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Chapter 4

Chapter 4. The Laws of Motion. Sir Isaac Newton. 1642 – 1727 Formulated basic concepts and laws of mechanics Universal Gravitation Calculus Light and optics. Classical Mechanics. Describes the relationship between the motion of objects in our everyday world and the forces acting on them

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Chapter 4

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  1. Chapter 4 The Laws of Motion

  2. Sir Isaac Newton • 1642 – 1727 • Formulated basic concepts and laws of mechanics • Universal Gravitation • Calculus • Light and optics

  3. Classical Mechanics • Describes the relationship between the motion of objects in our everyday world and the forces acting on them • Conditions when Classical Mechanics does not apply • very tiny objects (< atomic sizes) • objects moving near the speed of light

  4. Slide 4-3

  5. What Causes Motion? In the absence of any forces acting on it, an object will continue moving forever. Motion needs no “cause.” Slide 4-15

  6. Slide 4-4

  7. What Is a Force? A force... ... is a push or a pull. ... acts on an object. ... requires an agent. ... is a contact force or a long-range force. ... is a vector. Slide 4-17

  8. Fundamental Forces • Types • Strong nuclear force – attractive force that binds quarks to form protons & neutrons (strongest force) • Electromagnetic force – binding atoms and molecules to each other • Weak nuclear force – binding force between protons and neutrons • Gravity- attractive force between masses (weakest of all types of forces) • Characteristics • All field forces • Listed in order of decreasing strength • Only gravity and electromagnetic in mechanics

  9. Units of Force • SI unit of force is a Newton (N) • US Customary unit of force is a pound (lb) • 1 N = 0.225 lb • See table 4.1, Page 85

  10. Forces • Usually think of a force as a push or pull • Vector quantity • May be a contact force or a field force • Contact forces result from physical contact between two objects • Field forces act between disconnected objects • Also called “action at a distance”

  11. Contact and Field Forces • Elastic force of a spring • b) Component pulling force • c) Component pushing force • Gravitational force of two masses • b) Electrical force of attraction • c) Magnetic force

  12. External and Internal Forces • External force • Any force that results from the interaction between the object and its environment • Internal forces • Forces that originate within the object itself (molecular cohesiveness, frictions) • They cannot change the object’s velocity

  13. Force Vectors Slide 4-18

  14. A Short Catalog of Forces: Weight w Slide 4-19

  15. Spring Force Fsp Slide 4-20

  16. Tension ForceT Slide 4-21

  17. Normal Force n Slide 4-22

  18.  Friction fk and fs Slide 4-23

  19.  Drag D and Thrust Fthrust Slide 4-24

  20. Identifying Forces Slide 4-25

  21. Example Problem A block is dragged uphill by a rope. Identify all forces acting on the block. Slide 4-26

  22. Example Problem Block A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A. Slide 4-27

  23. Example Problem A ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release. Slide 4-28

  24. Reading Quiz • A “net force” is • the sum of the magnitudes of all the forces acting on an object. • the difference between two forces that are acting on an object. • the vector sum of all the forces acting on an object. • the force with the largest magnitude acting on an object. Slide 4-7

  25. Answer • A “net force” is • the sum of the magnitudes of all the forces acting on an object. • the difference between two forces that are acting on an object. • the vector sum of all the forces acting on an object. • the force with the largest magnitude acting on an object. Slide 4-8

  26. Reading Quiz • Which of the following is NOT one of the steps used to identify the forces acting on an object? • Name and label each force the object exerts on the environment. • Name and label each contact force acting on the object. • Draw a picture of the situation. • Identify “the system” and “the environment.” • Name and label each long-range force acting on the object. Slide 4-9

  27. Answer • Which of the following is NOT on of the steps used to identify the forces acting on an object? • Name and label each force the object exerts on the environment. • Name and label each contact force acting on the object. • Draw a picture of the situation. • Identify “the system” and “the environment.” • Name and label each long-range force acting on the object. Slide 4-10

  28. Reading Quiz • Which of these is not a force discussed in this chapter? • The tension force. • The normal force. • The orthogonal force. • The thrust force. Slide 4-11

  29. Answer • Which of these is not a force discussed in this chapter? • The tension force. • The normal force. • The orthogonal force. • The thrust force. Slide 4-12

  30. Reading Quiz • An action/reaction pair of forces • point in the same direction. • act on the same object. • are always long-range forces. • act on two different objects. Slide 4-13

  31. Answer • An action/reaction pair of forces • point in the same direction. • act on the same object. • are always long-range forces. • act on two different objects. Slide 4-14

  32. Newton’s First Law • An object moves with a velocity that is constant in magnitude and direction, unless acted on by a nonzero net force • The net force is defined as the vector sum of all the external forces exerted on the object

  33. Mass • A measure of the resistance of an object to changes in its motion due to a force • As a quantity of mass increases, it becomes more resistant to changes in its motion and will require greater force to overcome the inertia • Scalar quantity – direction does not matter • SI units are kg

  34. Inertia • Is the tendency of an object to continue in its original motion • Includes resting inertia and rotational inertia of a mass

  35. Summary Slide 4-39

  36. Slide 4-5

  37. Newton’s Second Law Slide 4-29

  38. Gravitational Force • Mutual force of attraction between any two objects • Expressed by Newton’s Law of Universal Gravitation: Gis the universal gravitational constant and has a value of 6.67 x 10-11 m3/kg-s-2

  39. Weight • The magnitude of the gravitational force acting on an object of mass m near the Earth’s surface is called the weight w of the object • w = mg is a special case of Newton’s Second Law • g is the acceleration due to gravity and has a value of 9.80 m/s2 • g can also be found from the Law of Universal Gravitation

  40. More about weight • Weight is not an inherent property of an object • mass is an inherent property • Weight depends upon location

  41. Example Problem • An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. • Is T greater than, equal to, or less than w? Or is there not enough information to tell? Slide 4-30

  42. Checking Understanding • An object, when pushed with a net force F, has an acceleration of 2 m/s2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be • ½ m/s2. • 1 m/s2. • 2 m/s2. • 4 m/s2. Slide 4-33

  43. Answer • An object, when pushed with a net force F, has an acceleration of 2 m/s2. Now twice the force is applied to an object that has four times the mass. Its acceleration will be • ½ m/s2. • 1 m/s2. • 2 m/s2. • 4 m/s2. Slide 4-34

  44. Checking Understanding • A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater? • The train. • The skier. • The net force is the same on both. • There’s not enough information to tell. Slide 4-35

  45. Answer • A 40-car train travels along a straight track at 40 mph. A skier speeds up as she skis downhill. On which is the net force greater? • The train. • The skier. • The net force is the same on both. • There’s not enough information to tell. Slide 4-36

  46. Checking Understanding • 10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is • greater than the force of Sarah on Jack. • equal to than the force of Sarah on Jack. • less than the force of Sarah on Jack. Slide 4-37

  47. Answer • 10-year-old Sarah stands on a skateboard. Her older brother Jack starts pushing her backward and she starts speeding up. The force of Jack on Sarah is • greater than the force of Sarah on Jack. • equal to than the force of Sarah on Jack. • less than the force of Sarah on Jack. Slide 4-38

  48. Summary Slide 4-40

  49. Applications of Newton’s Laws • Assumptions • Objects behave as particles • can ignore rotational motion (for now) • Masses of strings or ropes are negligible • Interested only in the forces acting on the object • can neglect reaction forces

  50. Solving Newton’s Second Law Problems • Read the problem at least once • Draw a picture of the system • Identify the object of primary interest • Indicate forces with arrows • Label each force • Use labels that bring to mind the physical quantity involved

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