Friction Ppt l34h

FRICTION  Force of Friction  Laws of Static and Dynamic Friction

 Limits of Friction  Friction – a necessity & evil

 INTRODUCTION  When an object (e.g., wooden block) moves over the

surface of another object (e.g., floor) , its motion is always opposed by a retarding force that resists motion.  This retarding force is called friction and always acts in a direction opposite to motion.  For example, an object sliding down an inclined plane experiences a frictional force directed up the inclined plane.  Similarly, an object that moves to the right on a level floor experiences the frictional force to the left.

 Friction can operate between two solids, between a

solid and a fluid or between two layers of a fluid.  However, in this chapter, we shall discuss only the friction between solid surface.

 FRICTION  When a solid object moves over the surface of

another solid object, its motion is always opposed by the retarding force, called friction.  Hence friction is an opposing force that comes into play when one body moves or attempts to move over the surface of another body. The frictional force is always parallel to the surfaces in .  Let us discuss how frictional force behaves.

 Consider a wooden block of mass m placed on a

horizontal surface as shown in Figure 1.

Fig.1

 The block pushes down on the horizontal surface

with a force equal to block’s weight (w = mg).  The ing horizontal surface pushes back with an equal and opposite force R (= N) so that R = mg.

 The force R is perpendicular to the surface and is

called normal reaction ( normal means perpendicular).  Suppose a small horizontal force F is applied on the block to the right but the block does not move. It means that a force has come into play to oppose the motion of the block.  This is the force of friction and is called static friction Fs because the blocks remains at rest or static even though we have applied force.  As we increase the applied force, the static friction Fs also increases till a stage is reached when the block is about to move.

 The value of static friction Fs is now maximum and is

called limiting friction or critical friction Fc .  Once the limiting friction (i.e., maximum static friction ) is overcome, the block begins to move and frictional force decreases. The friction is now called kinetic friction Fk .  Note that the word kinetic means that the block is moving. Thus the behaviour of friction can be summed up as under : 1) Before the body begins to move, the static friction plays the role ; the static friction increases as the applied force increases upto the value of limiting friction (i.e., maximum static friction).

2) Once limiting friction is overcome by the applied

force, the role of static friction is over. Now smaller frictional force Fk (=kinetic force) opposes the motion. k F 3) Both limiting friction ( Fc) and kinetic friction( ) are important. The limiting friction must be overcome to start the body moving and then smaller frictional force Fk to oppose the motion.

 TYPES OF FRICTION  There are two common types of friction viz.

1. Static friction 2. Kinetic friction. 1. Static friction :  The opposing force that comes into play when an object does not move over the surface of another even though force is applied to make it move is called static friction.  Consider a wooden block lying on a horizontal surface. If we apply a small horizontal force (say 1N) on the block and the block does not move, it means that a force (=1N) has come into play to oppose the motion.

 This is the force of friction and is called static friction

Fs  As the applied force is increased, the static friction

also increases (being always equal and opposite to the applied force) till a stage is reached when the block is about to move.  The value of static friction is now maximum and is called limiting friction or critical friction Fc .  Note that static friction is always adjusts itself equal to the applied force upto the value of limiting friction.  Thus limiting friction is the maximum static friction that comes into play when one body is just at the verge of moving over the surface of another body.

 Once the limiting friction is overcome by the applied force, the block begins to move and limiting friction Fc is converted into kinetic friction Fk .  The subsequent motion of the block is determined by

the applied force (F) and the kinetic friction Fk.  It is reminded that Fk < F c Discussion : The following point may be noted : 1. Static friction comes into play only when the applied force tends to move a body on the surface of another body. If the applied force is zero, static friction is also zero. 2. Static friction is a self –adjusting force. As the applied force increases, static friction also increases upto limiting friction.

