kg m/s2 or 9.8 newtons(N)

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That abominable English system

• Distances are measured in feet
• Time is measures in seconds
• g32 ft/s2
• The pound is the unit of force
• The slug is the unit of mass (really!)
• 1 pound 1 slug ft/sec2
• One slug has a weight of 32 pounds

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Apparent Weight

• Your apparent weight is the force that a scale
  must supply to make the net force on you equal to
  your mass x acceleration
• Think about elevators
• Free fall apparent weight is zero, gravitational
  force is generally not zero

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Mass and weight

• Mass can be thought of as the quantity of
  matter or resistance to acceleration. It does
  not change.
• Weight depends on where you are.
• Can measure mass by relative weighing.
• Question, how would baseball be different on the
  moon?

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Difference between mass and weight
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Space Age Misconceptions

• There is no gravity in a vacuum.
• NASA has built a zero gravity chamber.
• Gravity is the same as air pressure.
• Gravity pushes down.
• Gravity is the same as magnetism (William Gilbert
  1600 A. D.)

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Artificial Gravity

• Centrifugal force actually your tendency to
  move in a straight line while a restraint makes
  you accelerate.
• Measured in gs
• Amusement Parks
• Aerobatics
• Centrifuges

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Normal Force

• Perpendicular to solid surface
• As big as it needs to be!!! (unless surface
  breaks)
• Caused by microscopic deformations

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Friction

• Between solid surfaces
• Due to surface irregularities
• Two kinds static and dynamic (rolling is a
  third)
• Static friction depends on the other forces at
  work. There is a maximum proportional to normal
  force
• Kinetic friction is a set fraction of the normal
  force in size
• Friction is independent of contact area and speed

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Air Resistance

• In very complicated
• Proportional to speed at low speeds, so we can
  ignore it in many cases
• Proportional to square of speed at highway speeds
• Discuss
• Gas economy
• Parachutes

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Falling acceleration less than g

• a(down)(mg-Fair)/m
• At terminal velocity a0!
• Free fall ag

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Do not confuse

• Mass
• Weight
• Velocity
• Acceleration
• force