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PHSX 114, Monday, September 8, 2003

PHSX 114, Monday, September 8, 2003. Reading for this lecture: Chapter 4 (4-1 -- 4-4) Reading for Wednesday's lecture: Chapter 4 (4-5 -- 4-6) Homework for today's lecture: Chapter 4, questions 3, 5; problems 1, 65, 66. Exam results. How to find your exam score off the web.

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PHSX 114, Monday, September 8, 2003

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  1. PHSX 114, Monday, September 8, 2003 • Reading for this lecture: Chapter 4 (4-1 -- 4-4) • Reading for Wednesday's lecture: Chapter 4 (4-5 -- 4-6) • Homework for today's lecture: Chapter 4, questions 3, 5; problems 1, 65, 66

  2. Exam results • How to find your exam score off the web

  3. Dynamics -- the causes of motion • New terms: force, mass

  4. Aristotle's view of motion (accepted for 2000 years) • The natural state of an object is to be at rest. A force is required to make an object move. • Seems like common sense, matches our experiences

  5. Galileo (1564-1642) had a radical new view • Galileo imagines a world without friction • Natural state is constant velocity (motion in a straight line with constant speed) • Air track demo

  6. Newton's first law of motion • Borrows from Galileo • A body in motion (at rest) remains in motion (at rest) with constant velocity unless acted upon by a net external force. • Law of inertia

  7. Net external force • Symbol = ΣF • A vector sum • An object at rest can have forces acting upon it • Notebook on table has two forces acting upon it

  8. What happens when the net external force is not zero? • Answer: The object accelerates

  9. Newton's second law of motion • ΣF=ma • m is the mass • mass is the resistance of the body to being accelerated (inertial mass) • First two laws define force and mass

  10. Your turn • You observe an object at rest. Are there any forces acting upon it? • Answer: We know the net external force must be zero, but there could be forces acting upon it. • You observe an object moving with a constant velocity. Are there any forces acting upon it? • Answer: We know the net external force must be zero, but there could be forces acting upon it.

  11. Your turn • You observe an object accelerating. Are there any forces acting upon it? • Answer: We know the net external force cannot be zero. There must be at least one force acting upon it.

  12. Units of force and mass • SI units: mass in kilograms, force in Newtons • 1 N = 1 kg-m/s2 • In English units, a pound is a unit of force • Know what the English mass unit is? • The slug. 1 pound = 1 slug-ft/s2

  13. Mass is a scalar, force is a vector • Newton's second law is a vector equation • ΣFx=max, ΣFy=may, ΣFz=maz, • Example

  14. Your turn An object has mass 2 kg and two forces act upon it. F1= 3 N in the +x direction, F2= 4 N in the +y direction. Find the magnitude and direction of the acceleration vector. Answer: 2.5 m/s2 , 53.1 degrees from the x direction ax=( F1x+ F2x)/m=(3 + 0)/2 = 1.5 m/s2 , ay=( F1y+ F2y)/m=(0 + 4)/2 = 2 m/s2

  15. An object accelerates if some other object in the environment produces a force on it • Two basic types of force • 1. contact force • 2. action-at-a distance force • All the basic forces are of the second type

  16. Still to come: • Newton's 3rd (and final) law of motion • Discussing specific forces and their properties • Solving problems using Newton's laws

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