2. Kinetic friction : 

Kinetic friction is the opposing force that comes into play when one body moves over the surface of another body. It is sometimes called dynamic friction.  Once the limiting friction is overcome by the applied force, the body begins to move and the limiting friction is converted into kinetic friction.  Fig. 2 shows the graph between the applied force and force of friction.  The part OA of the graph is a straight line showing that static friction increases as the applied force increases upto limiting friction Fc .

 The body will remain at rest till the applied force does

not exceed OC. Once the applied force exceed OC, the body starts moving and friction decreases to Fk (kinetic friction).  Thus the limiting friction must be overcome to start the body moving and then a smaller kinetic frictional force Fk acts on the body to oppose the motion of the body.

Types of kinetic friction :  Kinetic friction is of two types viz. (a) Sliding friction (b) Rolling friction.  When the body slides over the surface of another body, then opposing force that comes into play is called the sliding friction.  For example, when a rectangular wooden block is moved on a level floor, the opposing force is the sliding friction.  When one body rolls over the surface of another body, then opposing force that comes into play is called the rolling friction. Thus when boll rolls over a surface, the opposing force is the rolling friction.

 LAWS OF FRICTION  Static friction as well as kinetic friction is a

complicated phenomenon. Many aspects of friction even today are not completely understood.  However, experiments show that to a good approximation, the force of friction obeys the following laws : 1) The frictional force (static as well as kinetic) depends upon the nature of the two surfaces in and their state of roughness. 2) The frictional force always acts parallel to the surfaces in and its direction is opposite to motion or attempted motion.

3) The frictional force is independent of the area of

of the two surfaces. 4) The magnitude of frictional force (kinetic and static) is directly proportional to the perpendicular force (normal reaction R) between the two surfaces in . 5) The frictional force is independent of the speed (applicable to kinetic friction only) of one surface relative to the other surface.

MAGNITUDE OF LIMITING FRICTION  Consider a wooden block of

m lying on a horizontal surface Applied force as shown in figure.  The normal reaction R is the ←Fc perpendicular force exerted by the surface on the block.  Clearly, R = mg. If Fc is the limiting friction (i.e., maximum static friction), then according to laws of friction, Fc is directly proportional to the normal reaction R i.e.,

Fc α R or Fc =  s R  Where  s is a constant of proportionality and is called coefficient of static friction. Its value depends upon the nature and condition of the surfaces in . Fc Limiting friction s   R Normal reaction

 Therefore, co-efficient of static friction between two

surfaces in is equal to the ratio of limiting friction and the normal reaction.  Since  s is a ratio of two forces, it has no units i.e., it is a pure number.

 ADVANTAGES AND DISADVANTAGES OF FRICTION 

Friction always opposes motion. It has advantages as well as disadvantages. Advantages : Although we often view friction as an undesirable evil and go to great pains to reduce it, we would really be in deep trouble without it. The following examples illustrate this point. 1) We would not able to walk if there had been no friction between the soles of our shoes and the ground. While walking, we push on the ground and the force of friction acts in the opposite direction. In the absence of friction, for example, on icy surface, the person’s shoes would slip when placed on the ground. The frictional force always opposes the motion of the shoes.

2) The frictional forces are very beneficial for cars and other moving vehicles. Without friction between tyres and the road, the car could not stop, start or turn corners. 3) The break of car or any moving devices depend upon friction. When we apply breaks, the car is stopped due to the force of friction between the break lining and the drum on the wheel. 4) Friction helps (a) hold screws in wood (b) drive nails in wood etc.

Disadvantages : Friction is associated with many disadvantages and much of our efforts are directed to reduce it. The following examples illustrate this point. 1) Much of energy is wasted in overcoming frictional forces. For example, as much as 20% of the gasoline burnt in car is used to overcome friction in the engine. 2) The energy used to overcome the frictional forces is converted into heat. This raises the temperature. In almost every machinery, the generation of heat causes several problems. 3) Since energy due to friction is converted into heat, it lowers the efficiency of very machinery.