NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
TEACHER SCHOOL
EDUCATION DEPARTMENT
JAMMU AND KASHMIR
Chapter14
FRICTION
KEYWORDS
Friction: Force which opposes the relative motion between the two surfaces in contact.
Spring Balance: A device used for measuring the force acting on an object.
Lubricants : Substances, which reduce friction.
Drag: Frictional force exerted by fluids
Streamlined: The shape, which would make body not to lose much energy in overcoming friction.
Rolling friction: The resistance to the motion of a rolling body over the surface of another body.
INTEXT QUESTIONS (PAGE 178-179)
Q.1. What do you understand by the term friction? Explain how it caused
Ans. Friction: It is the opposing force or retarding force which is called into play when a body, actually, moves or tends to move over the surface of an other body.
When an object is pushed with a small amount of force, it does not move at all. It means that the other surface, on which object is lying exerts some force on the object, which is in a direction opposite to our push. The opposing force is called friction.
Cause of friction: Every surface, howsoever, consist of irregularities in the form of mounds and grooves. When one surface tends to move over the other surface, these irregularities
get interlocked with one another and hence motion is opposed. This gives rise to frictional force. Thus, force of friction is due to roughness of the surface.
Q.2. Distinguish between the static friction and dynamic friction.
Ans. Static Friction: The friction, which is produced between two surfaces in contact with each at the position of rest is called static friction. This type of friction is more powerful than others. It is not easy to overcome the static friction. This type of friction needs a lot of energy to overcomes
Dynamic Friction (Sliding friction): The friction, which arises when two objects in contact are in motion is called sliding friction. It is lesser than static friction, but more than rolling friction. Inthis type of friction irregularities of surfaces do not get enough time get locked with each other as objects are already in motion.
Q.3. What is limiting friction ? State the laws of limiting friction?
Laws of Limiting Friction :
(i) Limiting friction always opposes the motion of a body and acts in the direction opposite to the direction applied force.
(i) Limiting friction depends upon the nature of surface in con- tact with each other.
(ii) Limiting friction increases with the increase in the weight of the body and vice-versa.
(iv) Limiting friction is independent of area of contact between two surfaces; provided the weight of the body and the nature of surface do not change.
Q.4. How is rolling friction caused ?
Ans. (1) The roller is the wheel sometime deforms the surface on which it is rolling, thereby causing a depression on account of its weight.
(ii) Sometimes, the roller or the wheel itself gets deformed temporarily at the point of contact of the surface of the body.
Because of the above deformations a kind of locking is formed in the direction of motion of the roller or the wheel. Thus, a force is required to overcome this temporary, but, continuously, forward moving lock of irregularities. This force applied is the equivalent of rolling friction.
Q.5. Under what conditions the rolling friction increases ?
Ans. In such process if the shape of the roller or wheel deforms, the. lock of irregualrities will form more so, if lock of irregularities increases the rolling friction will increase.
INTEXT QUESTIONS (PAGE 178-179)
Q.1. State one way of increasing the friction between two surfaces.
Ans. Friction between two surfaces can be increased by making a surface rough.
Q.2. Name four ways by which friction can be reduced between two surfaces in contact ?
Ans. The friction between two surfaces in contact can be reduced by adopting these ways:
(i) By polishing a rough surface.
(ii) By lubrication to put lubricant between the sliding surface.
(iii) By using wheels and ball bearings.
(iv) By streamlining.
Q.3. State two advantages of friction
Ans. Due to friction :
(i) We walk on the floor.
(ii) Teacher is able to write with chalk on the black-board.
Q.4. State two disadvantages of friction.
Ans. Two disadvantages of friction are:
(i) It causes wear and tear.
(ii) Heat produced due to friction damages the machines.
Q.5. Why is friction called a necessary evil?
Ans.
Friction is an evil. It causes a lot of problems for us. It wears out the objects or materials on which it acts. The soles of our shoes get weared due to friction. The floors, machines, their parts, clothes, metals, etc., get weared due to the friction they bear, but it is a necessary evil because we cannot do anything without friction while it also shows ill effects. No movement would come to an end without friction. Nobody would be at rest without friction. Friction is
necessary for making grip and bringing movements at rest.
Q.n.(a) What is lubricant ?
(b) How does a lubricant reduce friction ?
