Unit 3 - Dynamics

1
Unit 3 - Dynamics

• Introduction to Forces and Newtons three Laws of
  Motion

2
Unit Outcomes

• Demonstrate an understanding of Newton's Three
  Laws of Motion.
• Describe various types of forces.
• Define constant velocity, net force and inertia
  in terms of Newton's Laws.
• Calculate net force using the formula derived
  from Newton's Second Law.
• Calculate the equilibrant force of an object.
• Identify the three fundamental types of forces.
• Define and calculate "weight."
• Compare and contrast mass and weight in terms of
  physics.
• Describe normal force.
• Accurately draw normal force and tension vectors
  in physics diagrams.
• Describe how pulleys affect tension forces.
• Identify the different types of friction.
• Calculate the force of kinetic friction.
• Solve friction problems using the formula for
  friction and the friction coefficient.
• Draw free-body diagrams to show forces on a given
  object.
• Use free-body diagrams with Newton's Second Law
  to solve mechanics problems.
• Define momentum.
• Demonstrate an understanding of changes in
  momentum and Newton's Second Law.

3
Inertia

• The natural tendency of an object to resist any
  change in its motion
• The amount of inertia is directly related to mass
• Seatbelts, bus, subway

4
Mechanics

• Kinematics predicting and describing motion
  (displacement, velocity and acceleration)
• Dynamics Explaining why objects move the way
  they move
• The study of dynamics involves forces a push of
  a pull on an object that causes change in motion
  (see table 4.2 page 128)

5
Inertial Mass/ Gravitational Mass

• Inertial Mass - The net external force needed to
  act on a mass in order to change its motion
• Gravitational Mass the mutual force of
  attraction between every pair of masses in the
  universe. The forces vary directly with the
  magnitude of each mass and inversely with the
  square of the distance between the masses (Law of
  Universal Gravitation)

6
Common Forces

• Two Categories of Force
• Contact Forces friction, applied force,
  tension, normal force
• Non-Contact Forces gravity, electrical
  and magnetic attraction

7
Gravitational Force - Weight

• Because the Earth is so big (huge mass), its
  exerts a strong force on you and any other
  object. You exert an equal force on the Earth.
  Gravitational forces always act in pairs
• See figure 4.4 pg 131
• The magnitude of gravitational force varies

8
Weight

• Mass the amount of matter in an object
  (constant)
• Weight force of gravity action on an object. It
  varies with the distance the object is away from
  the center if the Earth
• Acceleration due to Gravity (g)
• 9.80 m/s2
• Is influenced by both mass of Earth and distance
  from the Earth. The value of g varies with
  location
• See table 4.3 page 132

9
Calculating Weight

• Weight Fg or W
• Fg mg Fg (W) force of gravity
  (weight)
• m mass of object
• g acceleration due to gravity
• Units kg x m/s2
• one newton (N)

10
Friction

• Friction is a contact force
• Frictional forces inhibit relative motion between
  objects in contact with each other
• Two types of friction
• Static frictional force exists when you start
  to move an object from rest
• Kinetic frictional force exists while the
  object is moving
• Static friction is greater that kinetic friction

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Static Frictional Force

• The strength of frictional force between two
  surfaces depend on the nature of the surfaces.
  All surfaces create some friction (no such thing
  as a smooth surface)
• Force of friction is an electromagnetic force
  acting between the surface atoms of one object
  and those of another

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• When two surfaces are at rest, the surface atoms
  interact to form relatively strong attractive
  forces. A push on one object creates a static
  frictional force that pushes back with exactly
  the same magnitude of force until the applied
  force is great enough to break the attractive
  forces between the surface atoms
• New bonds are formed when objects begin to move.
  The are continually being formed and broken as
  the object slides over the other.

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• Frictional force depends on two things
• Type of atoms and molecules making up the
  materials passing over each other
• Stickiness factor coefficient of friction (µ)
• The magnitude of the forces that are pressing the
  two surfaces together
• Normal force (Fn ) equal and opposite to weight
• Surprisingly friction is independent of velocity
  and area if contact (surface area) three
  assumption are necessary for this to be true

14
Calculating Friction

• Ff µFn
• Ff - force of friction (N)
• µ coefficient of friction (none)
• Fn - normal force (N)