Unit 2 Section 4 Notes

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Unit 2 Section 4 Notes

• Newtons Laws of Motion

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Newtons Laws of Motion

• Newtons First Law An object at rest stays at
  rest and an object in motion stays in motion
  unless acted upon by an outside force.
• In other words, the net force acting on the
  object is zero.
• Example A book sliding on any surface will
  eventually come to a stop due to friction. If it
  werent for friction, the book would continue to
  slide.

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Newtons First Law

• Inertia the tendency of an object at rest to
  remain at rest OR of an object moving at a
  constant velocity to continue moving at that
  speed.

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Inertia
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Example of Newtons First Law

• Famous Tablecloth Experiment
• Try It!
• Why does it work?
• The law of inertia says if we pull the tablecloth
  out really fast, it wont be able to pull the
  dishes for long enough to get them moving. The
  inertia of the dishes wants to keep them standing
  still.

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Newtons Second Law

• Newtons second law states that the unbalanced
  force acting on an object equals mass times
  acceleration
• Mathematically, net force mass x acceleration,
  or, Fnet ma.

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Newtons Second Law

• Newtons Second Law describes the relationship
  between 3 factors
• mass
• force
• acceleration

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Newtons Second Law

• Example Consider 2 shopping carts one empty
  and one full of groceries. If you push both
  carts with the same force, the one without the
  groceries will have a higher acceleration because
  it has less mass. Since the masses are
  different, the accelerations are different.
• If the masses are the same, a larger force gives
  a larger acceleration
• The SI unit for force is Newton,
• Symbolized by N
• 1 Newton 0.225 lbs
• 1 pound 4.448 N

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Newtons Second Law
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Sample Problem Newtons Second Law of Motion

• What is the net force necessary for a 1600 kg
  automobile to accelerate forward at 2.0 m/s2?
• Solution

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Gravity

• Remember that gravity depends on 2 factors
• Mass the larger the mass, the larger the
  gravitational pull direct relationship
• Distance the larger the distance, the smaller
  the gravitational force indirect relationship
• Think about an apple that falls from a tree. In
  terms of gravity, explain why the apple falls
  toward the earth and not toward the tree.
• The gravitational force is much greater between
  the apple and Earth than the apple and tree.

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Free Fall and Weight

• Free fall the motion of an object when gravity
  is the only force acting on it
• Free-fall acceleration directed toward the
  center of the Earth
• Abbreviation for free fall acceleration g
• On or near Earths surface, acceleration due to
  gravity is 9.8 m/s2

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Free Fall

• Astronauts in space appear to be weightless.
  This statement is NOT true because gravity exists
  everywhere in the universe it is the force of
  attraction between 2 objects due to mass.
• Astronauts in orbit experience apparent
  weightlessness because they are in free fall.
  The astronauts and vehicle are falling toward
  Earth with the same acceleration.

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Free Fall and Weight

• In the absence of air resistance, all objects
  falling near Earths surface accelerate at the
  same rate regardless of their mass.
• Why do all objects have the same free-fall
  acceleration?
• Remember that an objects weight (wmg) is equal
  to its force (Fma). So, mg ma, the masses
  cancel, so ga, which is 9.8 m/s2.

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Acceleration due to gravity

• Acceleration due to gravity changes depending on
  where you are
• Earth 9.8 m/s2
• Mars 3.7 m/s2
• Venus 8.8 m/s2
• Neptune 11.8 m/s2
• Example Suppose a person has a mass of 50 kg.
  What would be their weight on Earth? What about
  the moon?

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Terminal Velocity

• 2 factors are in affect when an object moves
  through the atmosphere air resistance and
  gravity.
• Terminal Velocity maximum velocity reached by a
  falling object occurs when air resistance
  force of gravity

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Mass and Weight

• Weight force on an object due to gravity
• Weight equals mass x free fall acceleration
• Equation w mg
• SI unit for weight Newton
• Mass and weight arent the same thing
• Mass measure of the amount of matter in an
  object
• Weight gravitational force an object experiences
  because of its mass

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Weight and shape of objects

• Weight influences the shape of objects
• On land animals have strong skeletons to support
  themselves against gravity
• In water some animals dont have skeletal
  systems because the downward force of gravity is
  balanced by the upward force of water.

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Newtons Third Law

• Newtons Third Law For every action force, there
  is an equal and opposite reaction force.
• This law of Newtons implies that forces always
  occur in pairs, but the forces act on different
  objects and occur at the same time.
• Newtons Third Law is used in rocketry.

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Question 1

• While driving down the road, a firefly strikes
  the windshield of a bus and makes a quite obvious
  mess in front of the face of the driver. This is
  a clear case of Newton's third law of motion. The
  firefly hit the bus and the bus hits the firefly.
  Which of the two forces is greater the force on
  the firefly or the force on the bus?

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Answer 1

• Trick Question! Each force is the same size. For
  every action, there is an equal and opposite
  reaction. The fact that the firefly splatters
  only means that with its smaller mass, it is less
  able to withstand the larger acceleration
  resulting from the interaction.

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Question 2

• Many people are familiar with the fact that a
  rifle recoils when fired. This recoil is the
  result of action-reaction force pairs. A
  gunpowder explosion creates hot gases which
  expand outward allowing the rifle to push forward
  on the bullet. Consistent with Newton's third law
  of motion, the bullet pushes backwards upon the
  rifle. The acceleration of the recoiling rifle is
  ...
• a. greater than the acceleration of the bullet.
• b. smaller than the acceleration of the bullet.
• c. the same size as the acceleration of the
  bullet.

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Answer 2

• B. The force on the rifle equals the force on the
  bullet. Yet, acceleration depends on both force
  and mass. The bullet has a greater acceleration
  due to the fact that it has a smaller mass.
  Remember acceleration and mass are inversely
  proportional.

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Newtons First Law

• Use Newtons First Law to describe the motion of
  the skateboarder and the motion of the person
  driving the car.

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Newtons Second Law

• Mike's car, which weighs 1,000 kg, is out of gas.
  Mike is trying to push the car to a gas station,
  and he makes the car go 0.05 m/s2. Using Newton's
  Second Law, how much force is Mike applying to
  the car?

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Newtons Third Law

• How does a rocket work?
• The rocket's action is to push down on the ground
  with the force of its powerful engines, and the
  reaction is that the ground pushes the rocket
  upwards with an equal force.