Newton

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Newtons Laws

• Three Laws of Motion

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Aristotles Motion

• Natural Motion is up or down
• Down for falling objects
• Up for smoke
• Circular for heavenly bodies since without end
• Violent Motion
• Due to imposed forces such as wind pushing a ship
  or someone pulling a cart
• Natural state of motion is rest
• A force is needed to keep something moving

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Aristotles Basic Error

• Friction not understood as a force

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Galileos Motion

• Force is a push or a pull
• Friction is a force that occurs when objects move
  past each other
• Friction due to tiny irregularities
• Only when friction is present is a force required
  to keep something moving

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Galileos Inclined Planes

• Ball rolling downhill speeds up
• Ball rolling uphill slows down
• He asked about ball on smooth level surface
• Concluded it would roll forever in absence of
  friction

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Inertia

• Resistance to change in state of motion
• Galileo concluded all objects have inertia
• Contradicted Aristotles theory of motion
• No force required to keep Earth in motion around
  sun because no friction

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Newton

• Born 1665
• Built on Galileos ideas
• Proposed three laws of motion at age of 23

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Newtons First Law
Ourtesy www.lakeheadu.ca/alumni/ hockey.gif

• Every object continues in its state of rest, or
  of motion in a straight line at constant speed,
  unless compelled to change that state by forces
  exerted on it.
• Also called Law of Inertia things move according
  to their own inertia
• Things keep on doing what they are doing
• Examples Hockey puck on ice, rolling ball, ball
  in space

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Mass

• Amount of inertia depends on amount of massor
  amount of material (number and kind of atoms)
• Measured in kilograms
• Question Which has more mass, a kilogram of
  lead or a kilogram of feathers?
• Mass vs. Volume volume is how much space
  something occupies

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Experiencing Inertia

• Inertia is resistance to shaking
• Which is easier to shake, a pen or a person?
• Why is it so hard to stop a heavy boat?

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Mass vs. Weight

• Mass is intrinsic property of any object
• Weight measures gravitational force on an object,
  usually due to a planet
• Weight depends on location of object
• Question 1 How does mass of a rock compare when
  on Earth and on moon?
• Question 2 How does its weight compare?

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Review Mass vs. Weight

• What is mass?
• Answer quantity of matter in something or a
  measure of its inertia
• What is weight?
• Answer Force on a body due to gravity

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Weight of 1 Kilogram

• 9.8 Newtons
• About 2.2 pounds
• Compare the weight of 1 kg nails with 1 kg
  styrofoam
• Answer Same

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Weight Examples

• What does a 70 kg person weigh?
• Weight mass x g(acceleration due to gravity)
• W mg 70 kg x 9.80 N/m2 686 N
• An object weighs 9800 n on Earth. What is its
  mass?
• m W/g 9800 / 9.8 m/s2 1000 kg

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Inertia in a Car

• Discuss three examples of inertia in a car

• Car hitting a wall
• Car hit from behind by a truck
• Car going around a corner

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Newtons Second Law

• Law of Acceleration
• The acceleration produced by a net force on an
  object is directly proportional to the magnitude
  of the net force, and is inversely proportional
  to the mass of the body.
• Acceleration net force mass
• F ma
• Acceleration is in direction of net force

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Net Force

• Net Force means sum of all forces acting
• Sum is Vector sum

F2
F1
Resultant force
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Understanding the Second Law
Force

• The cause of acceleration is
• _________ resists acceleration
• The greater the force, the ________ the
• ______________
• The greater the mass, the _________ the
  acceleration.

Mass or inertia
greater
acceleration
less
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Units

• F ma
• Unit of force is the Newton (N)
• 1 N 1 kg m/s2

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F ma is Three Equations

• F and a are vectors
• So F ma equation is really three
• SFx max SFy may SFz maz

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Examples

• What force is required to accelerate a 1000 kg
  car at 2.0 m/s2 ?
• Answer F ma 1000 kg x 2.0 m/s2 2000
  N.
• What is the acceleration of a 145 g baseball
  thrown with a force of 20.0 N?
• a F/m 20.0 N/0.145kg 138 m/s2

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F ma Example m unknown

• An astronaut puts a 500.0 N force on an object of
  unknown mass producing an accelerations of 0.462
  m/s2 . What was the mass?
• M F/a 500.0N/0.462 m/s2 1082 Kg 1.08 x
  103 Kg

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Newtons Third Law

• Forces always come in pairs
• Two forces on different objects
• Whenever one object exerts a force on a second
  object, the second exerts an equal and opposite
  force on the first
• Example hammer hits nail

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Example pushing on wall

• What are the forces when you push on a wall?
• You exert force on wall
• You accelerate in the opposite direction
• Wall must have exerted a force on you in the
  direction you accelerated (by 2nd Law)

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Example person walking

• Foot exerts force backward on ground
• Ground exerts force forward on foot

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Example Throwing ball

• Pitcher exerts force on ball
• Ball exerts equal and opposite force on pitcher
• Why doesnt pitcher move?

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Example Rocket

• Rocket engine exerts rearward force on gas
  molecules
• Molecules exert forward force on rocket.

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Horse and Cart

• Horse exerts force on cart
• Cart exerts equal and opposite force on horse
• Net force zero
• Cart cant move
• Huh?

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Book on Table

• The mass of the book is one kg. What is the
  force (magnitude and direction) on the book?
• 9.8 N upward

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More Examples

• Can you think of some more examples of Newtons
  Third Law in Action?

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Review Newtons Laws of Motion

• Newtons First Law
• Every object continues in its state of
  rest, or of motion in a straight line at constant
  speed, unless compelled to change that state by
  forces exerted on it.
• Newtons Second Law
• The acceleration produced by a net force
  on an object is directly proportional to the
  magnitude of the net force, and is inversely
  proportional to the mass of the body.
• Newtons Third Law
• Whenever one object exerts a force on a
  second object, the second exerts an equal
  opposite force on the first