Chapter 5


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Chapter 5

  1. 1. Jaydeep PatelSchool of Technology,5-1
  2. 2. Friction When a body moves over another body, its motion is opposed by a resisting force along the surfaces of contact.This resisting force is called Friction. Magnitude of this frictional (resisting) force depends mainly upon the materials of the two bodies, upon the quality of the surface roughness and upon the pressure between the two surfaces of contact.5-2
  3. 3. Force of the friction will be zero for 100% smooth or perfectly smooth surfaces. Practically it is impossible to make ideal or 100% smooth surfaces. Best possible smooth surfaces do have minute irregularities on the surfaces and hence force of friction will be negligible but will not be zero5-3
  4. 4. Definition Of Friction Force When a motion occurs or about to occur between the two surfaces in contact, at that time because of interlocking of irregularities these surfaces oppose the motion. Resisting force, generated due to interlocking, opposite to the motion is called The Force of Friction.5-4
  5. 5. If contact surface is smooth. friction force (F) will be less. If contact surface is rough, friction force (F) will be more. In machine parts like piston, bearings, liner etc. attempts are made to reduce friction to increase life of parts and efficiency of machine. Outer surface of vehicle tyres is made rough to increase friction between tyre and road surface for safe driving. High friction (lots of friction) – will slow something down Low friction (not much friction) – will keep things moving5-5
  6. 6. High friction or Low friction? Ski’s on the snow Car tyre Brakes on a bike Water on a slide Pencil and rubber5-6
  7. 7. Limiting Friction Consider a body resting on the surface. If a little force is applied on the body, it will not move, because friction force (F) will resist the motion. With further increase in external force, body will not move, But there is a limit of developing friction force. If external force becomes greater than that friction force body will move. The maximum friction force that can be developed at the contact surface, when body is just on the point of moving is called limiting force of friction.5-7
  8. 8. Types of Friction Friction Static Dynamic Friction Friction Sliding Rolling Friction Friction5-8
  9. 9. Static friction : Friction experienced by a body when it is at rest is called static friction. In case of static friction, if (Applied force < Friction force) then there is no Motion. Dynamic friction : Friction experienced by a body, when it is in motion is called dynamic friction. Dynamic friction is always less than static friction. Sliding friction : Friction experienced by a body when it slides over another body, is called sliding friction. e.g. to push the table on floor. Rolling friction : Friction experienced by a body when it rolls over another body is called rolling friction. e.g. - to move the drum by rolling, Ball bearing used in machines.5-9
  10. 10. striking a match against a rough surface A Flower pot on top of a table Rolling friction slows down the motion of a tire rolling along the pavement5 - 10
  11. 11. Angle of Friction5 - 11
  12. 12. Coefficient Of Friction5 - 12
  13. 13. Angle Of Repose5 - 13
  14. 14. Variation Of Friction (F) With Applied Force (P) If the force P is increased, the friction force F also increases, continuing to oppose P, until its magnitude reaches a certain maximum value Fm. If P is further increased, the friction force cannot balance it any more and the block starts sliding. As soon as the block has been set in motion, the magnitude of F drops from Fm to a lower value Fk. This is because there is less interpenetration between the irregularities of the surfaces in contact when these surfaces move with respect to each other. From then on, the block keeps sliding with increasing velocity while the friction force, denoted by Fk called the kinetic-friction force, remains approximately constant.5 - 14
  15. 15. Laws of static friction Following are the laws of static friction. The friction force always act in a direction, opposite to that in which the body tends to move. The magnitude of friction force is equal to the external force. F=P The ratio of limiting friction (F) and normal reaction (N) is constant. F/N = The friction force does not depends upon the area of contact between the two surfaces. The friction force depends upon the roughness of the surfaces.5 - 15
  16. 16. Laws of dynamic friction Following are the laws of dynamic friction : The friction force always act in a direction, opposite to that in which the body is moving. The ratio of limiting friction (F) and normal reaction (N) is constant & it is known as co-efficient of friction ( ). F/N = =Constant For moderate speeds, the friction force remains constant. But, it decreases slightly with the increase of speed.5 - 16
  17. 17. Ladder Friction The ladder is a device used to climb on the wall. The ladder is made of wood or iron. There are two long pieces in the ladder to which many small cross pieces called rungs are connected at equal distances. If the ladder is uniform throughout its length then the C.G. of the ladder will be at the mid- point of it.5 - 17
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  19. 19. 1. WL ( )Weight of the ladder through C.G. of ladder [G] 2.WM ( )Weight of the man through [M], where man is standing. 3. P ( ) Applied force P through [C] at a height h from the floor. 4. RA ( ) Reaction by the floor on the ladder. 5. FA ( ) Friction force on the ladder at point A. Ladder slips rightward on the floor and hence FA is leftward. 6. RB ( ) Reaction by the wall on the ladder. 7. FB ( ) Friction force on the ladder at point B. Ladder slips downward on the wall and hence FB is upward.5 - 19
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  21. 21. A 5 metre long ladder, weighing 750 N is resting as shown in the fig. Check whether ladder will be stable or not. Suggest options to make it stable, if it is unstable. The vertical wall is smooth and floor has coefficient of friction 0.25.5 - 21
  22. 22. Wedge Friction Wedges are simple machines used to raise large stone blocks and other heavy loads. These loads can be raised by applying to the wedge a force usually considerably smaller than the weight of the load. In addition, because of the friction between the surfaces in contact, a properly shaped wedge will remain in place after being forced under the load. Wedges can thus be used advantageously to make small adjustments in the position of heavy pieces of machinery.5 - 22
  23. 23. Wedges are used as shown to split tree trunks because the normal forces exerted by the wedges on the wood are much larger than the forces required to insert the wedges.5 - 23
  24. 24. WEDGE FRICTION5 - 24
  25. 25. Problems of wedges are the problems of equilibrium of forces. When wedge is inserted frictional forces are generated on the rubbing surfaces of the wedge. Problems of equilibrium of forces acting on the wedge are solved by anyone of the following three method. 1. LamisTheorem 2. By taking (i) ∑H = 0 (ii) ∑V = 0 i.e., by resolution of forces. 3. Graphical Method5 - 25
  26. 26. Thank You5 - 26