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Physics 1901 (Advanced)

Physics 1901 (Advanced). A/Prof Geraint F. Lewis Rm 560, A29 gfl@physics.usyd.edu.au www.physics.usyd.edu.au/~gfl/Lecture. Physics@Sydney. World renowned research Astronomy & Astrophysics Optics & Photonics Theoretical Physics Plasma & High Energy Physics Brain & Medical Physics

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Physics 1901 (Advanced)

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  1. Physics 1901 (Advanced) A/Prof Geraint F. Lewis Rm 560, A29 gfl@physics.usyd.edu.au www.physics.usyd.edu.au/~gfl/Lecture http://www.physics.usyd.edu.au/~gfl/Lecture

  2. Physics@Sydney • World renowned research • Astronomy & Astrophysics • Optics & Photonics • Theoretical Physics • Plasma & High Energy Physics • Brain & Medical Physics Take advantage of this expertise http://www.physics.usyd.edu.au/~gfl/Lecture

  3. Physics 1901 (Advanced) Three module course consisting of • Mechanics (15 lectures) • Thermal Physics (10 lectures) • Waves & Chaos (13 lectures) It is assumed you have prior physics knowledge. Stream changes made by the HECS deadline. http://www.physics.usyd.edu.au/~gfl/Lecture

  4. Learning What you learn from this course depends upon the effort you put in • Lectures are a guide to course material • University Physics by Young & Freeman • Online resources: WebCT & Junior Physics • 6hrs/week independent study http://www.physics.usyd.edu.au/~gfl/Lecture

  5. Tutorials • Interactive Workshop Tutorials • Work in small groups (upto 4) • Worksheets & Hands-on demonstrations • A chance to ask questions • A place to clarify ideas • Not assessed; up to you. • No worksheets if you don’t attend. http://www.physics.usyd.edu.au/~gfl/Lecture

  6. Labs • Labs are 3 hours • Work in groups of 4 • Read in advance • Get it done faster • Better chance of learning something • Level 4, Carslaw Building • Lab manuals from the CO-OP http://www.physics.usyd.edu.au/~gfl/Lecture

  7. Assessment Lab 20% Mastering Physics 10% Progressive Test 5% Lab Skills Test 5% Exam 60% It is important to know concepts & ideas, not just manipulate formulae. It is important to know the meaning of Academic Honesty http://www.physics.usyd.edu.au/~gfl/Lecture

  8. If you need help • Talk to me or a duty tutor • Consult the web resources • Serious personal problems or illness it is important to complete a Special Consideration Form ASAP! http://www.physics.usyd.edu.au/~gfl/Lecture

  9. Physics 1901: Mechanics http://www.physics.usyd.edu.au/~gfl/Lecture

  10. Physics • is the study of the changeable properties of natural objects • Position, mass, temperature, charge Physics is predictive Know the properties of something now, calculate the properties of something later http://www.physics.usyd.edu.au/~gfl/Lecture

  11. Classical Mechanics (why classical?) • Modern physics • General Relativity • Quantum Mechanics • Classical mechanics • Physics of “human experience” http://www.physics.usyd.edu.au/~gfl/Lecture

  12. Classical Mechanics (what & why?) Simply put, classical mechanics is “how do things respond to forces?” • The concepts of classical mechanics underpin the rest of physics • Have implications in all sciences! • Applied classical mechanics = Engineering? http://www.physics.usyd.edu.au/~gfl/Lecture

  13. Course Layout http://www.physics.usyd.edu.au/~gfl/Lecture

  14. Kinematics (Review Ch 1-3) • Kinematics is the description of motion Let’s start with motion in one dimension xo is the initial position of an object vo is the initial velocity of an object a is the (constant) acceleration of an object What are its properties after a time t ? http://www.physics.usyd.edu.au/~gfl/Lecture

  15. Velocity & Acceleration Velocity is the change of distance over time Acceleration is the change of velocity over time (Differential equations!) http://www.physics.usyd.edu.au/~gfl/Lecture

  16. Kinematic Equations You do not need to memorize such equations as they will be given in an exam. You should be able to derive them from the definitions of velocity and acceleration! http://www.physics.usyd.edu.au/~gfl/Lecture

  17. Non-Constant Acceleration We will consider only constant acceleration. Remember this is not generally true. is called the jerk Can use these to derive more general kinematic equations. http://www.physics.usyd.edu.au/~gfl/Lecture

  18. More than one dimension: Vectors Once we consider motion in more than one dimension, vectors make life simpler. The kinematic equations can be applied in each direction separately. You decide the coordinate system! http://www.physics.usyd.edu.au/~gfl/Lecture

  19. Decomposing Vectors Vectors have a length & direction. To use them we need to decompose the vector into its components. (this is important!) http://www.physics.usyd.edu.au/~gfl/Lecture

  20. Adding Vectors http://www.physics.usyd.edu.au/~gfl/Lecture

  21. Monkey & Hunter http://www.physics.usyd.edu.au/~gfl/Lecture

  22. Galileo & Inertia The Principle of Inertia If a body is left along, it remains where it is or continues along with uniform motion. Why the universe behaves like this is a mystery, but without it science would be quite tricky. http://www.physics.usyd.edu.au/~gfl/Lecture

  23. Isaac Newton Developed concept of Dynamics Considered the motion of a body as it is being influenced by something. Developed three fundamental laws of motion. Amongst the most powerful scientific laws! http://www.physics.usyd.edu.au/~gfl/Lecture

  24. What is the ‘something’? “In order to use Newton’s laws, we have to find some formula for the force; these laws say pay attention to the forces. If an object is accelerating, some agency is at work; find it” Richard Feynman Lectures on Physics http://www.physics.usyd.edu.au/~gfl/Lecture

