Chapter 4 Force and The Laws of Motion

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Chapter 4 Force and The Laws of Motion

• Physics
• Teacher Mrs. R. Williams

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Background

• 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.

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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
  ma).
• 3. For every action there is an equal and
  opposite reaction.

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SECTION 4.1--Force

• Definition Force is the cause of an
  acceleration or the change in an objects
  velocity
• 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

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Newtons

•  
• 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

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Forces

• 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

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Force Diagram

• Force Diagrams
• FORCE IS A VECTOR with both a magnitude and a
  direction
• 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
  diagrams.

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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
  sum.

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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.

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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.
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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.
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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.

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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

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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
  altitudes!)

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THE NORMAL FORCE

• 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
  perpendicular.
• The normal force is always perpendicular to the
  surface of contactNOT always opposite the force
  of gravity.
• Fn m g cosT

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FRICTION

• 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
  surfaces.
• Two Types of Friction
• Static Friction is the friction experienced by
  two or more objects that are in contact and at
  rest.
• 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.

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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
  force)
• 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
  µ)
• 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
  normal
• µ Friction force
• Normal force