Physics Chapter 4: Motions and Force

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Physics Chapter 4 Motions and Force

• Christopher Chui

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Motions and Force Dynamics

• Newtons 1st law of motion Every body continues
  in its state of rest or of uniform speed in a
  straight line unless acted on by a nonzero net
  force law of inertia holds in inertial ref
  frames
• Mass is a measure of inertia
• Weight is the force of gravity acting on a body
  mg
• Newtons 2nd law The acceleration of an object
  is directly proportional to the net force acting
  on it and is inversely proportional to its mass.
  The direction of the acceleration is in the
  direction of the net force acting on the objecta
  SF/m or SF m a
• Force is an action capable of accelerating an
  object

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Units for Mass and Force

• System Mass Force (incl. Weight)
• SI kg newton (kg-m/s2)
• cgs g dyne (g-cm/s2)
• British slug pound (lb)

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Newtons 3rd Law FGP - FPG

• Whenever one object exerts a force on a second
  object, the second exerts an equal and opposite
  force on the first
• Action and reaction act on different objects
• Inanimate objects can exert a force
• For each force, be clear on which object it acts,
  and by which object it is exerted
• SF m a applies only to forces acting on a body

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Weightthe Force of Gravity and the Normal Force

• Weight and normal force are not action-reaction
  pairs
• Force exerted by the table is called a contact
  force
• When a contact force acts perpendicular to the
  common surface of contact, it is the normal force
• The net force is the vector sum of all forces
  acting on the object
• A free-body diagram or force diagram shows all
  forces acting on each object involved

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Problem Solving (Newtons Laws Free-Body
Diagrams)

• Draw a sketch of the situation
• Consider only one object at a time
• Draw a free-body diagram for each body showing
  all forces acting on the body
• Do not show any forces that the body exerts on
  other bodies
• Draw the arrow for each force vector as
  accurately as possible for direction and
  magnitude
• Label each force, including the unknowns, as to
  its source
• Only forces acting on a given body can be
  included in the F ma equation for that body
• Resolve vectors in x and y components
• For each body, the Fma can be applied to x and y
  separately

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Friction

• Kinetic friction Ffr mk FN
• Ffr is perpendicular to FN
• Static friction refers to a force parallel to the
  two surfaces when they are not sliding
• Static friction is usually larger than kinetic
  friction in value, Ffr lt or ms FN
• The equal sign refers to maximum friction

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Physics Assignment 3

• Read and memorize Summary on p. 103
• Do even numbered Questions on p. 103-104
• Do even numbered Problems on pp. 104 - 107
• Turn in Physics Assignment 3 seven days from
  today
• Prepare for Physics Quiz 3
• Physics Quiz 3 will take place 7 days from today