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Chapter 4

Chapter 4. Forces and Mass. does not apply for very tiny objects (< atomic sizes) objects moving near the speed of light. Classical Mechanics.

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Chapter 4

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  1. Chapter 4 Forces and Mass

  2. does not apply for very tiny objects (< atomic sizes) objects moving near the speed of light Classical Mechanics

  3. If the net force SF exerted on an object is zero the object continues in its original state of motion. That is, if SF = 0, an object at rest remains at rest and an object moving with some velocity continues with the same velocity. Contrast with Aristotle! Newton’s First Law

  4. Usually a push or pull Vector Either contact or field force Forces

  5. Contact and Field Forces

  6. Types Strong nuclear force Electromagnetic force Weak nuclear force Gravity Fundamental (Field) Forces

  7. QCD (Quantum chromodynamics) confines quarksby exchaning gluons Nuclear force: binds protons and neutronsby exchanging pions Strong Nuclear Force

  8. Electromagnetic Forces • Opposites attract, like-signs repel • Electric forces bind electrons in atoms • Magnetic forces arise from moving charges

  9. Involves exchange of heavy W or Z particle Responsible for decay of neutrons Weak Nuclear Force

  10. Attractive force between any two bodies Proportional to both masses Inversely proportional to square of distance Gravity

  11. Tendency of an object to continue in its original motion Inertia (Newton’s First Law)

  12. A measure of the resistance of an object to changes in its motion due to a force Scalar SI units are kg Mass

  13. Acceleration is proportional to net force and inversely proportional to mass. Newton’s Second Law

  14. SI unit is Newton (N) US Customary unit is pound (lb) 1 N = 0.225 lb Units of Force

  15. Weight is magnitude of gravitational force mass weight Weight

  16. Mass is inherent property Weight depends on location Weight vs. Mass

  17. Single isolated force cannot exist For every action there is an equal and opposite reaction Force on “1” due to “2” Newton’s Third Law

  18. Newton’s Third Law cont. • F12 is action force F21 is reaction force • You can switch action <-> reaction • Action & reaction forces act on different objects

  19. Action-Reaction Pairs

  20. Define the OBJECT (free body) • Newton’s Law uses the forces acting ON object • n and Fg act on object • n’ and Fg’ act on other objects

  21. Objects behave as particles ignore rotational motion (for now) Consider only forces acting ON object neglect reaction forces Assumptions for F=ma

  22. Definition of Equilibrium

  23. Example 4.1a A Ford Pinto is parked in a parking lot There is no net force on the Pinto A) True B) False

  24. Example 4.1b A Ford Pinto is parked in a parking lot The contact force acting on the Pinto from the parking lot surface ______________ . A) Points upwards B) Is zero C) Points downward

  25. Example 4.1c A Ford Pinto drives down a highway on the moon at constant velocity (where there is no air resistance) The Pinto’s acceleration is __________ A) Less than zero B) Equal to zero C) Greater than zero

  26. Example 4.1d A Ford Pinto drives down a highway on the moon at constant velocity (where there is no air resistance) The force acting on the Pinto from the contact with the highway is vertical. A) True B) False

  27. Strings, ropes and Pulleys Gravity Normal forces Friction Springs (later) Mechanical Forces

  28. Force from rope points AWAY from object Magnitude of the force is tension Tension does not change when going over frictionless pulley Some Rules for Ropes and Pulleys

  29. Example 4.2 a) Find acceleration b) Find T, the tension above the bowling ball c) Find T3, the tension in the rope between the pails d) Find force ceiling must exert on pulley a) a = g/6 = 1.635 m/s2b) T= 57.2 Nc) T3=24.5 Nd) Fpulley=2T = 114.5 N

  30. Example 4.3a 2) Which statements are correct?Assume the objects are static. T1 is _____ T2 A) Less than B) Equal to C) Greater than cos(10o)=0.985 sin(10o)=0.173

  31. Example 4.3b 2) Which statements are correct?Assume the objects are static. T2 is ______ T3 A) Less than B) Equal to C) Greater than cos(10o)=0.985 sin(10o)=0.173

  32. Example 4.3c 2) Which statements are correct?Assume the objects are static. T1 is _____ Mg A) Less than B) Equal to C) Greater than cos(10o)=0.985 sin(10o)=0.173

  33. Example 4.3d 2) Which statements are correct?Assume the objects are static. T1+T2 is ______ Mg A) Less than B) Equal to C) Greater than cos(10o)=0.985 sin(10o)=0.173

  34. Example 4.4 Given that Mlight = 25 kg, find all three tensions T3 = 245.3 N, T1 = 147.4 N, T2 = 195.7 N

  35. Cable Pull Demo

  36. Inclined Planes • Choose x along the incline and y perpendicular to incline • Replace force of gravity with its components

  37. Example 4.5 Find the acceleration and the tension a = 4.43 m/s2, T= 53.7 N

  38. M Example 4.6 Find M such that the box slides at constant v M=15.6 kg

  39. RESISTIVE force between object and neighbors or the medium Examples: Sliding a box Air resistance Rolling resistance Forces of Friction

  40. Parallel to surface, opposite toother forces ~ independent of the area of contact Depends on the surfaces in contact Sliding Friction

  41. Coefficients of Friction

  42. Static Friction, ƒs • ms is coefficient of static friction • N is the normal force f F

  43. Kinetic Friction, ƒk • mk is coefficient of kinetic friction • Friction force opposes F • n is the normal force f F

  44. Friction Demo

  45. Example 4.7 The man pushes/pulls with a force of 200 N. Thechild and sled combo has a mass of 30 kg and the coefficient of kinetic friction is 0.15. For each case:What is the frictional force opposing his efforts? What is the acceleration of the child? f=59 N, a=3.80 m/s2 / f=29.1 N, a=4.8 m/s2

  46. Example 4.8 Given m1 = 10 kg and m2 = 5 kg: a) What value of ms would stop the block from sliding? b) If the box is sliding and mk = 0.2, what is the acceleration? c) What is the tension of the rope? a) ms = 0.5 b) a=1.96 m/s2 c) 39.25 N

  47. Example 4.9 What is the minimum ms required to prevent a sled from slipping down a hill of slope 30 degrees? ms = 0.577

  48. Other kinds of friction • Air resistance, F ~ Area  v2 • Rolling resistance, F ~ v Terminal velocity:

  49. Coffee Filter Demo

  50. Example 4.9 An elevator falls with acceleration a = 8.0 m/s2. If a 200-lb person stood on a bathroom scale during the fall, what would the scale read? 36.9 lbs

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