Newton

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Newtons Laws

• Inertia

2
What is a Force?

• The idea the forces caused motion was professed
  by Aristotle in the 4th century B.C.
• Force a push or pull that one body applies to
  another
• A force is a vector, it has magnitude and
  direction

3
What is a Force?

• A force can cause an objects motion to change
• When two or more forces combine at the same time,
  they create a net force

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Net Forces

• Multiple forces create a net force that means
  they act as a single force
• Balanced forces are equal is size but opposite in
  direction
• Balanced forces act as if there is no force

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Example of Balanced Forces
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Example of Unbalanced Forces
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Inertia

• Galileo professed the first modern ideas about
  motion
• The inclined plane experiment

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Inertia

• Inertia an objects resistance to any change in
  motion
• All objects have inertia
• Objects with a greater mass have a greater inertia

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Newtons 1st Law

• Newton built on Galileos ideas by stating An
  object moving with a constant velocity keeps
  moving with that velocity unless a net force acts
  on it an object at rest will stay at rest unless
  a net force acts on it.

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Mass

• Inertia depends on mass
• Mass is the amount of matter an object contains
• Newtons laws are based on mass

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Weight

• Weight is a force that results from gravity
• Weight is not the same as mass
• Weight equals mass times acceleration - mg

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SI and English, a Comparison
SI English
SI English Kilograms Slugs
SI English Kilograms Slugs Newtons Poun
ds
13
Newtons Laws

• Fma

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Forces and Motion are Connected

• An object will have greater acceleration if a
  greater force is applied to it
• Which will accelerate faster, a car with one
  person pushing it or the same car with eight
  people pushing it?

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Forces and Motion are Connected

• The mass of an object also affects acceleration
• Which will accelerate faster, a car with two
  people pushing it or the same two people pushing
  a fully loaded gravel truck?

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Newtons Second Law of Motion

• Connects force mass and acceleration in the
  equation acceleration equals net force divided by
  mass A F/m

F ma
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Quick Review

• 1st Law Things at rest stay at rest, things in
  motion continue to move with the same velocity
  unless acted on by a net force
• 2nd Law F ma

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Summary

• Things will speed up, change direction or stop
  only if acted on by a net force and the rate of
  change of the objects motion
• is governed by the size of the net force and the
  amount of inertia the object has

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Newtons Laws

• Friction

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Question

• Why does my car slow down when I take my foot off
  of the gas?
• Answer Friction

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Friction

• Friction is a force that opposes motion between
  two surfaces that are touching
• Friction changes the energy of motion into heat
  energy rub your hands together and feel the
  heat!

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Types of Friction

• Static the friction between two surfaces that
  are in contact but not moving
• Sliding The friction that opposes the motion of
  two surfaces that are in contact and sliding past
  each other

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Types of Friction

• Rolling friction between an object rolling and
  the surface it is rolling on
• Fluid Friction - the resistance of a gas or
  liquid as an object passes through it

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Calculating Friction

• Static and kinetic friction depend on the
  surfaces in contact and the force between the
  surfaces

FfµFn
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More Impediments to Motion

• Air resistance the opposition to something
  moving through the air
• Air resistance depends on shape, size, and speed.
  A good portion of the gas your car burns is to
  overcome air resistance

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Extreme Air Resistance

• Terminal Velocity an objects weight matches the
  air resistance

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Newtons Laws

• Action
• Reaction

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Newtons Third Law of Motion

• To every action force, there is an equal and
  opposite reaction force
• Action reaction forces act on different objects
  and differ from balanced forces

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Examples of Action Reaction Pairs

• Rocket propulsion is based on Newtons third law
• When I throw a ball, my hand pushes against the
  ball and accelerates the ball. The ball actually
  pushes back just as hard on my hand

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Simple but Tricky

• When I jump up, my feet push against the earth.
  Since the earth is so massive, it has a lot of
  inertia and doesn't move. However, the earth
  pushes back just as hard and throws me into the
  air.