Lecture 3 Newton

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Lecture 3Newtons Three Laws of Motion
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Origins of Modern Astronomy

• Sir Isaac Newton (1642-1727)
• Formulated the laws of motion and gravitation
  that govern all bodies in the universe.

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Newtons First Law of Motion

• The Law of Inertia
• Every object continues in a state of rest or of
  uniform speed in a straight line unless acted on
  by a nonzero force.
• Inertia The property of objects to resist
  changes in motion.

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The Earth in Motion

• Nicolaus Copernicus (1473-1543)
• Proposed that the Earth revolved around the Sun
  from observations of the motion of planets.
• Because the concept of inertia was unknown at his
  time, the idea of a moving Earth was difficult to
  comprehend.

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If the Earth moves at 30 km/s, how can the bird
drop down and catch the worm?
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Thanks to inertia, you can flip a coin in an
airplane without having it fly into your face at
500 mph.
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Newtons Second Law of Motion

• The acceleration produced by a net force on an
  object
• is directly proportional to the net force,
• is in the same direction as the net force,
• and is inversely proportional to the mass of the
  object.

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Newtons Second Law of Motion
Acceleration net force / mass a F / m
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Newtons Second Law of Motion

• Net Force mass x acceleration

F ma Units 1 N 1 kgm/s2
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Newtons Second Law of Motion
F ma
F
F
F
m
a
a
m
a
m
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You apply the same amount of force on two
separate carts one cart with mass of 1 kg and
another with a mass of 2 kg. Which of the
following is correct?

1. The acceleration of the 2 kg cart will be ½ as
   much as that of the 1 kg cart.
2. The acceleration of the 2 kg cart will be 2 times
   greater than that of the 1 kg cart.
3. The acceleration will be the same for both carts.
4. The acceleration of the 2 kg cart will be ¼ as
   much as that of the 1 kg cart.

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A jumbo jet cruises at a constant velocity of
1000 km/h when the thrusting force of its engines
are a constant 100,000 N. What is the force of
air resistance on the jet?

1. 0 N
2. 100,000 N
3. 1,000 N
4. There is not enough information to answer this
   question.

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How much force, or thrust, must a 20,000-kg jet
plane develop to achieve an acceleration of 2
m/s2?

1. 10,000 N
2. 10,000 m/s2
3. 20,000 N
4. 20,000 m/s2
5. 40,000 N
6. 40,000 m/s2

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Homework Assignment 2

• A few rules to remember
• At rest or constant velocity no change in
  motion
• No change in motion no acceleration No Net
  Force (SF 0)

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A constant g on Earth

• Galileo was the first to measure the acceleration
  of objects in free fall, but could not explain
  why they all fall equally.
• Greater the mass stronger gravitational pull.

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A constant g on Earth
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A constant g on Earth

• g (10m/s2) is independent of an objects mass.

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In a vacuum, a coin and a feather fall equally,
side by side. Would it be correct to say that
equal forces of gravity act on both the coin and
the feather in a vacuum?

1. Yes
2. No

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A constant g on Earth
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Weight

• Calculating Weight using Newtons Second Law
• F ma
• Weight mg
• g acceleration due to gravity on Earth

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Falling Objects and Air Resistance

• On Earth, air-resistance must be considered for
  falling objects.
• As falling speed increases so does the opposing
  force of air-resistance.
• Net force (SF) Weight Air-resistance

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Falling Objects and Air Resistance

• Acceleration of falling object calculated using
  Newtons 2nd Law

a SF / m a (mg - R )/ m R force due to Air
Resistance mg weight
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Terminal Velocity

• Terminal velocity reached when the force of
  air-resistance the falling objects weight.
• No net force (SF 0) no acceleration no
  change in velocity

a SF / m 0 a (mg - R )/ m 0
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Terminal Velocity

• Varies from 150 to 200 km/h for a human skydiver.

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A bowling ball and a feather are dropped from the
same height at the same time. Which reaches
terminal velocity first?

1. Bowling Ball
2. Feather

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A bowling ball and a feather are dropped from the
same height at the same time. Which has the
greater terminal velocity?

1. Bowling Ball
2. Feather

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Terminal Velocity

• Greater force of air resistance (R) needed to
  cancel out the weight (mg) of heavier objects in
  free fall
• Greater R requires a greater velocity which
  requires acceleration for a longer period of time.

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Effect of air-resistance on falling objects
Initially velocity is 0 Air-resistance is 0
Velocity has increased Air-resistance increases
Velocity continues to increase Air-resistance
increases
R 80 N
R 40 N
R 0 N
Weight 100 N
Weight 100 N
Weight 100 N
Net Force 100 N Initial Acceleration is 10m/s2
Net Force 60 N Acceleration is less due to
smaller net force
Net Force 20 N Acceleration has decreased more
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Effect of air-resistance on falling objects
Velocity no longer changes (Terminal
Velocity) Air-resistance is 100 N
R 100 N
No net force no acceleration no change in
velocity!
Weight 100 N
Net Force 0 N Acceleration 0 m/s2
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A bowling ball and a feather are dropped from the
same height at the same time. Which would strike
the ground first if it were on the Moon?

1. Bowling Ball
2. Feather
3. Both at the same time

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Forces and Interactions

• A force is not a thing in itself but makes up an
  interaction between one thing and another.
• Force Pair two forces that are equal in
  magnitude and opposite in direction.
• Constitutes a single interaction.

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Forces and Interactions

• You can only exert as much force on an object as
  it can exert back on you.

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Forces and Interactions
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Newtons Third Law of Motion

• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first.
• Action force and reaction force

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To every action there is always an opposed equal
reaction
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Action and Reaction

• Earth is pulled up by the boulder with just as
  much force as the boulder is pulled down by
  Earth.
• Forces are equal in magnitude but what about the
  acceleration of the two objects?

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A speeding bus and an insect experience a head-on
collision. The force of the bus on the insect
splatters it on the windshield. Is the
corresponding force of the insect on the bus
greater, less, or the same?

1. Greater
2. Less
3. The same

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What about the resulting acceleration that the
bug experiences?

1. Greater than the acceleration of the bus.
2. Less than the acceleration of the bus.
3. The accelerations are the same.

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Equal and opposite forces does not always mean
equal and opposite accelerations.
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Action and Reaction
a 20 m/s2
a 500 m/s2
F 10,000 N
F 10,000 N
Cannonball 20 kg Cannon 500 kg
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Chuck Norris delivers a roundhouse kick with a
force of 8,000 N to an opponent. Assuming that
the laws of physics apply to Chuck Norris, how
much force is exerted back on his foot?

1. Less than 8,000 N
2. More than 8,000 N
3. 8,000 N
4. Im too intimidated by Chuck Norris to answer
   this question.

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Assume that Chuck Norris has a mass of 100 kg and
his opponent has a mass of 80 kg. The force
exerted on each was 8,000 N in the previous
question. What is the acceleration of his
opponent during impact?

1. 8,000 m/s2
2. 20 m/s2
3. 100 m/s2
4. 80 m/s2
5. 640,000 m/s2

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Defining a System

• If action and reaction forces on an object are
  equal and opposite, then how can an object
  accelerate?
• An acceleration of a system is only possible if a
  force external to the system is involved.

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Flight

• Lift an upward reaction force that allows for
  flight.
• When the force of lift exceeds an objects weight
  it will accelerate upward.
• A helicopters whirling blades are shaped to
  force air downward and the air forces the blades
  up.

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A birds wing pushes down on the air and the air
pushes back on the wing.