Chapter 5: Newtons Laws

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Chapter 5 Newtons Laws

• Brent Royuk
• Phys-111Concordia University

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Introduction

• Kinematics vs. Dynamics
• Newton Philosophiae Naturalis Principia
  Mathematica (The Mathematical Principles of
  Natural Philosophy), 1687

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Newton

• Newton

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Newton

• Newton

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The First Law

• The Ooze Tube
• Aristotelian Mechanics
• Natural vs. Violent Motion
• Impetus
• Scholasticism
• Galileo
• The tendency to stop is overstated
• Empiricism and idealization
• Outer space air tracks

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The First Law

• The Law
• Lex I Corpus omne perseverare in statu suo
  quiescendi vel movendi uniformiter in directum,
  nisi quatenus a viribus impressis cogitur statum
  illum mutare.
• Every body perseveres in its state of being at
  rest or of moving uniformly straight forward,
  except insofar as it is compelled to change its
  state by force impressed.
• Inertia What is it?
• What keeps Earth moving in its orbit around the
  sun?
• So whats the big deal? Isnt this law obvious?

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The First Law

• The pen hoop
• The penny the bottle
• The dinner table

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ConcepTest
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The Second Law

• How would you define force?
• Do all forces cause accelerations?
• The Net Force
• What if you push harder?
• a ? F
• What if the object is more massive?
• a ? 1/m
• The Law
• The Force Unit
• Example Push a 15-kg cart with a force of 36 N.
  How fast is it going in 3.0 seconds?

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Free-Body Diagrams

• An accounting system for forces
• Figure 5-5, p. 113
• Let a system of masses be a point, draw all
  vectors tail-to-tail so you can see whats going
  on.
• What is a system?
• Internal vs. External Forces
• What would a FBD look like for a car coasting
  down a highway?
• Example A box that weighs 49 N (and therefore
  has a mass of 5.0 kg) is pushed sideways with a
  force of 8.6 N. This push is resisted by a
  frictional force of 4.3 N. Find the acceleration
  of the box.

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Newtons Third Law

• For every _____ there is an equal and opposite
  _____.
• The bathroom scale law.
• F12 -F21
• Example Lets say you apply a 50-N force to a
  car in neutral on a level surface. The car
  pushes back on you with a 50-N force as well.
  These forces are balanced, so no acceleration
  takes place, right?
• Figure 5-8, p. 118

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Newtons Third Law
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ConcepTest

• Two boxesone large and heavy, the other small
  and light are touching each other at rest on a
  smooth, level floor. You push with a force F on
  either the small box or the large box. Is the
  contact force between the two boxes
• the same in either case,
• larger when you push on the large box, or
• larger when you push on the small box?

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ConcepTest

• Two kissing adolescents have their braces lock.
  In order to become unstuck they wish to push
  against each other with the greatest force
  possible. Should they
• Have the stronger boy push against the weaker
  girls shoulders?
• Have the weaker girl push against the stronger
  boys shoulders?
• Increase the force by pushing against each
  others hands?

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ConcepTest
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Forces in 2-D

• When forces act in 2 dimensions, use a FBD with a
  convenient coordinate system and treat the
  components separately.
• Examples
• Example 5-5, p. 122
• Active Example 5-2

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Weight

• W mg
• A newton-weight
• Mass vs. weight how are they different?
• A weightless hammer in the space shuttle.
• Galileo and the Tower of Pisa again
• Two kinds of mass?
• Example
• You are holding a 5.0-kg mass in your hand, upon
  which you are exerting an upward force of 79 N.
  What happens to the mass?

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Normal Forces

• Forces exerted by surfaces are always normal to
  the plane of the surface.
• Flat surfaces
• Draw a FBD for a mass on a flat table.
• Example Find the normal force for a 15-kg box
  pulled by a rope inclined 25o above horizontal
  with a force of 12 N.
• Inclined surfaces
• Find the normal force and acceleration for a mass
  on a frictionless plane inclined at angle ? with
  respect to horizontal.