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Chapter 2 Newton’s First Law of Motion

Chapter 2 Newton’s First Law of Motion

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Chapter 2 Newton’s First Law of Motion

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  1. Part I: Mechanics Chapter 2Newton’s First Law of Motion Physics 1 (Garcia) SJSU

  2. Aristotle & Galileo • Aristotle was great philosopher but not such a good scientist. • Aristotle’s theory of motion is wrong. • Took 2000 years before Galileo got motion right. Physics 1 (Garcia) SJSU

  3. Motion according to Aristotle (I) • Every object has a “natural” state. • In “natural motion”, “Earth” elements (stone, apple, you, etc.) are drawn to the Earth. • Heavier objects are more strongly attracted so they fall faster (stone falls faster than a feather). Aristotle Important: These are Aristotle’s ideas, but he’s wrong! Reality Physics 1 (Garcia) SJSU

  4. Motion according to Aristotle (II) • Pushing or pulling an object causes “unnatural” motion (or “violent” motion). • If cause is removed (stop pushing) then object returns to “natural” state and stops moving. Pushed brick slides but then comes to a stop BRICK BRICK Physics 1 (Garcia) SJSU

  5. Natural & Unnatural Motion Aristotle thought objects in unnatural motion would first stop, then fall. He was wrong! “Unnatural” “Natural” Reality Physics 1 (Garcia) SJSU

  6. Galileo & Falling • Pull of gravity (weight) obviously larger for a bowling ball than for a baseball. • YET the two balls fall with the same speed. • What about a feather? (Answer in Chapter 4) Physics 1 (Garcia) SJSU

  7. Galileo’s Inclines (I) Downhill: Speed increases Uphill: Speed decreases Flat surface: Speed increases, decreases, or constant? Questions existence of “natural Earth” state of not moving. Physics 1 (Garcia) SJSU

  8. Galileo’s Inclines (II) Imagine ball rolling down hill then back uphill Again, questions existence of “natural Earth” state of not moving. Physics 1 (Garcia) SJSU

  9. Newton’s First Law of Motion An object at rest remains at rest & an object in motion remains in uniform motion*, unless a force acts on the object. *Moving in a straight line with constant speed. Physics 1 (Garcia) SJSU

  10. Newton’s First Law of Motion Object at rest (not moving) then No force on object then Uniform motion Note: Object at rest is just uniform motion at zero speed. Physics 1 (Garcia) SJSU

  11. Demo: Riding Light Rail When a moving train stops, you continue moving forward. When the stopped train starts moving again, you remain stationary and are thrown backwards Physics 1 (Garcia) SJSU

  12. Demo: Rolling, rolling, rolling WALL All moving together Bottom car stops. Other two continue With same speed. Physics 1 (Garcia) SJSU

  13. Demo: Tablecloth Pull Flower Vase Yank quickly Tablecloth Physics 1 (Garcia) SJSU

  14. Demo: Inertia Balls Upper String Which string breaks in each case? Heavy Cannon Balls Lower String Slow pull Quick yank Physics 1 (Garcia) SJSU

  15. Demo: Paper Pull What happens in each case? Which roll has more inertia? Quick yank Physics 1 (Garcia) SJSU

  16. Net Force When several forces act on an object, the forces add together. Sum of forces called net force or total force 3 Newtons 8 Newtons BRICK 5 Newtons same as The Newton is metric unit of force (about 1/5 pound). Physics 1 (Garcia) SJSU

  17. Check Yourself ? Physics 1 (Garcia) SJSU

  18. Equilibrium Rule If an object is at rest then the net force must be zero. Similarly if in uniform motion. Zero Newtons (No Force) 3 Newtons 3 Newtons BRICK same as When this happens we say that forces “balance.” Physics 1 (Garcia) SJSU

  19. Demo: Up+Up=Down+Down Scale reads 7 Newtons Scale reads 5 Newtons 5+7=10+2 STICK (2 Newtons) 10 Newton Iron mass Stick is at rest. The two upward forces (by people holding the scales) must always be equal to the two downward forces (weight of stick and iron mass). Physics 1 (Garcia) SJSU

  20. Support Force Solid surfaces exert a force, called a support force, on objects pressed against them. Downward force (weight) balanced by upward force (support). 100 Newton Gold Brick 100 Newton Support force How much is the net force on the brick? Physics 1 (Garcia) SJSU