Ans. (a) Lubricant is a substance, which can reduce friction.
(b)The lubricant separates two surfaces in, such a way that the interlocking of irregularities are, greatly, reduced as, the spaces between them are filled with the lubricant.
(c) What kind of lubricant is used in
(i) a sewing machine,
(ii) the axle of a tractor?
Ans. (i) In a sewing machine, oil is used.
(ii) In the axle of a tractor, grease is used.
Q.7. Name two solid lubricants and state where they are used?
Ans. Graphite and finely powdered talcum are the two solid lubricants. Graphite is used in those parts of the machinery where oil cannot applied. Finely, powered talcum is used on the carrom-board.
Q.8. What do you understand by the term streamlining? Name a few machines which use streamlining to reduce friction ?
Ans. Giving a shape to the bodies, such that they offer the least resistance to the air or water is called streamlining. Modern aircrafts, boats and ships are the machines, which use streamlining to reduce friction.
Q.9. Why is friction called a perverse force ?
Ans. Force opposing the motion is force of friction that is why it is called a perverse force.
TEXT BOOK EXERCISE
Q.1. Answer the following questions:
(i) Why are the worn out tyres discarded?
Ans. There must be a friction between the tyres and the road. Due to constant use, grooves from the tyres disappear and unable to produce required amount of friction that's why the worn out tyres are discarded.
(ii) Why do carrom coins move faster on carrom board when dusted with talcum powder ?
Ans. Carrom coins move faster on carrom-board when dusted with talcum powder because talcum powder is used as a lubricant, which reduces friction between the carrom-board and the coins.
(iii) Why is the surface of conveyor belt made rough?
Ans. The surface of conveyor belt is made rough due to providing maximum friction.
(iv) Why is the sewing machine often oiled?
Ans. The sewing machine is often oiled because when it is applied it reduces the force of friction between them and the interlocking of irregularities is, greatly, reduced as the spaces between them are filled with the oil.
(v)Why do new automobiles tyres have deep grooves?
Ans. New automobile tyres have deep grooves, so that they after the required amount of friction.
(vi) Why does a ball rolling on the ground slow down ?
Ans. A ball rolling on the ground slows down because rolling ball push the ground backward and the ground reacts back only on account of friction.
(vii) Why are the boats and aeroplanes have special shape?
Ans. The boats and aeroplanes are given special shape, so that they experience minimum amount of friction, while moving through water or air.
(viii) Why do meteors burn on entering into the atmosphere?
Ans. The meteors enter the atmosphere of the earth at a very high speed. At such speeds the friction due to air, extremely, high and the temperature of meteor rises to, such a high degree that they catch fire.
(ix) Why do painters use sand paper in polishing doors?
Ans. Painters use sand papers in polishing doors to make them rough for friction. There is maximum limiting friction between them.
(X)Why is it easier to tie a knot with cotton string as compared to silky string?
Ans. When two bodies are placed upon one another, the irregularities of the surface of two bodies interlock more the irregularities on the two surfaces in contact, i.e., the rougher are the surfaces in contact; more is the force of friction. The same rule of friction is in the process of the a knot. Due to the force of friction it is easier to tie a knot with cotton string as compared to silk string.
Q.2. Fill in the blank spaces by choosing words from the list given below:-
Movement, Sliding, Streamlined, Static, Opposite.
(i)Friction always acts in the direction.................
applied force.
(ii)..................friction is a self-adjusting force.
(iii)Friction is very useful as it helps in the..............to the direction of the bodies.
(iv)The. ....... friction is 10 times the rolling friction.
(v)The boats and aeroplanes are................so as to reduce fluid friction
Ans. (i) - Opposite; (ii) - Static; (iii) - Movement; (iv) - Sliding;
Q.3. Statements given below are incorrect, write the correct statements :-
(i)Sliding friction is slightly more than the limiting friction.
Correct Statement :- Sliding friction is slightly less than the limiting friction
(v)The conveyor belts are made rough, in order to decrease friction.
Correct Statement :- The conveyor belts are made rough, in order to increase friction.
(iii) The friction between two surfaces, decreases with the increase in the weight of a body.
Correct Statement :- The friction between two surfaces increases with the increase in the weight of a body.
(iv) The friction offered by the wheels is called sliding friction.
