Presentation on theme: "Wed. Feb. 9th1 PHSX213 class Class stuff –Questionnaire (hand in at end of class) –Homework comments –Midterm Exam –Problem Solving Session tonight ? More."— Presentation transcript:
Wed. Feb. 9th1 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
Wed. Feb. 9th2 Questionnaire (Overall Goals) 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. Name: (optional) PTO
Wed. Feb. 9th3 Questionnaire (Specifics) 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 ? Your opinions / suggestions please
Wed. Feb. 9th5 Midterm Exam ( r, t ) = ( Budig 120, Wed. Feb. 16 th 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.
Wed. Feb. 9th6 Problem solving session tonight ? Malott 1089. 6:00 PM - > 7.15 PM. If I’m not there check my office.
Wed. Feb. 9th7 Reading Quiz Which is the only one of the following that is a force ? A) inertia B) weight C) mass D) pressure E) torque
Wed. Feb. 9th8 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
Wed. Feb. 9th9 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
Wed. Feb. 9th10 A worked example Problem 5.42 What is m 2 ? Ah hah, a “rope problem”. Maybe Newton III is useful here …
Wed. Feb. 9th11 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
Wed. Feb. 9th12 In the figure is the tension in the string greater than, less than, or equal to the weight of block B? A.Greater than B.Less than C.Equal to Check-Point 2
Wed. Feb. 9th13 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, s, k –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)
Wed. Feb. 9th14 Frictional Forces Depend on the magnitude of the normal force, F N Static friction : F s fr ≤ s F N = F s fr max Kinetic friction : F k fr = k F N N.B. These are NOT vector equations (F fr and F N are in different directions !). The coefficients are dimensionless numbers.
Wed. Feb. 9th15 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, s > k
Wed. Feb. 9th16 Static friction on incline Show that s = tan
Wed. Feb. 9th17 Another worked example Problem 6.23 M M 2M M When the three blocks are released, they accelerate at 0.5 m/s 2. What is the coefficient of kinetic friction between block 2 and the table ?
Wed. Feb. 9th18 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
Wed. Feb. 9th19 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