Solved examples for class 8 science : Chapter 12
I. Multiple Choice Questions (MCQs).
Q.1. The statement frictional force is a self adjusting force is
a) true only in the case of static friction
b) true only in the case of rolling friction
c) true only in the case of sliding friction
d) a false statement
Answer: (a) true only in the case of static friction
Q.2. Friction always opposes the relative motion between the two bodies in contact irrespective of the direction of
c) frictional force
d) none of these
Answer. (a) motion
Q.3. The frictional force is directly proportional to
b) nature of surface
d) none of these
Answer. (c) weight
Q.4. A fast moving vehicle, whose engine is switched off comes to rest on a rough road, because of
a) static friction
b) sliding friction
c) rolling friction
d) all of these
Answer. c) rolling friction
Q.5. Friction can be minimised by
a) polishing the surfaces
b) streamlining the bodies
c) applying lubricants on the surfaces
d) all of these
Answer. d) all of these
II. Fill in the blanks by selecting appropriate words from the given list: List : opposite, adhesion, streamlined, force, contact, friction
Q.1. Frictional force comes into play when two surfaces are in physical………………………… .
Q.2. Frictional force acts between the surfaces of the bodies and always acts in ……………….direction.
Q.3. If you apply the……………. along the right direction, the……………………..acts along the left direction.
Q.4. The force of………………. between the two surfaces give rise to friction.
Q.5. The symmetrical shape of a body which offers least resistance due to friction is called ………………………shape.
3- force, friction,
III. Match the items of Column A with those of Column B.
Answer. 1-c, 2-d, 3-b, 4-e, 5-a,
IV. Write ‘T’ for the true and ‘F’ for the false statements.
Q.1. While walking the force of friction acts along the direction of motion.
Q.2. The friction is due to the roughness of the two surfaces in contact.
Q.3. The force of cohesion between the two surfaces gives rise to friction.
Q.4. When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.
Q.5. Friction between various parts of the machine leads to a gain of energy and thereby increases the efficiency of machines.
Answer. 1-F, 2-T, 3-F, 4-T, 5-F
V. Very Short Answer Questions.
Q.1. Which force always opposes the relative motion between the bodies in contact?
Answer. Friction force
Q.2. Write the factors on which friction depends.
Answer. The force of friction depends on the following factors.
o Frictional force is directly proportional to the weight of the body which moves.
o The force of friction depends on the nature of the surface in contact.
Q.3. Define self-adjusting frictional force.
Answer: The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction or self-adjusting frictional force.
Q.4. What are sliding and rolling friction?
Answer: Sliding Friction is the frictional force which resists the relative sliding motion between two contact surfaces.
When one body rolls over the surface of another body, the resistance to its motion is called rolling friction.
Q.5. Write the name of the shape of aeroplane for minimising friction.
Answer. Streamlined shaped
VI. Short Answer Questions.
Q.1. How can you say friction produces heat?
Answer. When the palms of the hands are rubbed together for a few seconds, they become warm. So, we can say that friction produces heat.
Q.2. Why is it difficult to walk on a smooth and wet floor?
Answer. It is difficult to walk on a smooth or wet floor because friction is very less. The required grip needed for walking is not available on a smooth or wet floor. So, it is difficult to walk on a smooth and wet floor.
Q.3. Why are spikes provided in the soles of shoes used by football players and athletes?
Answer. Spikes are provided in the soles of shoes used by football players and athletes to increase friction so that they get a firm grip on the ground.
Q.4. What are the factors affecting friction?
Answer. The force of friction depends on the following factors.
• The force of friction depends on the nature of the surface in contact. For example the force of friction between a pair of highly polished surfaces is very small as compared to the force of friction between two rough surfaces.
• On a horizontal surface, the force of friction is directly proportional to the weight of the body which moves.
Q.5. What are causes of friction?
Ans. The following factors cause the friction between two surfaces:
(i) Due to the interlocking between the surfaces: No surface can be considered perfectly smooth. The degree of roughness varies from surface to surface. Some are most rough whereas some are less. When two surfaces slide over each other, the rough edges of one surface interlock with rough edges of the other. This interlocking opposes the relative motion between the two bodies. This gives rise to a frictional force. Thus, friction is due to the roughness of the two surfaces in contact.