Correct Statement :- The friction offered by the wheels is called rolling friction.
(v)The friction increase with the increase in the area of contact at the two surfaces.
Correct Statement :- The friction decrease with the increase in the area of contact at the two surfaces.
ADDITIONAL SHORT ANSWER TYPE QUESTIONS
Q.1. What is the cause of friction?
Ans. Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces, which appear very smooth have a large number of minute irregularities on them. Irregularities on the two surfaces lock into one another. When we attempt to move any one surface over the other, we have to apply a force to over- come interlocking. On rough surfaces, there are larger number of irregularities. So, the force of friction is greater, if a rough surface is involved.
Q2. Can we reduce friction to zero by polishing surfaces or using large amount of lubricants ?
Ams. Friction can never be, entirely, eliminated. No surface is perfectly smooth. Some irregularities are always there
Q3. What is rolling friction?
Ans. Rolling Friction: A friction, which plays between two rolling materials is called rolling friction. Rolling movement of both ob- jects does not allow the locking of irregularities, so, it is lesser than sliding friction even.
Q4. You spill a bucket of soapy water on the marble floor acci- dently, Would it make easier or more difficult for you to walk on the floor? Why?
Ans. Due to soapy surface, the frictional force between our feet and surface, reduces due to which the grip of our feet with surface is not enough to walk. So, it is difficult to walk on a soapy floor.
Q.5. Explain why sportsman use shoes with spikes?
Ans. Sportsman have rough spikes on his shoes bottom to make the grip of feet with surfaces as due to spikes the frictional force between shoes and surface increases.
Q.6. Explain why is sliding friction always less than the static friction?
Ans. When the object starts sliding, the contact points on its surface do not get enough time to lock into the contact points on the floor. So, the sliding friction is, slightly, less than the static friction and you find it easier to move the box already in motion than to get it started.
Q.7. Enlist some methods of reducing friction.
Ans. The friction can be reduced by :
(i) By polishing the surfaces.
(i) By lubricating the surfaces.
(ii) By proper selection of material.
(iv) By streamlining.
(v) By using ball bearing.
ADDITIONAL LONG ANSWER TYPE QUESTIONS
Q.8. Explain various types of frctions.
Ans. There are following types of frictions:
(i) Static frictions
(ii) Sliding friction
(iii) Rolling friction
Static Friction: The friction, which is produced between two surfaces in contact with each at the position of rest is called static friction. This type of friction is more powerful than others. It is not easy to overcome the static friction. This type of friction needs a lot of energy to overcome.
Dynamic Friction (Sliding friction): The friction, which arises when two objects in contact are in motion, is called sliding friction. It is lesser than static friction, but more than rolling friction. In this type of friction, irregularities of surfaces do not get enough time get locked with each other as objects are already in motion.
Rolling Friction: The friction arises, when two bodies are rolling on each other is called rolling friction. Rolling movement of, both, objects do not allow the locking of irregularities, so, it is smallest friction of three types of friction.
Q.9. Describe the advantages and disadvantages of friction.
Ans. Advantages of Friction :
(1) Walking will not be possible without friction. Our foot pressing the ground will only slip.
(i) No two bodies will stick to each other, if there is no friction.
(ii) Brakes of the vehicles will not work without friction.
(iv) Writing on black-board or on paper will also not be possible without friction.
(v) If an object stated moving, it would never stop, if there were no friction.
Dis-advantages of Friction :
(1) Friction causes wear and tear of the parts of machinery in contact. Thus, their lifetime reduces.
(ii) Frictional forces result in the production of heat which causes damage to the machinery.
(iii) Friction always opposes the relative motion between any two surfaces. So, extra energy has to be spending in overcoming friction.
Q.10. Show with the help of an example, how liquids exert less force of friction?
Ans. The frictional force exerted by fluids (liquids or gas) is also called drag. It is established that liquids exert less force of functions compared to the solids. The shape of a body around which a fluid (liquid or gas) can easily flow, offering minimum friction, is called streamlined shape. In nature, the body of fishes is streamlined such that they encounter least friction in water. Even a swimmer tries to streamline his body as far as possible so that he faces least friction due to water. The body of boats and ships is also streamlined so that they encounter minimum friction while moving through water.
0 Comments