Book

1

• Book Piece of Paper Demo

2
Recall Newtons 1st Law

• Newtons First Law of Motion
• Every object continues in a state of rest, or of
  motion in a straight line at constant speed,
  unless it is compelled to change that state by
  forces exerted on it.
• What are some of the natural forces that might be
  exerted on an object?
• Friction
• Air Resistance
• Gravity

3
Aristotle

• Aristotle believed that heavier objects fell
  faster.
• But try this thought experiment
• According to Aristotle, a 20kg object would fall
  faster than a 10 kg object
• What would happen if you connected the two masses
  together with a string?

4
Galileo

• 1st possibility would the masses combine to
  form a 30 kg object? or
• 2nd possibility since the 20 kg object would
  fall faster than the 10 kg would the string
  break?
• Galileo did not buy Aristotles theory because he
  could not resolve this dilemma in his head.
• That is when he designed his experiment to throw
  a cannonball and the stone off of the Leaning
  Tower of Pisa.

5
Galileo

• He found that mass did not matter, but he never
  explained why.
• Recall Newtons Second Law.
• F ma
• Rewrite as
• a

6
Newton Explains Freefall

• a
• Acceleration is dependent on the ratio of the
  Force to the mass (of an object)
• So, Even though the cannonball has a larger
  force, its mass is greater too.
• The stone has a smaller force, but also has a
  smaller mass.
• The ratios are both the same though and therefore
  the accelerations are the same.

7
Free Fall

• Free Fall When the only force acting on an
  object is gravity
• An object in free fall accelerates as it falls.
• Why?
• In free fall, gravity is an unbalanced force and
  unbalanced forces accelerate

8
Free Fall

• Near Earths surface acceleration due to gravity
  is 9.8 m/ s2
• (For every second an object falls, its velocity
  increases by 9.8 m/s)
• All objects in free fall accelerate at the same
  rate regardless of mass

9
Ideal vs. Real World

• There is a difference between ideal conditions
  and the real world.
• What other force would be acting on an object
  being dropped off of the tower?
• Air Resistance
• Objects that fall through the air experience a
  type of fluid friction called air resistance
• Recall friction works in the opposite direction
  of motion

10
Air Resistance

• Air Resistance is not the same for all objects
• 1. The greater the surface area, the greater the
  air resistance
• (Example skydiving)
• 2. Air Resistance increases with velocity
• So as a falling object speeds up, the air
  resistance against it increases

11
Air Resistance

• Eventually, the air resistance equals the force
  of gravity
• Remember when forces are balanced, there is no
  acceleration
• Objects continue to fall, but their velocity does
  not increase

12
Air Resistance

• Air Resistance lowers the net force acting on an
  object.
• The net force decreases very little for heavy
  objects and a lot for lighter objects.
• This can be seen more clearly by comparing the
  book and the piece of paper
• The papers downward acceleration is brief,
  because the air resistance needed to counteract
  its light weight builds up relatively quickly.

13
Air Resistance

• When acceleration stops, Terminal Velocity is
  reached.
• Terminal Velocity the greatest velocity that a
  falling object reaches
• Conceptual Physics Falling Air Resistance
  Chapter 5

14
Air Resistance

• Differentiate between the amount of force air
  resistance creates and the affect it has
• The same amount of air resistance has more of an
  effect on light objects than heavy objects.
• It is the ratio of the force of air resistance to
  the mass that matters.
• NTQ 5-3 Feather Elephant

15
Gravity

• Gravity Force that pulls objects toward
  each other.
• Universal Law of Gravitation
• The force of gravity acts between all objects in
  the Universe

16
Gravity

• Strength of the force depends on
• 1. The masses of the objects involved
• 2. The distance between the objects involved
• F G
• where G 6.67 x 10-11