(ii) Due to force of adhesion between the two surfaces: When two surfaces are placed in contact with each other, the atoms and molecules in contact with each surface attract each other with a force called force of adhesion or adhesive force.
When a body moves over another surface of this force (adhesive force) opposes the motion; which gives rise to a force of friction. Therefore, the force of adhesion between the two surfaces gives rise to friction.
Q.6. What do you understand by fluid friction?
Answer. In science, liquids and gases are referred as fluids. Fluids also offer resistance to the motion of objects moving through them. This resistance offered by fluids is called fluid friction. Fluid friction on an object depends on the shape of the object, speed of the object and nature of the fluid.
VII. Long Answer Questions.
Q.1. What are kinds of friction? Describe them with examples.
Answer: There are three kinds of frictions: Static friction, sliding friction and rolling friction. To move or slide a box along the floor, you have to overcome the force of friction. The friction acting on an object at rest is called static friction. The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction.
If you stop pushing the box when is started sliding, the box will come to stop again. Thus, a friction is acting on the box even when it is sliding. The force required to keep the object moving with the same speed is a measure of sliding friction. The force required to keep the box moving at the same speed is a measure of the sliding friction.
Sliding friction is slightly smaller than the static friction.
Sliding friction: When one body rolls over the surface of another body, the resistance to its motion is called rolling friction. It is easier to roll a large log of wood than to drag it . Thus rolling decreases friction and makes movement of things easier. This is why suitcase and bags are fitted with wheels.
Thus, we can summarise
Static friction > Sliding friction > Rolling friction
Q.2. (a) Discuss the methods of increasing friction.
(b) What are methods of reducing friction? Describe with examples.
Answer. a) Increasing friction: It is sometimes desirable to increase friction to avoid slipping. There are two methods to increase friction:
(i) By making the surface rough:
• Kabaddi player rub their hands with soil for a better grip of their opponents.
• The tyres of the vehicles have grooves to increase resistance with the road.
• Spikes are provided in the soles of shoes used by football players and athletes to increase friction so that they get a firm grip on the ground.
(ii) By increasing the mass of the object that is moving.
b) Reducing friction: In some situations, however, friction is undesirable and can be minimise it by the following methods:
(i) By polishing the surfaces: Rough surfaces can be made smooth by polishing. It will reduce friction.
(ii) By streamlining the bodies: To reduce fluid friction, aeroplanes, rockets, sail boats are shaped into streamlined body. Birds have streamlined body.
(iii) By applying oils, greases or lubricants on the surfaces: Use of oils, lubricants and greases makes the surface smooth. In machines, friction is reduced by using above-mentioned things. When oil, grease or lubricant is applied between the moving part of a machine, a thin layer is formed there and the moving surfaces do not directly rub against each other.
(iv) By sprinkling a soft, slippery line powder on the surface:
A small quantity of talcum powder on a wooden surface of carrom board reduces friction between striker and carrom board surface. Graphite powder is used in machines to reduce friction.
(v) Wheels: If a suitcase is fitted with wheels, it is easier to move it, because the friction between the wheels and the ground is the least.
(vi) Ball-bearing: These are made in various designs to reduce friction.
They reduce friction because rolling friction is smaller than sliding friction. These bearings are also lubricated to further improve their function.
Q.3. (a) What do you understand by friction?
(b) How can you say friction is a necessary evil. Describe with examples.
Answer. (a)Friction is an invisible force that acts between tow surfaces in physical contact. It always acts in a opposite direction to the direction of motion. This slows down moving objects. So, we can say that friction opposes motion.
b) Friction has both good and bad aspects. It is the friction acting between our sole (of foot) and surface of the ground that we are able to walk, run or move fast. Our day-to-day life could have been impossible without frictional force.
On the other hand, friction between various parts of the machines leads to a loss of energy and thereby reduces the efficiency of machines. It also causes wear and tear of moving parts of the machines. Therefore, friction is considered as a necessary evil.