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Newton st Law of Motion

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					Newton’s   1st   Law of Motion

            Inertia
                 Objectives
• Explain the relationship between the motion
  of an object and the net external force acting
  on the object.
• Determine the net external force on an
  object.
• Calculate the force required to bring an
  object into equilibrium.
Newton’s 1st Law of Motion States:

 • An object at rest tends to stay at rest and an
   object in motion tends to stay in motion with
   the same speed and in the same direction
   unless acted upon by an unbalanced force.
1st Law
                1st Law

All objects resist changes in their state
of motion – they tend to "keep on
doing what they're doing."
1st Law
               1st Law
Inertia is the resistance an object has
to a change in its state of motion.
1st Law
                   1st Law
• Galileo, developed the concept of inertia.
• Galileo reasoned that moving objects
  eventually stop because of a force called
  friction.
1st Law
                   1st Law
• The tendency of an
  object to resist
  changes in its state
  of motion is
  dependent upon its
  mass.
                1st Law
• Mass is not volume.
• Mass is measured in kilograms.
                 1st Law
• Mass is not weight.
• It is more fundamental than weight.
• Mass is a measure of the actual material
  in an object. It depends only the number
  of atoms that compose it.
• Weight is a measure of the gravitational
  force that acts on a material and
  depends on where the object is located.
1st Law
                 1st Law
• Mass: the quantity of matter in a body.
• The measure of inertia.
• Weight: the force due to gravity upon a
  body.
                 1st Law
• The SI unit of force is the NEWTON.
• 1 kg = 9.8 N = 2.2 lbs.
       Check Your Understanding

1. Imagine a place in the cosmos far from
   all gravitational and frictional influences.
   Suppose an astronaut in that place
   throws a rock. The rock will:
   A. gradually stop.
   B. continue in motion in the same
        direction at constant speed.
       Check Your Understanding
• An 2-kg object is moving horizontally with a
  speed of 4 m/s.
• How much net force is required to keep the
  object moving with the same speed and in the
  same direction?
        Check Your Understanding
• Mac and Tosh are arguing in the cafeteria.
  Mac says that if he throws his jello with a
  greater speed it will have a greater inertia.
• Tosh argues that inertia does not depend
  upon speed, but rather upon mass.

• With whom do you agree? Why?
        Check Your Understanding
• If you were in a weightless environment in
  space, would it require a force to set an object
  in motion?
        Check Your Understanding
• Mr. Wegley spends most Sunday afternoons at
  rest on the sofa, watching pro football games
  and consuming large quantities of food.

• What effect (if any) does this practice have
  upon his inertia? Explain.
        Check Your Understanding
• Ben is being chased through the woods by a bull
  moose which he was attempting to photograph.
• The enormous mass of the bull moose is
  extremely intimidating.
• Yet, if Ben makes a zigzag pattern through the
  woods, he will be able to use the large mass of
  the moose to his own advantage.
• Explain this in terms of inertia and Newton's first
  law of motion.
            State of Motion
• Inertia is the tendency of an object to
  resist changes in its velocity.
             State of Motion
• We learned before that an object which
  is not changing its velocity is said to have
  an acceleration of 0 m/s2.
• Thus, an alternate definition of inertia
  would be:
• Inertia is the tendency of an object to
  resist accelerations.
       Check Your Understanding
• Several physics teachers
  are taking some time off
  to play a little mini golf.
  The 15th hole has a large
  metal rim which putters
  must use to guide their
  ball towards the hole.
• Mrs. Menken guides her
  golf ball around the metal
  rim. When the ball leaves
  the rim, which path (1, 2,
  or 3) will the golf ball
  follow?
      Check Your Understanding
• A 4.0 kg object is moving across a frictionless
  surface with a constant velocity of 2 m/s.
  Which one of the following horizontal forces is
  necessary to maintain this state of motion?
• 0N
• 0.5 N
• 2.0 N
• 8.0 N
   Balanced and Unbalanced Forces
• An object at rest tends to stay at rest and an
  object in motion tends to stay in motion with
  the same speed and in the same direction
  unless acted upon by an unbalanced force.
• What is an unbalanced force?
Balanced and Unbalanced Forces
 Balanced and Unbalanced Forces
• Since these two forces are of equal magnitude
  and in opposite directions, they balance each
  other.
• The book is said to be at equilibrium.
Balanced and Unbalanced Forces
Balanced and Unbalanced Forces
   Balanced and Unbalanced Forces

• The book is not at equilibrium and it
  subsequently accelerates.
• Unbalanced forces cause accelerations.