Newtons 1st Law

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Newtons 1st Law

• An object at rest remains at rest and an object
  in motion continues in motion with a constant
  speed in a straight line, assuming no net force
  is acting.

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Newtons 2nd Law

• The acceleration of an object is directly
  proportional to the net force acting on it and
  inversely proportional to its mass.
• SF ma
• SF vector sum of forces net force
• Unit of F Newton (N)
• m mass in kg
• a acceleration in m/s2

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Newtons 3rd Law

• The force object A exerts on object B is equal in
  magnitude and opposite in direction to the force
  object B exerts on object A.
• F12 -F21
• Example a rifle exerts a force to accelerate a
  bullet, while the bullet exerts a kickback
  force in the opposite direction on the rifle

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Horse-Sled Paradox

• Why does the sled move at all?

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Horse-Sled Paradox Resolution

• To solve the paradox , we simply have to draw
  the free body diagram showing all the forces
  acting on sled and horse. A net force on both
  will cause the system to accelerate according to
  Newtons 2nd Law.

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Examples of Forces
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Force of Gravity

• The force that gravity exerts on an object is (a)
  called weight , (b) applied to the center of
  gravity of the object.
• Mass is the quantity of matter (in kilogram)
• Weight formula begins with F ma
• F Force Weight W
• a acceleration due to gravity g 9.8m/s2
• W mg

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Mass vs. weight

• Despite the obvious difference between weight
  and mass, our culture
• Is littered with confusing miseuses of these
  words. Canned foods have such
• Labels as net weight 320 grams . We often
  speak of weighing an object,
• Yet report the answer in kilograms. It is
  important that you keep mass and
• Weight separate and not confuse the 2 concepts,
  despite what you may hear
• In the supermarket from textbook Crummett and
  Western
• MASS QUANTITY OF MATTER NUMBER OF ATOMS IN AN
  OBJECT (kg)
• WEIGHT FORCE OF GRAVITY ON OBJECT (UNIT
  NEWTON)
• W mg

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Mass vs. Weight example
Mass is the same but weight is different. Weight
is due to planets gravity.
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Elephant vs. Feather
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Normal Force

• A normal force is perpendicular (at right angles)
  to a given surface

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Normal Force(2)
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Normal force (3)

• Normal force not necessarily equal to weight

N Fg F N W F
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Elevator Problem
T mg ma
T ma mg
T m ( a g )
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Elevator Problem(2)
T mg m(-a) T mg ma T m ( g
a) Note if a g, T 0 and we have free
fall, also called weightlessness
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Atwoods Machine without Friction Table type
m1
T
m2
m2g m1a m2 a
T m1a m2g T m2a
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Continued

• Solve previous slide with system approach
• asystem Fsystem / Total Mass

In this example a W2 / (m1 m2)
a m2g / (m1 m2)
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Atwood MachineSee active figure 5.14
m2g T m2a T m1g m1a
Add above 2 eqns. to get m2g m1g m2a m1a
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Systems Approach

• In previous slide
• a (W2 W1) / (m1 m2)
• a ( m2g m1g ) / (m1 m2)
• a g (m2 m1) / ( m1 m2)

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Friction concepts
f mN
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ESP(Electronic Stability Program)

• This quote is from the Jeep SUV handbook The
  ESP cant prevent the natural laws of physics
  from acting on the vehicle, nor can it increase
  the traction afforded by prevailing road
  conditions. ESP cannot prevent accidents,
  including those resulting from excessive speed in
  turns, driving on very slippery surfaces, or
  hydroplaning
• Interpretation 1 The company needs a legal
  disclaimer, cant blame them for that.
• Interpretation 2 They are exactly correct
  Newtons Laws ultimately determine the
  performance of the car, irregardless of how fancy
  the engineering devices might be

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Atwood Machine on Table with Friction
m1
m2
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Inclined Plane without Friction
See active figure 5.18
mgsinQ ma
a gsinQ
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Angle of repose

• This is the minimum angle such that a body just
  begins to slide down plane
• In this case a 0, so
• Wx f
• mgsinq mN mWy mmgcosq
• sinq mcosq
• m sinq /cosq tanq

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Inclined Plane with Friction
See active figure 5.18
mgsinq mmgcosq ma
a g (sinq mcosq)
Note above is for object sliding down the
plane. If object is sliding up the plane,
replace above minus sign with a plus sign.