Friction
# Friction :
If the switch off the engine of car it will stop after moving a certain distance. It means that some retarding force are acting on the car which stops it . the force opposing the motion of the car (body) is called "force of friction"
- The frictional force is tangential to the surface in contact and always in a direction Opposite to the direction of motion of the object .
- Frictional force is a force opposing the relative motion between two surfaces which are in contact with each other.
- The force of friction arises due to the interlocking of irregular projections on the two surfaces and it is called as "force of contact"
# Types of Friction :
(A) Static force (Up to limited friction) :
The Frictional force that exists between the body and the surface so long as they are relatively at rest even when the external force acts , is called the static force .
- When there is no external force acting on the body, the frictional force is zero.
- If we increase the magnitude of the external force, the static friction is also increases.
- At one point, when there is a slight increase in the magnitude of the external force , The body just starts sliding over the surface.
- The force of friction at limiting stage it's called the 'limiting friction.'
- The limiting fiction depends upon the nature of the two surfaces in contact.
(B) Kinetic or Sliding friction :
- When the external force exceeds the limiting friction , the body just starts sliding.
- The minimum force required to maintain the motion of the body over the other surface is called as "kinetic friction".
- the frictional force that exists between the two surfaces when there is a uniform relative motion between them is called the "sliding friction".
- Kinetic friction is a smaller than the limiting friction.
C) Rolling Friction :
- The frictional force that exist between the two surfaces when a body rolls over the other is called the "rolling friction".
- It deforms the surface over which it rolls.
- rolling body it-self get deformed at the point of contact over the surface.
# Vibration of friction force with applied force :
- The part OA of the graph shows that the static friction increases with the increase in applied force.
- The force of friction is maximum at a point A which is equal to AP.
- If the applied force is increased further, the body will continue to be in motion but the force of friction is slightly lesser.
- After this the force of friction remains constant.
# Factors affecting friction :
The force of friction depends upon the following factors :
- Materials of the bodies in contact.
- Roughness of the surface in contact.
- Force of friction is independent of the area of the two surfaces.
- The limiting frictional force is directly proportional to the normal reaction.
# Reducing friction :
Frictional force can be reduced in the following way :
- Use of lubricants - In machines , friction can be reduced by applying lubricants between the contact surfaces to fill the fine pores or depression in the surface and make them smooth thereby reducing friction.
- Polishing - unevenness of the surfaces can be reduced by polishing, thereby reducing the friction.
- By streamlining - Air friction is reduce by designing streamlined bodies of cars or airplanes. Similarly, if the bodies of boats and ships are streamlined, friction of water can be reduced.
.