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Chapter 4

Chapter 4. Newton’s Laws of Motion. Stationary Earth. Earth is heavy Easier to imagine the sky can move It looks like the sky is moving We have no sensation of our motion Rotating Earth would make objects fly off of the surface Stellar Parallax. June. Stellar Parallax.

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Chapter 4

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  1. Chapter 4 Newton’s Laws of Motion

  2. Stationary Earth • Earth is heavy • Easier to imagine the sky can move • It looks like the sky is moving • We have no sensation of our motion • Rotating Earth would make objects fly off of the surface • Stellar Parallax

  3. June Stellar Parallax • Hipparchus 150 BC Sun January

  4. Aristotelian UniverseTerrestrial Realm • Composition predicts motion • natural tendencies Fire and Air tend to rise Earth and Water tend to sink • Overall tendency to seek rest • Objects are corruptible (changing)

  5. 1. ARISTOTLE ON MOTION Aristotle attempted to understand motion by classification. Two Classes: Natural and Violent

  6. Aristotelian UniverseCelestial Realm • Celestial Objects composed of Aether • Self luminous but does not consume • Motion is constant, circular • Objects are incorruptible (not changing) Meteors and comets were phenomena of the Earth’s atmosphere

  7. The falling speed of an object was supposed to be proportional to its weight. • Galileo’s Experiment

  8. Nicholas Copernicus1473 - 1543 (Niklas Koppernigk) Developed a mathematical model for a Sun-centered solar system

  9. Galileo Galilei1564-1642 • Among the first to turn a telescope to the sky • Developed the Scientific Method • Believed in the popularization of science • Developed the Law of Inertia

  10. He tested with planes. • Demo - Ball and incline plane • The change in speed depended on the slope of the incline. • Seemed that the ball was trying to achieve the same vertical height.

  11. Isaac Newton1642 - 1727

  12. Newton’s Laws The 1st Law A body continues to move as it has been moving unless acted upon by an external force.

  13. Newton’s First Law • No mention of chemical composition. • No mention of terrestrial or celestial realms • Force required when object changes motion. • Acceleration is the observable consequence of force acting.

  14. 5 N 10 N 10 N 5 N 5 N 5 N 5 N 5 N Net Force • Force is a vector • We must add all the forces acting on the body 0 N

  15. Equilibrium • When the net force is zero (SF = 0) • Acceleration is zero • Velocity may not be zero

  16. Mass and Inertia • Inertia is a property of a body that resists changes in motion • Mass is a measure of the amount of matter in a body • Inertia and Mass are the same concept • Units – Kg or slugs

  17. Weight • Force of gravity pulling on the mass of the body • Units – Newtons or Pounds • A newton is about the weight of a small apple • One kg weighs 2.2 lbs. • W = mg • g = acceleration due to gravity = 32 ft/s2 = 9.8 m/s2

  18. Inertia Demonstrations • Demo - Table setting • Demo - Bottle, hoop, and chalk

  19. Newton’s Laws The 2nd Law The Sum of the Forces acting on a body is proportional to the acceleration that the body experiences S F  a S F = (mass) a

  20. m m m m m M M M m 2. NEWTON'S 2nd LAW OF MOTION F a F a F a F a F a F a

  21. If the net force is perpendicular to the velocity, the direction of the velocity changes. • Acceleration is always in the direction of the net force.

  22. When Acceleration Is Zero - Equilibrium Static Equilibrium Velocity is zero Examples: Scales pushing up Normal up Weight down Computer setting on a table Weighing yourself on a set of scales Hanging from a tree Tree pulling up Weight down Car parked on an incline Normal Friction Weight down Weight down

  23. Dynamic Equilibrium Velocity is nonzero and constant Examples: Driving at constant velocity Normal up Friction Force from road Weight down

  24. Newton’s Laws The 3rd Law For every action force there is an equal and opposite reaction force (You cannot touch without being touched)

  25. Dissipative Forces Oppose motion or the tendency of motion • Friction • Air Resistance

  26. Friction • Depends upon... • Materials that are in contact • Forces that press surfaces together (Normal forces) • Static friction (no motion) • Proportional to force used until maximum is reached • Kinetic friction (motion) • Nearly constant for normal speeds

  27. Friction Static F f Kinetic f F

  28. Air Resistance • Depends on • Surface Area • Velocity • When weight = air resistance we have SF= 0 Þ a = 0 Weight Air Resistance Terminal Velocity!

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