  25. Universal Forces • Gravity • Electro-magnetic Forces • Strong Force • Weak Force All forces are some form of the above! http://www.physics.usyd.edu.au/~gfl/Lecture

  26. Newton’s First Law “A body acted on by no net force moves with constant velocity (which may be zero) and zero acceleration” This just reiterates Galileo’s ideas of inertia. http://www.physics.usyd.edu.au/~gfl/Lecture

  27. Newton’s Second Law “If a net external force acts on a body, the body accelerates. The direction of the acceleration is the same as the direction of the net force. The net force vector is equal to the mass of the body times its acceleration” http://www.physics.usyd.edu.au/~gfl/Lecture

  28. What is Mass? • The amount of substance in a body • The source of gravity • The ‘coefficient’ of inertia Why these quantities are the same is another mystery of the Universe. http://www.physics.usyd.edu.au/~gfl/Lecture

  29. Newton’s Third Law “If body A exerts a force on body B (an ‘action’), then body B exerts a force on body A (a `reaction’). These two forces have the same magnitude but are opposite in direction. These two forces act on different bodies” (Be careful with the minus sign! This is a vector equation!) http://www.physics.usyd.edu.au/~gfl/Lecture

  30. Newton’s Third Law http://www.physics.usyd.edu.au/~gfl/Lecture

  31. Using Newton’s Laws • With no net force, a body remains at rest or at constant velocity. • With a net force, a body accelerates in the direction of the net force, dependent upon its mass. • To every action, there is an equal and opposite reaction. http://www.physics.usyd.edu.au/~gfl/Lecture

  32. Complications: Weight All masses are attracted to the centre of the Earth. Gravity produces an acceleration of g=9.8m/s2 which means the force is For example: a 51kg gymnast has a weight of 500N (remember your units). http://www.physics.usyd.edu.au/~gfl/Lecture

  33. Complications: Normal Forces Weight acts through the centre of mass, but as I am not accelerating when I stand on the ground, the net force=0! Hence, there is another force balancing weight, supplied by the ground, called the normal force. Are weight & the normal force represent an Action-Reaction pair? http://www.physics.usyd.edu.au/~gfl/Lecture

  34. Complications: Normal Forces • Normal forces are due to the repulsion of atoms • Normal forces are normal to a surface http://www.physics.usyd.edu.au/~gfl/Lecture

  35. Complications: Tension Tension occurs in ropes and strings and depends upon the particular configuration of the forces. For a massless rope, the tension is the same throughout the rope. http://www.physics.usyd.edu.au/~gfl/Lecture

  36. Complications: Tension http://www.physics.usyd.edu.au/~gfl/Lecture

  37. Complications: Tension When considering a rope with mass, its weight must be considered. In the static case Remember, weight is a force so its direction is important!! http://www.physics.usyd.edu.au/~gfl/Lecture

  38. Free-Body Diagrams Split the problem into smaller pieces. Consider the forces on particular parts. Keeping track of action-reaction pairs is vital. http://www.physics.usyd.edu.au/~gfl/Lecture

  39. Free-Body Diagram http://www.physics.usyd.edu.au/~gfl/Lecture

  40. Free-Body Diagram: Example A trolley of mass m1 is place on a slope inclined at 15o. It is attached via a light string and pulley to a hanging sand bucket. What mass of sand m2 is needed such that the trolley possesses uniform motion? (Assume no friction) http://www.physics.usyd.edu.au/~gfl/Lecture

  41. Free-Body Diagram: Example http://www.physics.usyd.edu.au/~gfl/Lecture

  42. Solving Problems: A Guide • Draw a ‘free-body’ diagram • Consider all of the forces acting • Choose axes to ease the solution • ‘Decompose’ the forces • Equations of motion http://www.physics.usyd.edu.au/~gfl/Lecture

  43. Complications: Friction Microscopically, surfaces are not smooth but consist of pits & peaks. When you try and move something these can lock like a jigsaw puzzle and resist movement. What force is actually causing the friction? http://www.physics.usyd.edu.au/~gfl/Lecture

  44. Complications: Friction Metals can have a more complicated friction. As surfaces come into contact, atoms undergo cold welding. Pull these apart adds to the friction. The number of atoms in contact depends upon how hard the surfaces are pressed together. http://www.physics.usyd.edu.au/~gfl/Lecture

  45. Complications: Friction Experimentally the amount of friction is found to be proportional to the component of weight perpendicular to the surface (equivalently the normal force). Static Friction: The frictional force resisting a force attempting to move an object. Kinetic Friction: The frictional force experience by a moving object. http://www.physics.usyd.edu.au/~gfl/Lecture

  46. Static Friction As the object is not moving, there must be no net force. where s is the coefficient of static friction. The frictional force Ff balances the applied force until a point where F=Ff. http://www.physics.usyd.edu.au/~gfl/Lecture

  47. Kinetic Friction Kinetic friction opposes a moving object. where K is the coefficient of kinetic friction. Unlike static friction, kinetic friction has a fixed value independent of the applied force. (Is this really true?) http://www.physics.usyd.edu.au/~gfl/Lecture

  48. Friction http://www.physics.usyd.edu.au/~gfl/Lecture

  49. Coefficients of Friction Generally, s is larger than K (e.g. steel upon steel; s=0.74 and K=0.57) http://www.physics.usyd.edu.au/~gfl/Lecture

  50. Worked Example (5-91) Block A, with a weight of 3w, slides down an inclined plane S of slope angle 36.9o at a constant speed, while plank B with weight w rests on top of A. The plank is attached by a cord to the top of the plane. • Draw a diagram of the forces acting on block A • If the coefficient of kinetic friction is the same between A & B and A & A, determine its value. http://www.physics.usyd.edu.au/~gfl/Lecture

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