Friction

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Friction

  1. 1. SECTION : C CLASS : XI ROLL NUMBER : 21 ADMITION NUMBER : 2002/167 NAME : DIPTANGSU GOSWAMI
  2. 2. FRICTION: INTRODUCTION  “Friction is a Force that always pushes against an object when it touches another object”  “When 2 things are in contact with each other, there will be friction acting between them” Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. The classic rules of sliding friction was discovered by Leonardo da Vinci.
  3. 3. Friction due to the Surface m N=mg mg F Ffriction
  4. 4. m N=mg mg Sliding Friction 2N 2N
  5. 5. m N=mg mg Sliding Friction 3N 3N
  6. 6. m N=mg mg Sliding Friction 4N 4N
  7. 7. m N=mg mg Sliding Friction 5N 4N
  8. 8. m Sliding Friction 2N 2N
  9. 9. m Sliding Friction 3N 3N
  10. 10. m Sliding Friction 4N 4N
  11. 11. m Sliding Friction 5N 4N
  12. 12. m Sliding Friction 5N 4N It is seen that when the force is gradually increased, the friction also increases, this is called static friction. But it does so only to an extent, this is called the limiting friction. When the body is in motion the frictional force acting against it is called the kinetic friction. The kinetic friction is less than the limiting friction. This proves that friction is a self adjusting force till a limit.
  13. 13. Friction opposes Motion Force on person by box Force on floor by box Force on box by floor Force on box by person←Acceleration
  14. 14. •Dry friction resists relative lateral motion of two solid surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and kinetic friction between moving surfaces. •Fluid friction describes the friction between layers within a viscous fluid that are moving relative to each other. •Lubricated friction is a case of fluid friction where a fluid separates two solid surfaces. •Skin friction is a component of drag, the force resisting the motion of a solid body through a fluid. •Internal friction is the force resisting motion between the elements making up a solid material while it undergoes deformation. There are several types of frictions:
  15. 15. When surfaces in contact move relative to each other, the friction between the two surfaces converts kinetic energy into heat. This property can have dramatic consequences, as illustrated by the use of friction created by rubbing pieces of wood together to start a fire. Kinetic energy is converted to heat whenever motion with friction occurs, for example when a viscous fluid is stirred. Another important consequence of many types of friction can be wear, which may lead to performance degradation and/or damage to components. Friction is not itself a fundamental force but arises from fundamental electromagnetic forces between the charged particles constituting the two contacting surfaces. The complexity of these interactions makes the calculation of friction from first principles impossible and necessitates the use of empirical methods for analysis and the development of theory.
  16. 16. Dry friction Dry friction resists relative lateral motion of two solid surfaces in contact. The two regimes of dry friction are 'static friction' ("stiction") between non- moving surfaces, and kinetic friction (sometimes called sliding friction or dynamic friction) between moving surfaces. Coulomb friction, named after Charles-Augustin de Coulomb, is an approximate model used to calculate the force of dry friction. It is governed by the equation: Where, Ff is the force of friction exerted by each surface on the other. It is parallel to the surface, in a direction opposite to the net applied force, μ is the coefficient of friction, which is an empirical property of the contacting materials, Fn is the normal force exerted by each surface on the other, directed perpendicular (normal) to the surface.
  17. 17. The normal force is defined as the net force compressing two parallel surfaces together; and its direction is perpendicular to the surfaces. In the simple case of a mass resting on a horizontal surface, the only component of the normal force is the force due to gravity, where . In this case, the magnitude of the friction force is the product of the mass of the object, the acceleration due to gravity, and the coefficient of friction. However, the coefficient of friction is not a function of mass or volume; it depends only on the material. For instance, a large aluminum block has the same coefficient of friction as a small aluminum block. However, the magnitude of the friction force itself depends on the normal force, and hence on the mass of the block. mg Friction N θ
