SECTION 4'2 USING NEWTONS LAWS

1
SECTION 4.2 USING NEWTONS LAWS

•  Objectives
• Describe how the weight and the mass of an object
  are related.
• Differentiate between actual weight and apparent
  weight.

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USING NEWTONS SECOND LAW

• Weight force of gravity on an object. It is
  measured in Newtons like all other forces. It is
  a Vector quantity. It uses the symbol W or Fw
  or Fg and you find the weight using the equation
• W mg or Fw mg or Fg mg
  (new book uses Fg)
•  
• The weight of any object is proportional to its
  mass.
•  
• Weight would be a negative quantity but often
  times when the direction is given it is given as
  down instead of as a negative value.
• So we can say something has a weight with a
  negative value or with a value followed by the
  word DOWN.
•  
• Because the value of g is much less on the moon
  than it is on Earth, you would Weigh less on the
  moon than on Earth, but your mass would be the
  same.

3
SCALES

• A spring scale measures Weight and not mass.
•  
• Do not forget that mass is in kg.
•  
• Do Example 1 p. 97
• FNet ma
• 1 .3a
• 3.33 m/s2 a in direction of Anudja
• Do Practice Problems p. 97 15-18

4
APPARENT WEIGHT

• What would a scale read if you are not in
  equilibrium?
• If an elevator accelerates upward then the upward
  force on the scale is greater than the downward
  force of your weight and thus the scale reading
  is greater than your weight.
• If an elevator accelerates downward then you feel
  lighter and thus the scale has a lower reading
  than your weight.
• If someone pushes down on you then you add your
  weight with the force of them pushing down on you
  to get the Net Force.
• If someone pulls up on you then you subtract the
  force with which they pull on you from your
  weight to get the Net Force.
• Note Need to know these conceptually not to list
  on the test.

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APPARENT WEIGHT

• Apparent Weight the force experienced by an
  object resulting from all the forces acting on
  it, giving the object an acceleration.
•  
• Weightlessness an objects apparent weight of
  zero that results when there are no contact
  forces pushing up on the object.
• Do Example 2 p. 99
• At Rest While Accelerating
• Fg mg FNet FScale - Fg
• Fg 75(9.8) FScale FNet Fg
• Fg 735 N FScale ma mg
• FScale 75(2) 75(9.8)
• FScale 150 735
• FScale 885 N
•  
• So scale reads greater value when accelerating up
  than when at rest.
• Do Practice Problems p. 100 19-20

6
DRAG FORCE AND TERMINAL VELOCITY

• Air exerts a huge force, but in most cases, it
  exerts a balanced force on all sides, and
  therefore has no net effect.
•  
• Drag Force the force exerted by a fluid on the
  object moving through the fluid. It is dependent
  on the motion of the object, the properties of an
  object, and the properties of the fluid that the
  object is moving through.
• As velocity increases so does the drag force.
• Terminal Velocity is the constant velocity that
  is reached when the drag force equals the force
  of gravity (Weight). When Terminal Velocity is
  reached the object no longer accelerates to the
  ground.

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DRAG FORCE AND TERMINAL VELOCITY

• Read p. 101 with the Terminal Velocity examples.
•  
• If mass of a person on Earth is 20 kg, what will
  be his mass on moon? (Gravity on Moon is six
  times less than the gravity on Earth
• 20 kg mass does not change, weight does
•  
• Your mass is 100 kg, and you are standing on a
  bathroom scale in an elevator. What is the scale
  reading when the elevator is falling freely?
• ZERO Newtons (Weightlessness)
•  
• When in an elevator on a scale when will your
  apparent weight be greater than your real weight?
• When it accelerates up.
• Do 4.2 Section Review p. 101 21-27