Forces and Newtons Laws

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Forces and Newtons Laws

• Physics

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What is a Force?

• A Push or a Pull!
• Standard Unit is the NEWTON (N)
• Types
• Contact Forces (macroscopic)
• Normal contact stuff (support forces, pushes,
  pulls,..)
• Friction (always in the opposite direction of
  motion)
• Air Resistance (again always opposite in
  direction)
• Tension (Ropes, Wires, Cables)
• Field Forces
• Electric, Magnetic, Nuclear
• Gravity (exists between any two objects)
• Requires lots of mass to be significant!
• On earth 1 kg is pulled on with a force of 9.8
  Newtons
• Practice .

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Using Gravitational Field to Determine Weight /
Mass

• 12 kg __________ N
• 100 grams ________ N
• 10 N __________ kg
• I weigh 175 pounds.
• How much mass is that in kg (earth)?
• What is that weight in N?

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Overview of Newtons Laws

• First Law What happens to the motion of an
  object if there is no total, net force acting on
  it?
• Second Law What happens to the motion of an
  object if there are forces acting on it?
• Third Law What happens to the motion of several
  objects when they interact?

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Newtons First Law of Inertia

• Every object continues in its state of rest, or
  of uniform motion in a straight line, unless it
  is compelled to change that state by forces
  impressed upon it.
• In other words if there is no NET FORCE acting
  on the object, its motion should remain
  unchanged!
• The amount of Inertia an object has depends on
  its mass
• Exs
• Inertial Balance

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NET FORCE

• The sum of all forces acting on an object is the
  objects NET FORCE.
• When the NET FORCE is zero, the object is said to
  be at equilibrium.
• Sometimes Equilibrium occurs and the object is at
  rest
• Sometimes Equilibrium occurs and the object is
  moving AT CONSTANT VELOCITY!
• Exs Support forces and scales

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Review

• 80 kg ___________ Newtons
• 15 kg ___________ Newtons
• 1000 N __________ Kg
• 50 grams ________ N
• 20 pounds _______ Newtons
• Will a scale reading increase or decrease as you
  accelerate up in an elevator?
• Will a scale reading increase or decrease as you
  move downward at constant velocity in an elevator?

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Diagrams and Vectors

• Force Diagrams
• Should consist of a small picture of the object
• Also consist of scaled arrows that represent
  force acting on the object. Longer the arrow,
  greater the force.
• Each Force Arrow should be labeled as Object B
  on Object A, where Object A is the object your
  concerned with.
• Exs. Practice . Practice
• Vectors Force is a VECTOR therefore we should
  follow the rules for adding vectors.
• If in the same direction they just add up.
• If in the opposite direction they just subtract.
• If at an angle, line the tails up together and
  create a parallelogram. The diagonal of the
  parallelogram (starting from the tails) is the
  Resultant.
• Exs Practice Practice

HONORS
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Check for Understanding

• Examine the diagram below of 3 books of different
  masses atop a scale.
• What is the mass of the middle book?
• With what force does Book C exert on Book B?
• Draw a complete force diagram for each book !!!

Book A
Mass 1 kg
Book B
Mass ?
Mass 2.5 kg
Book C
Scale 65 Newtons
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Jar of Flies

• A bunch of flies are in a capped jar. You place
  the jar on a scale. The scale will register the
  most weight when the flies are

a) Sitting on the bottom of the jar. b) Flying
around inside the jar. c) . Weight of the jar
is the same in both cases.
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Check for Understanding Again..

• Examine the diagram below of 3 planters of
  different masses hanging from each other.
• Plant B exerts 120 Newtons of Force on Plant A.
• What is the mass of Plant B?
• Draw a complete force diagram for each plant !!!

Ceiling
Mass 4 kg
Mass ?
Mass 7 kg
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Kink

• Water is shooting out the end of a pipe. The end
  of the pipe is bent into a figure 6. Ignoring
  the effects of gravity, which of the following is
  true?

a) The water shoots out in a curved arc. b)
The water shoots out in a straight line.
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Friction (Honors)

• Two types, static and kinetic
• Static Friction describes the resistive forces
  between two objects at rest.
• Kinetic Friction describes the resistive forces
  between two objects that slide past each other.
• Kinetic Friction is always less than static.
• Exs Tires and road
• Friction depends only on two things Normal Force
  (support force) and the coefficient of friction
• Coefficient of friction describes how sticky
  the two objects are. These are listed in tables
  somewhere.
• Ffriction µ Fnormal
• Ex

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Newtons Second Law

• If there is a net force on an object it will
  accelerate. (Understand Motion first!)
• The acceleration will be proportional to the net
  force on the object
• But the acceleration will be inversely
  proportional to the mass (inertia) of the object
• Acceleration Net Force / Mass
• Exs

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Free Fall (a 9.8 m/s2)

• Must ignore air resistance and friction
• Velocity gained each second is the SAME.
• After 10 seconds velocity gained ?
• A ball thrown down at 20 m/s will be going how
  fast after 10 seconds?
• Distance gained each second is NOT the SAME!
• D ½ a t2 vo t

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Non Free-Fall (a lt 9.8 m/s2)

• Objects falling through the air will experience a
  drag force (air resistance)
• The amount of Air drag depends on
• The cross-sectional area of the object (size)
• The speed of the object
• The density of the air
• The texture of the surface of the object
  (coefficient of drag, CD)
• Ex
• At one moment a skydiver (50 kg) is falling with
  an acceleration of 5 m/s/s, what is the force of
  air drag?

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Newtons Third Law

• For every action force there exists a reaction
  force that is the same in magnitude but opposite
  in direction.
• Exs
• Action My feet push on the floor with 650 N of
  force
• Reaction The floor pushes on my feet with 650 N
  of force
• Action My cars tires push backwards on the
  pavement with 10,000 N of force.
• Reaction

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Vectors (Honors)

• To resolve multiple vectors at various angles
• Setup a x-y coordinate system
• Break down each vector into x and y components
• X component hyp x Cos ?
• Y component hyp x Sin ?
• Add up the xs and ys and follow Pythagorean to
  get resultant. X2 y2 R2