  18. 18. Coefficient of friction The coefficient of friction (COF), often symbolized by the Greek letter µ, is a dimensionless scalar value which describes the ratio of the force of friction between two bodies and the force pressing them together. The coefficient of friction depends on the materials used; for example, ice on steel has a low coefficient of friction, while rubber on pavement has a high coefficient of friction. Coefficients of friction range from near zero to greater than one. Coefficient of static friction Coefficient of kinetic friction
  19. 19. Coefficient of Friction Material on Material s = static friction k = kinetic friction steel / steel 0.6 0.4 add grease to steel 0.1 0.05 metal / ice 0.022 0.02 brake lining / iron 0.4 0.3 tire / dry pavement 0.9 0.8 tire / wet pavement 0.8 0.7
  20. 20. Free Rolling or Inertial Rolling  Continuum assumption  Rigid Cylindrical Roller  Rigid Horizontal Surface  Velocity remains constant W N Free body diagram v ω FrictionForce
  21. 21. v ω v ω Pure Rolling and Impure Rolling Pure Rolling Impure Rolling This happens when the friction present between the disc and the surface is sufficient to prevent the wheel from sliding. This happens when the friction present between the disc and the surface is not sufficient and therefore the wheel slides.
  22. 22. Static friction Static friction is friction between two or more solid objects that are not moving relative to each other. For example, static friction can prevent an object from sliding down a sloped surface. The coefficient of static friction, typically denoted as μs, is usually higher than the coefficient of kinetic friction. The static friction force must be overcome by an applied force before an object can move. The maximum possible friction force between two surfaces before sliding begins is the product of the coefficient of static friction and the normal force Kinetic friction Kinetic (or dynamic) friction occurs when two objects are moving relative to each other and rub together (like a sled on the ground). The coefficient of kinetic friction is typically denoted as μk, and is usually less than the coefficient of static friction for the same materials.
  23. 23. “Normal” Forces and Frictional Forces Weight of block Decompose Vector Normal Force Friction Force Weight of block Reaction Force From Ramp “Normal” means perpendicular Friction Force = Normal Force  (coefficient of friction) Ffriction = Fnormal Normal Force Friction Force Reaction Force From Ramp
  24. 24. The Static Friction Force In this module, when we use the following equation, we refer only to the maximum value of static friction and simply write: fs = sn When an attempt is made to move an object on a surface, static friction slowly increases to a MAXIMUM value. s sf n n fs P W
  25. 25. 2 N Friction and the Normal Force 4 N fk = knfs = sn 12 N 6 N 8 N 4 N
  26. 26. Friction forces are independent of area. 4 N 4 N If the total mass pulled is constant, the same force (4 N) is required to overcome friction even with twice the area of contact. For this to be true, it is essential that all other variables be rigidly controlled.
  27. 27. Angle of friction For certain applications it is more useful to define static friction in terms of the maximum angle before which one of the items will begin sliding. This is called the angle of friction or friction angle. It is defined as: θ is the angle from vertical and µs is the static coefficient of friction between the objects. This formula can also be used to calculate µs from empirical measurements of the friction angle.
  28. 28. • As P increases, the static-friction force F increases as well until it reaches a maximum value Fm. NF sm  • Further increase in P causes the block to begin to move as F drops to a smaller kinetic-friction force Fk. NF kk  Force vs. Time t F Maximum Static Friction Constant Kinetic Friction
  29. 29. • Maximum static-friction force: NF sm  • Kinetic-friction force: sk kk NF   75.0  • Maximum static-friction force and kinetic-friction force are: - proportional to normal force - dependent on type and condition of contact surfaces - independent of contact area
  30. 30. • Four situations can occur when a rigid body is in contact with a horizontal surface: • No friction, (Px = 0) • No motion, (Px < Fm) • Motion impending, (Px = Fm) • Motion, (Px > Fm)
  31. 31. • It is sometimes convenient to replace normal force N and friction force F by their resultant R: • No friction • Motion impending • No motion ss sm s N N N F      tan tan • Motio n kk kk k N N N F      tan tan
  32. 32. • Consider block of weight W resting on board with variable inclination angle q. • No friction • No motion • Motion impending • Motion
  33. 33. Conclusion ◦ Every force has an equal, opposing force. ◦ Friction opposes motion, requiring continued application of force to maintain constant velocity. ◦ Air resistance produces terminal velocity, alters trajectories of projectiles.
  34. 34. Thank You

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