Welcome back to Physics 211

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Welcome back to Physics 211

• Todays agenda
• More on Newtons Laws
• Free Body Diagrams

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Newtons Second Law

• Second Law
• SFon object Fnet m aof object
• where Fnet is the vector sum of all external
  forces on the object considered
• m (inertial) mass

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The force of friction is described by

• the law of friction.
• the theory of friction.
• a model of friction.
• the friction hypothesis.

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Static Friction
The box is in static equilibrium, so the static
friction must exactly balance the pushing force
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Static Friction

• An object remains at rest as long as fs lt fs max.
• The object slips when fs fs max.
• A static friction force fs gt fs max is not
  physically   possible.

where the proportionality constant µs is called
the coefficient of static friction.
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Kinetic Friction
The kinetic friction force is proportional to the
magnitude of the normal force.
where the proportionality constant µk is called
the coefficient of kinetic friction.
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Block on Incline
f
N
W
q
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The magnitude of the force of kinetic friction
between two objects
What if ? gt tan-1ms ?

• depends on the type of surfaces of the objects
• depends on the normal force that the objects
  exert on each other
• does not depend on the surface area where the two
  objects are touching
• does not depend on the speed with which one
  object is moving relative to the other

fk µkn
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What if ? gt tan-1 µs ?
f
n

• Block begins to slide
• Resolve along plane
• µkWcos? - Wsin? ma
• Or
• a g(µkcos? - sin?)

W
q
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Example Problem 43

• A baggage handler drops your 10 kg suitcase onto
  a conveyor belt running at 2.0 m/s. The materials
  are such that µs0.50 and µk0.30. How far is
  your suitcase dragged before it is riding
  smoothly on the belt?

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Example Problem 43

• A baggage handler drops your 10 kg suitcase onto
  a conveyor belt running at 2.0 m/s. The materials
  are such that µs0.50 and µk0.30. How far is
  your suitcase dragged before it is riding
  smoothly on the belt?

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Relationship between Mass, Weight and Gravity
Mass

• scalar quantity that describes an objects
  inertia
• describes the amount of matter in an object
• intrinsic property of an object
• tells us something about the object, regardless
  of where the object is, what its doing, or
  whatever forces may be acting on it

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Gravity
Somewhat more loosely, gravity is a force that
acts on mass. When two objects with masses m1 and
m2 are separated by distance r, each object pulls
on the other with a force given by Newtons law
of gravity.
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Gravity cont.
where G 6.67 x 10 -11 Nm2/kg2
near Earth
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Chapter 7. Newtons Third Law

• Topics
• Interacting Objects
• Analyzing Interacting Objects
• Newtons Third Law
• Ropes and Pulleys
• Examples of Interacting-Object Problems

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Interacting Objects
If object A exerts a force on object B, then
object B exerts a force on object A. The pair of
forces, as shown, is called an action/reaction
pair.
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Newtons 3rd Law
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Physics Education Research
fact that a large majority can recite Newtons
third law when they enter the class. Halloun and
Hestenes (1985b) have characterized student
beliefs about interactions in terms of a
dominance principle the larger (or faster or
more active) object exerts a larger force than
the smaller (or slower or less active) object.
Students tend to view an interaction as a
conflict in which the stronger wins. Its not
hard to understand how this common-sense view
comes about. After all, the effect of the
collision on the compact car is much larger than
its effect on the truck.
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• Some of the more specific difficulties students
  have with Newtons third law and with interacting
  objects are
• Students dont believe Newtons third law. Its
  too contrary to common sense.
• Students have difficulty identifying
  action/reaction force pairs
• They match two forces on the same object.
• They place forces on the wrong objects.
• They dont believe that long-range forces (e.g.,
  gravity) have reaction forces.
• Students confuse equal force with equal
  acceleration.

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Students dont understand tension

• They think that tension is the sum of the forces
  exerted at the two ends of a string.
• They think that tension exerts a force only in
  the direction of motion.
• They think that tension can pass through an
  object to another string on the other side.
• Students often dont recognize that objects
  connected by an inextensible string must have
  accelerations of equal magnitude.

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FHW7 Due F 10/16/09

• Ch.6 EP 1, 5, 18, 20, 21

WHW8 Due F 10/21/09

• Ch.6 EP 29, 37, 46 Ch. 7 CQ 12