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Chapter 4 Force and The Laws of Motion


Chapter 4 Force and The Laws of Motion Physics Teacher: Mrs. R. Williams – PowerPoint PPT presentation

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Title: Chapter 4 Force and The Laws of Motion

Chapter 4 Force and The Laws of Motion
  • Physics
  • Teacher Mrs. R. Williams

  • Sir Isaac Newton (1643-1727) an English scientist
    and mathematician famous for his discovery of the
    law of gravity also discovered the three laws of
    motion. He published them in his book
    Philosophiae Naturalis Principia Mathematica
    (mathematic principles of natural philosophy) in
    1687. Today these laws are known as Newtons
    Laws of Motion and describe the motion of all
    objects on the scale we experience in our
    everyday lives.

Newtons Laws of Motion
  • 1. An object in motion tends to stay in motion
    and an object at rest tends to   stay at rest
    unless acted upon by an   unbalanced force.
  • 2. Force equals mass times acceleration (F
  • 3. For every action there is an equal and
    opposite reaction.

SECTION 4.1--Force
  • Definition Force is the cause of an
    acceleration or the change in an objects
  • A force can be a push, a pull, and pressure
    force can act directly through physical contact
    or at a distance like a magnetic field.
  • We measure force in NEWTONS.
  • A newton is the force needed to accelerate a 1
    kilogram mass by 1 meter per second squared
  • 1N 1kg x 1m/s2

  • Newtons ARE NOT units of weight or mass!
  • Weight What you weigh is a measure of your mass
    on the planet earth. Your weight can change
    relative to your environment! Mr. Price weighs
    200 pounds on earth, but on the moon he would
    weigh only 33 pounds!
  • While your weight can change your mass is
    constant regardless of your environment.
    Although Mr. Price would weigh 33 pounds on the
    moon his MASS would still be 200 pounds on the
    moon, Jupiter or in a black hole!
  • Newtons are a measure of forcewe can use newtons
    to measure how much force the earth is exerting
    on your body (your weight) but newtons are units
    of force!
  • Conversion Factors
  • 1 pound 4.448 newtons
  • 1 newton 0.225 pounds

  • Two Types of Forces
  • 1. Contact Forces The force that arises from
    the physical contact of two objects.
  • 2. Field Forces The force that can exist
    between objects even in the absence of physical
    contact (like gravity and magnetism)
  • The 4 Fundamental Forces of the Universe
  • Gravity
  • Electromagnetism
  • The Strong Nuclear Force
  • The Weak Nuclear Force

Force Diagram
  • Force Diagrams
  • FORCE IS A VECTOR with both a magnitude and a
  • Forces are represented by arrows which indicate
    the direction of the force. The length of the
    arrow indicates the magnitude of the force.
  • Diagrams are often used to analyze situations
    where more than one force is acting on an object.
    These are known as force diagrams
  • Simple force diagrams of single objects and the
    forces acting on them are called free-body

Section 4-2Newtons First Law
  • Inertia An object at rest remains at rest, and
    an object in motion continues in motion unless
    the object experiences a net external force.
  • Acceleration is determined by net external force.
  • Newtons first law implies that the net external
    forces on an object (with a constant velocity)
    must be equal to zero.
  • net external forces means the sum of all of the
    individual forcesadd those vectors!
  • The Greek letter sigma (S) is used to designate a

What does this mean?
  • Basically, an object will keep doing what it was
    doing unless acted on by an unbalanced force.
  • If the object was sitting still, it will remain
    stationary. If it was moving at a constant
    velocity, it will keep moving.
  • It takes force to change the motion of an object.

What is meant by unbalanced force?
If the forces on an object are equal and
opposite, they are said to be balanced, and the
object experiences no change in motion. If they
are not equal and opposite, then the forces are
unbalanced and the motion of the object changes.
Some Examples from Real Life
  • A soccer ball is sitting at rest. It takes an
    unbalanced force of a kick to change its motion.

Two teams are playing tug of war. They are both
exerting equal force on the rope in opposite
directions. This balanced force results in no
change of motion.
Newtons First Law
  • Mass is a measurement of inertia
  • The inertia of an object is proportional to its
    massbigger objects are harder to move and harder
    to stop.
  • Dont confuse size, mass and density
  • Equilibrium
  • Equilibrium exists when the net forc3es on an
    object equal zero.
  • Equilibrium can occur when an object is at rest
    or moving with a constant velocity.

Section 4-3--Newtons 2nd and 3rd Laws
  • Newtons 2nd Law Force Mass x Acceleration
  • SF ma
  • For objects at equilibrium a0
  • Newtons 3rd Law For every action there is an
    equal and opposite reaction.
  • Newtons third law implies that forces always
    exist in pairs.
  • We often split these pairs up into the action
    force and the reaction force, which are always
    have the same magnitude but opposite directions.
  • Action/reaction pairs can cancel each other out
    which results in equilibrium.
  • Action/reaction pairs can also accelerate objects
    (see action/reaction handout)
  • Field Forces also Exist in Pairs

Section 4-4Everyday Forces
  • Weight
  • Weight is defined in physics as the force of
    gravity exerted on a mass. Weight has magnitude
    but no direction making it a scalar quantity.
  • Remember the force of gravity is g 9.81m/s2
  • This value changes inversely to the distance from
    the center of the earth (you weigh less at higher

  • The Normal Force
  • The Normal Force A force exerted by one
    object on another in a direction perpendicular to
    the surface of contact. (The Floor Force)
  • The mathematical definition of normal is
  • The normal force is always perpendicular to the
    surface of contactNOT always opposite the force
    of gravity.
  • Fn m g cosT

  • The Force of Friction
  • Whenever two objects come into contact some
    energy is lost to friction.
  • Friction is defined as the resistive force that
    opposes the relative motion of two contacting
  • Two Types of Friction
  • Static Friction is the friction experienced by
    two or more objects that are in contact and at
  • Kinetic Friction is the friction experienced by
    two or more objects that are in contact and are
    moving relative to each other.
  • Kinetic friction is less than static friction
  • Friction must always be considered in order to
    exactly calculate net forces.

Force of Friction
  • The force of friction is proportional to the
    normal force (i.e. more massive objects
    experience more friction and a larger normal
  • Friction depends on the surfaces in contact (i.e.
    smooth surfaces experience less friction than
    rough surfaces.)
  • The Coefficient of Friction (the Greek letter mu
  • The coefficient of static friction (µs) is the
    ratio of the maximum value of the force of static
    friction to the normal force.
  • The coefficient of kinetic friction (µk) is the
    ratio of the force of kinetic friction to the
  • µ Friction force
  • Normal force
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