Introducing Forces

1
Introducing Forces

• Chapter 4 (MHR Physics)

2
Forces

• Essentially thought of as pushes or pulls on our
  everyday level
• Are responsible for motion
• Cause objects to accelerate
• Vector quantity-has magnitude direction
• Units are Newtons, (N) dynes, pounds

3
Common types of forces

• Gravitational
• Frictional
• Electric
• Magnetic
• Mechanical

4
Key Terms

• Inertia
• Kinematics
• Dynamics
• Mechanics
• Force
• Inertial Mass
• Gravitational Mass

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Inertia (4.1)

• Is the tendency of an object to remain in its
  current state of motion. The amount of an
  objects inertia is directly related to its mass.
• Examples you dont want to get out of bed on a
  Saturday am
• A granite boulder rests on a rocky beach

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Inertia (4.1)

• Galileos thought experiments led to our
  understanding of inertia-objects rolling down
  inclines will speed up, those rolling up inclines
  will slow down and those rolling along a
  horizontal surface will continue to roll. His
  work built on Aristotles and Buridan who
  believed respectively that constant force
  produced constant speed and that objects remained
  moving because they had impetus inside

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The study of motion

• Kinematics is the branch which describes an
  objects motion in terms of displacement,
  velocity, and acceleration
• Dynamics explains why objects move i.e respond to
  forces
• Mechanics Kinematics Dynamics

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Inertia (4.1)

• Inertial mass is the property of matter causing
  it to resist a change in its motion
• Gravitational mass is the property of matter that
  determines the strength of the gravitational
  force. The more mass, the stronger the
  gravitational attraction for that mass.
• Einstein showed that these two are essentially
  the same.

9
Common Forces (4.2)

• Forces which involve direct physical contact are
  contact forces e.g. pushing, pulling, and
  frictional
• Forces which do not involve direct physical
  contact but which act over a distance are called
  non contact forces e.g. electric, magnetic,
  gravitational

10
Common Forces (4.2)

• Weight (gravitational force) is the force of
  gravity acting on a mass. We can jump higher on
  the moon than on earth because the moons gravity
  is about 1/6 th that of the earth.
• Fg mg where F is force in N, m is mass in kg
  and g is acceleration due to gravity
• Since g varies with location on the earth, your
  weight (not mass) will vary by location

11
Common Forces (4.2)

• Friction is a force which opposes motion. It is
  measured in N and is a contact force
• Two kinds of friction
• Static friction involves no net motion
• Kinetic friction involves movement
• The coefficient of friction is a pure number
  which indicates how much friction exists between
  surfaces.
• The symbol is µ
• The formula for friction is µ Ff /FN or Ff
  µFN

12
Friction

• Depends on nature of the surfaces-rougher
  surfaces have more friction than smoother ones
• Actually an electromagnetic interaction at the
  atomic level. Surface atoms of objects in contact
  interact and form attractive forces leading to a
  stick and slip process when one object slides
  over another e.g. marker on whiteboard, chalk on
  chalkboard, squealing tires

13
Friction

• The coefficient of friction is a stickiness
  value for specific combinations of surfaces
• Friction also depends on the magnitude of the
  forces pressing the surfaces together.
• The force that a surface exerts back on an object
  is called the normal force. It acts perpendicular
  to the surface.

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Friction

• Force of friction is independent of velocity
• Force of friction is independent of the area of
  contact (only if mass is evenly distributed). The
  surfaces must be flat.
• Certain plastics and rubbers have natural
  properties which do not agree with the standard
  model of friction e.g. ice-gripping tires,
  adhesive tape

15
Free Body Diagrams (FBD)

• Represent all the forces acting on an object.
• Only contains forces and angles at which they
  act.
• Draw them roughly to scale so a bigger force will
  have a bigger line.