The Law of Inertia

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• The Law of Inertia

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Who was Isaac Newton?

• Sir Isaac Newton (1643-1727), an English
  scientist and mathematician famous for his
  discovery of the law of gravity, also discovered
  the three laws of motion. He published them in
  his book Mathematic Principles of Natural
  Philosophy in 1687. Today these laws are known
  as Newtons Laws of Motion and describe the
  motion of all objects we experience in our
  everyday lives.

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Newtons First Law States

• An object at rest will remain at rest, and an
  object in motion will remain in motion unless
  acted upon by an outside force.

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What does this mean?

• Basically, an object will keep doing what its
  doing unless it is acted on by an unbalanced
  force.
• If the object was sitting still, it will remain
  stationary. If it was moving at a constant
  velocity, it will keep moving.
• It takes force to change
• the motion of an object.

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What is Force?

• Force is any push or pull which causes something
  to move or change its speed or direction
• Forces can be balanced or unbalanced

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What is a Balanced Force?

• Balanced forces are equal in size and opposite in
  direction
• They result in no change in velocity

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What is an Unbalanced Force?

• Unbalanced forces are not equal and opposite in
  direction.
• They result in a change in motion of the object.

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Some Examples from Real Life

• A soccer ball is sitting at rest. It takes an
  unbalanced force of a kick to change its motion.
• Two teams are playing tug of war. They are both
  exerting equal force on the rope in opposite
  directions. This balanced force results in no
  change of motion.

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Newtons First Law is also called the Law of
Inertia

• Inertia is the tendency of an object to resist
  changes in its state of motion.
• The First Law states that all objects have
  inertia. The more mass an object has, the more
  inertia it has (and the harder it is to change
  its motion).
• Mass is the measure of the amount of matter in
  an object It is measured in Kilograms.

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More Examples from Real Life

• A powerful locomotive begins to pull a long line
  of boxcars that were sitting at rest. Since the
  boxcars are so massive, they have a great deal of
  inertia and it takes a large force to change
  their motion. Once they are moving, it takes a
  large force to stop them.

On your way to school, a bug flies into your
windshield. Since the bug is so small, it has
very little inertia and exerts a very small force
on your car (so small that you dont even feel
it).
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Newtons First Law

• Dont let this be you. Wear seat belts.
• Because of inertia, objects (including you)
  resist changes in their motion. When the car
  going 60 m/hour is stopped by the brick wall,
  your body keeps moving at 60 m/hour.

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If objects in motion tend to stay in motion, why
dont moving objects keep moving forever?

• Things dont keep moving forever because theres
  almost always an unbalanced force acting upon it.
• A book sliding across a table slows down and
  stops because of the force of friction.
• If you throw a ball upwards it will eventually
  slow down and fall because of the force of
  gravity.

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What is Friction?

• Friction is a force that opposes motion between 2
  surfaces.
• The amount of friction depends on the
• types of surfaces
• force between
• the surfaces

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What is Air Resistance?

• Air can create friction and cause a moving object
  to slow down. This is called air resistance.
• Air resistance is the force of air exerted on a
  falling object. The air pushes up as gravity
  pulls down. It is dependent upon the shape and
  surface area of the object.
• In the absence of air resistance, all objects
  will fall at the same rate due to gravity.

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What is Gravity?

• Gravity is an attraction force between all
  masses. According to Newton, every object in the
  universe exerts a gravitational attraction to all
  other objects in the universe.
• The amount of gravitational force depends upon
  the mass of the objects and the distance between
  the objects.

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What is Gravity?

• The greater the mass.
• the greater the force of gravity.
• the greater the inertia the object is said to
  have.

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• Who experiences more gravity - the astronaut or
  the lumberjack?
• The lumberjack!
• Which exerts more gravity on the astronaut- the
  Earth or the moon?
• The Earth!

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In outer space, away from gravity and any sources
of friction, a rocket ship launched with a
certain speed and direction would keep going in
that same direction and at that same speed
forever.
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Comprehension Check

• Ben and Jerry are arguing in the cafeteria. Ben
  says that if he throws ice cream with a greater
  speed, it will have a greater inertia. Jerry
  argues that inertia doesnt depend on an objects
  speed, but upon its mass. Why do you agree with
  and why?
• Imagine that you are in space, far away from
  Earth. If you were to throw a rock, will the
  rock gradually stop or continue in motion in the
  same direction and speed? Explain why.
• Lori Lostalot is being chased through the woods
  by a grizzly bear. The large mass of the bear is
  intimidating, but Lori believes she can use the
  bears mass to her advantage. Instead of running
  in a straight line, she makes a zigzag pattern
  through the woods. Explain why her plan will
  work and help her to escape from the bear.