Newtons First Law of Motion Inertia

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Chapter 4

• Newtons First Law of Motion - Inertia

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Aristotle

• Born in 384 B.C.
• Attended Platos academy
• Wrote about Logic, Physics, Philosophy, Rhetoric,
  and Psychology
• During the time of Aristotle, the ideas of
  science were still being developed.
• Aristotle was one of the primary scientists, and
  he was mainly concerned with how and why objects
  move the sun, stars, a ball rolling down an
  inclined plane, an apple falling from the tree,
  etc.
• He came up with two categories for motion
  imposed motion, which he called violent motion,
  and natural motion.

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Aristotle

• An example of natural motion would be an object
  falling freely to the ground.
• An example of imposed motion is someone hammering
  a nail into the wall there is some external
  force or cause for the motion.
• Aristotle thought that objects in their natural
  resting places could not move by themselves, they
  had to be pushed or pulled.

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• For 1600 years, people thought that there was no
  force big and strong enough to move earth, so
  they assumed that the earth just did not move.
• Copernicus was an astronomer that formulated the
  theory that the earth and other planets moved
  around the sun.
• Galileo was a supporter of Copernicus. He
  believed that a force was not necessary to keep
  an object moving. A force is any push or pull.
  He argued that only when friction is present, is
  a force needed to keep an object moving. Without
  friction, an object in motion tends to stay in
  motion unless acted on by an outside force.

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Inclined Planes

• A ball rolling down an incline rolls up the
  opposite incline and reaches its initial height.
• As the angle of the incline is made smaller (so
  that the incline becomes more horizontal), the
  ball will still reach its initial height.
• Galileo stated that every object resists change
  to its state of motion. This resistance is
  called inertia.
• An objects resistance to change in its motion is
  called inertia.

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Newtons First Law(Law of Inertia)

• 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 acted upon it.
• Things tend to keep doing what they are doing
  unless a force that is big enough causes them to
  change their motion.

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Journal 1/9

• Explain how inertia, an objects resistance to
  change in its state of motion, affected the
  hardboiled egg and the raw egg. Which egg
  (hardboiled or raw) had a greater inertia, or a
  greater resistance to change? (A greater
  resistance to change would mean that it will take
  a larger force to stop its motion). Write at
  least 4 sentences.

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Do Now 1/12

• 1.) What is inertia? Give an example of an
  object that has a large inertia. Give an example
  of an object that has a small inertia.
• 2.) What is Newtons First Law of Motion?
• 3.) If the force of gravity between the sun and
  planets suddenly disappeared, what type of path
  would the planets follow?

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Mass

• The quantity of matter in an object
• Measured in kilograms (kg)
• The more mass an object has, the greater its
  inertia and the more force it takes to change its
  state of motion
• Mass is the measure of the inertia of an object
  greater mass greater inertia

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Weight

• Mass is different from weight
• Weight is the force of gravity on an object. It
  is an objects gravitational pull to the earth.
• Weight and mass are not the same, but they are
  proportional.
• Twice the mass weighs twice as much

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Weight (in Newtons)

• Since weight is the force of gravity on an
  object, we can measure weight in Newtons.
• 1 kg 9.8N
• If you know the mass of an object, measured in
  kilograms, and you want to know its weight in
  Newtons, just multiply the mass by 9.8.
• If 1 kg 9.8N then 10 kg ___ N
• Weight mass acceleration due to gravity (g)
• Weight (N) mass (kg) x g (N/kg) g is always
  9.8 N/kg

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• If a 1 kg bag of nails weighs 9.8 N at the
  earths surface, is it true that 1 kg of orange
  juice weighs 9.8 N?
• Find the weight of the following objects
• 1.) A can of soup with a mass of 2 kg
• 2.) A pillow with a mass of ½ kg
• 3.) A person with a mass of 45 kg
• 4.) A car with a mass of 230 kg

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Do Now 1/13

• 1.) A book has a mass of 10 kg. A stone has a
  mass of 8 kg. Which object has a greater
  inertia? Which object weighs more?
• 2.) Find the weight (in N) of the following
  objects
• (a) 2 kg of yogurt
• (b) A bicycle with a mass of 32 kg
• (c) A person with a mass of 49 kg

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Net Force

• The combination of all the forces acting on the
  object
• If one person pushes against an object with a
  force, and another person pushes against the
  object with the same force but in the opposite
  direction, the combination of the forces is zero.
  Therefore, the net force is equal to zero. When
  the net force is equal to zero, the object is in
  a state of equilibrium.
• The net force changes an objects state of motion.

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7 N
7 N
3 N
8 N
8 N
8 N
5 N
1 N
4 N
10 N
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Tension

• Tension is a force that is transmitted through a
  rope, string, or wire.
• When you hang motionless from a rope, your body
  is in equilibrium which means that your net force
  is zero.
• The force of gravity is acting on your body as
  you hang (it pulls your body weight down to the
  ground). But if your net force is zero, there
  must be some other force pulling you upwards to
  cancel out that force of gravity.
• The tension in the ropes is the force that is
  pulling your body up.

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• 1.) 100 g (0.10 kg) ______ N
• Hang 100 g from one spring scale. What does the
  scale read? (in Newtons and Grams)
• 2.) Hang 100 g (0.10 kg ______ N) from two
  scales, held vertically side by side. What does
  each scale read? (in Newtons and Grams)

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• 5.) If 500 g (0.5 kg) hangs from two scales held
  vertically side by side, what will each scale
  read in Grams?
• 6.) If a weight of10 N hangs from two scales held
  vertically side by side, what will each scale
  read in Newtons?

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Do Now 1/14

• 1.) If a 10 kg mass hangs from a scale, what will
  the scale read?
• 2.) If the same 10 kg mass hangs from two scales,
  what will each scale read?
• 3.) A bathroom scale measures your weight. You
  step onto two bathroom scales one foot on one
  scale, and the other foot on the other scale.
  What will each scale read? If you put more
  weight on one foot than the other, what will each
  scale read?

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5 N
5 N
10 N
10 N
10 N
10 N
10 N
20 N
20 N
As the angle between the spring scales increases,
the scale readings increase to maintain the 10 N
upward resultant.
10 N
10 N
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• 1.) 20 N hangs from a scale held vertically.
  What does the scale read?
• 2.) Suppose the same 20 N hangs from 2 scales,
  scale 1 and scale 2, both held at a 60 angle.
  Another 20 N hangs from 2 different scales, scale
  3 and scale 4, held at a 25 angle. In which
  case is the tension greater? Which reading will
  be greater the reading from scale 1 and scale 2
  or the reading from scale 3 and scale 4?