If the switch off the engine of car it will stop after moving a certain distance. It means that some retarding force is acting on the car which stops it. The force opposing the motion of the car 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.Reducing friction
Frictional force can be reduced in the following ways:Use of lubricants :
In machines, friction can be reduced by applying lubricants between the contact surfaces to fill the fine pores or depressions in the surfaces and make them smooth thereby reducing friction.Polishing :
unevenness of the surfaces can be reduced by polishing, thereby reducing the friction.Use of ball bearings :
In rotating machines, shafts are mounted on ball bearings. By doing so, rolling friction occurs lesser than sliding friction, thereby reducing the friction.By streamlining :
Air friction is reduced by designing streamlined bodies of cars or aeroplanes. Similarly, if the bodies of boats and ships are streamlined, friction of water can be reduced.Disadvantages of friction :
Wear and Tear : In machine parts like gears, brakes when they come in contact with each other continuously, they wear out gradually, which should be replaced time to time.
Friction reduces efficiency of the machine.
Friction in machine produces heat and undesirable noise which damages the machine. To avoid excessive heating, water is circulated in machines generally.