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PHSX213 class

PHSX213 class. Class stuff Questionnaire (hand in at end of class) Homework comments Midterm Exam Problem Solving Session tonight ? More Newton III Friction Drag force Circular motion. Questionnaire (Overall Goals). Name: (optional).

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PHSX213 class

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  1. PHSX213 class • Class stuff • Questionnaire (hand in at end of class) • Homework comments • Midterm Exam • Problem Solving Session tonight ? • More Newton III • Friction • Drag force • Circular motion

  2. Questionnaire (Overall Goals) Name: (optional) • 1. What do you expect to learn from this course? • 2. What do you hope to do with this new knowledge? • 3. What do you expect the class time to do for you? • 4. What do you expect the book to do for you? • 5. How many hours per week do you think it will take to learn all you need to know from this course? Include class-time, lab, homework etc. PTO

  3. Questionnaire (Specifics) Your opinions / suggestions please • 1. What do you think of the overall difficulty/pace? • 2. What do you think of the personal response system (clickers)? • 3. What do you think of the eGradePlus homework assignments? • 4. If you were teaching this class, what would you do differently ? • 5. Any other comments ?

  4. HW Comments

  5. Midterm Exam • ( r, t ) = ( Budig 120, Wed. Feb. 16th 8:00 – 9:30 PM ) • We will have some small corner of this (huge) room. • Chapters 1-6. • If you cannot make this exam time because of some university authorized activity – you need to let me know asap, and preferably document it with an E-mail. • The majority of the exam will consist of multiple choice questions (5 options) similar to the ones from the practice exam I’ll distribute on Friday. • Questions will be a mix of conceptual and short calculations. • Will contain a brief formula sheet (see practice exam). • Only calculator and pencil allowed in exam. • The remainder will probably consist of 1 or 2 questions a bit like the homeworks which should be answered more in the style of my written homework solutions.

  6. Problem solving session tonight ? • Malott 1089. 6:00 PM - > 7.15 PM. • If I’m not there check my office.

  7. Reading Quiz • Which is the only one of the following that is a force ? • A) inertia • B) weight • C) mass • D) pressure • E) torque

  8. Monday’s Check-Point 2 • A lead block is suspended from your hand by a string. The reaction to the force of gravity on the block is the force exerted by: • A. the string on the block • B. the block on the string • C. the string on the hand • D. the hand on the string • E. the block on the Earth

  9. Check-again • How many forces are involved, if you were to draw free-body diagrams for all the forces acting on the hand, string, block and Earth in the context of that problem ? • A) 5 • B) 7 • C) 9 • D) 11 (by my counting) • E) 13

  10. A worked example Ah hah, a “rope problem”. Maybe Newton III is useful here … Problem 5.42 What is m2 ?

  11. Problem notes • Draw a “free-body diagram”, considering ONLY the forces acting on one single body. • i.e. only the forces that may cause this body to accelerate. • Sometimes, it may be advantageous to draw a free-body diagram for a system of objects

  12. Check-Point 2 In the figure is the tension in the string greater than, less than, or equal to the weight of block B? • Greater than • Less than • Equal to

  13. Frictional Forces • For now we’re mostly concerned with a macroscopic description of friction. • Discuss how things move • Recognize : • Different surfaces slide differently on other surfaces • Characterize with coefficients of friction, ms, mk • Frictional forces are involved in opposing objects starting to move (static (ie. stationary) friction) • Frictional forces are also involved in opposing the motion of moving objects (kinetic (ie. moving) friction)

  14. Frictional Forces • Depend on the magnitude of the normal force, FN • Static friction : Fsfr≤ ms FN = Fsfrmax • Kinetic friction : Fkfr = mk FN • N.B. These are NOT vector equations (Ffr and FN are in different directions !). The coefficients are dimensionless numbers.

  15. Friction : microscopic explanation • Bonds or “cold-welds” form at points of contact between the surfaces • Force is required to break the welds to • Get the body moving • Keep the body moving as welds continue to form This is also qualitatively explains why usually, ms > mk

  16. Static friction on incline Show that ms = tanq

  17. Another worked example Problem 6.23 2M When the three blocks are released, they accelerate at 0.5 m/s2. What is the coefficient of kinetic friction between block 2 and the table ? M 2M M M

  18. Check-Point 3 • Why do raindrops fall with constant speed during the later stages of their descent? • A. The gravitational force is the same for all drops • B. Air resistance just balances the force of gravity • C. The drops all fall from the same height • D. The force of gravity is negligible for objects as small as raindrops • E. Gravity cannot increase the speed of a falling object to more than 9.8m/s

  19. Check-Point 3 • Why do raindrops fall with constant speed during the later stages of their descent? • A. The gravitational force is the same for all drops • B. Air resistance just balances the force of gravity • C. The drops all fall from the same height • D. The force of gravity is negligible for objects as small as raindrops • E. Gravity cannot increase the speed of a falling object to more than 9.8m/s

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