DAV Class 8 Science Solution Chapter 5 Friction
DAV School Books Class 8 Science Solution Chapter 5 Friction all Question Answer. DAV Class 8 5th Chapter Friction full Chapter explanation by expert teacher.
DAV School Books Class 8 Science Solution Chapter 5 Friction
A.) Fill in the blanks.
1.) Friction always ___________ any relative motion between the two surfaces that are in contact with each other.
2.) Static friction is always ___________ than sliding friction.
3.) Sportsmen use shoes, fitted with spikes, to ___________ friction between their shoes and the ground.
4.) Sprinkling of talcum powder, on the carrom-board, helps to ___________ friction.
5.) Frictional force, on an object moving in a fluid, depends on its ___________.
6.) The shape of an aeroplane is ___________ to reduce, the effects of friction, due to air.
Answer: (1) opposes (2) greater (3) increase (4) decrease (5) speed (6) streamlined
B.) State True or False for the following statements.
1.) When a body slides over smooth and wet surfaces, the amount of interlocking of irregularities, of the two surfaces in contact, increases.
2.) If a car moves eastwards, the force of friction acts southwards.
3.) Friction depends on how hard the two surfaces press against each other.
4.) The sole of shoes and the tyres of the vehicles are ‘treaded’ to decrease the effects of friction.
5.) Lubricants can be liquids, semi-liquids or solids.
6.) Friction can be increased by using ball bearings between the moving parts of machines.
7.) Fluid friction can be minimised by giving suitable shapes to the objects moving through the fluid.
Answer: (1) False, it does not happen so since the friction is less (2) False, No it will act westwards (3) True (4) False, they are treated to increase the force of friction between them and the surface (5) True (6) False, no they are semi solid. (7) True
C.) Tick the correct option.
1.) Suppose your writing desk is tilted to a position where a book kept on it just starts sliding down. The figure, showing the correct direction of frictional force acting on it, is-
Answer: option D ( since frictional force opposes the direction of motion)
2.) Two boys are applying oppositely directed, and equal in magnitude, forces on a box as shown in the figure. In such a case, the force of friction, that would exist between the lower end of the box and the ground, would be called-
Answer: static friction
3.) Four students were asked to arrange the (frictional) forces, due to rolling, static and sliding friction, in an increasing order. Their arrangements are listed below. The correct arrangement is –
Answer: rolling, sliding, static
4.) The energy, ‘used up’ in overcoming friction, gets converted mainly into-
Answer: heat energy
5.) Rahul rolls a ball on a wooden surface. The ball covers a certain distance before coming to rest. To make the same ball cover a (much) longer distance, before coming to rest, Rahul should-
Answer: sprinkling talcum powder on the wooden surface could make the surface smoother and reduce interlocking of the surfaces in contact as result it will reduce the force of friction.
6.) A block is sliding on a horizontal surface. The force of friction between the two can be increased by-
Answer: putting a second identical block on top of the given block would increase the force of friction
D.) Answer the following questions in brief.
1.) Define the terms (a) static friction (b) sliding friction.
Answer: (a) Static Friction- The Frictional force that exists between one body which is stationary or at rest and the surface on which it has been kept is called static friction.
(b) Sliding friction-The force of friction that generally acts between an object when it is sliding over another object the resultant frictional force is known as sliding friction.
2.) State the meanings of the terms (a) rolling friction (b) fluid friction.
Answer: (a) Rolling friction- Rolling friction comes to play when a body rolls over another body it causes friction.
(b) Fluid Friction – Like in other mediums , in case of fluids when two objects are moving through them an opposing force of friction is caused.
3.) State the likely cause of friction.
Answer: The frictional force is the result of interlocking of the irregularities of the surfaces coming in contact. The relative motion between these two surfaces is the secondary cause of friction.
4.) A box is resting on the floor. To move it, a (variable) force is applied as shown in the diagram below.
(a) Give the (likely) value of the force of limiting friction.
Answer: Limiting force= >2F and <3F
(b) Why does the box stay at rest in the cases corresponding to Fig (a) and Fig (b)?
Answer:The force applied on the box is lesser than the limiting force , thus the applied force cannot overcome it.
(c) Mark the direction of frictional force, and mention the kind of frictional force that comes into play, in each case.
Answer: The direction of the frictional force is opposite to the motion of the object and in the given case it is opposite to the direction of motion of the box as given in the figure.
Types of friction observed
- Case a-Static Friction
- Case b-Static Friction
- Case c-Sliding Friction
5.) Name the kind of frictional friction, that comes into play, when a book, kept on a collection of cylindrical pencils, is moved by pushing it.
Answer: Rolling friction
6.) Why are objects given special shapes when they are moving through fluids? Write the name given to these special shapes?
Answer: It is seen that then different organisms and objects move throughout fluids at high speed and use large force to counterbalance the opposing motion. due to this the bodies have to use a lot of energy which is not desired, in order to minimise this loss bodies are given a specific shape. This shape is known as streamlined shape.
E.) Answer the following questions.
1.) Define Force of friction. List the factors affecting the force of friction. Explain with examples.
Answer: In a situation where an object moves over the surface of any other object the force acting between the two surfaces that are in contact with each other is known as force of friction this force of friction opposes the motion of the two surfaces that are in contact.
- the important factors that affect the force of friction are
- irregular surfaces that are in contact with each other
- that are in contact with each other
- when two surfaces are firmly put up against each other
when we try to move a cart across a rough road it is much more difficult but when we try to move the same cart across a road that is much smoother it becomes easier for us to move it with lesser force applied
2.) A ball is allowed to roll down an inclined wooden plane from a given height. At the foot of the inclined plane, it moves on a horizontal surface, differently ‘covered’, one by one, as shown in the following figures.
In which case, is the ball likely to move the longest distance? Give reason for your answer.
Answer: In case ‘c’ (glass sheet), the ball will move for the longest distance because, the glass surface is the smoothest, it does not have any irregularity to interlock with the surface of the ball and thus, friction will be the least. As friction will be the least in this case the ball will move the farthest
3.) Explain why sliding friction is slightly less than static friction.
Answer: In case of static friction two objects are in close contact with each other and in fact they are interlocked whereas in case of sliding friction two word is are just in contact with each other and are not interlocked as a result sliding friction is lesser than static friction
4.) Give reasons for the following-
(a) We tend to slip when we step on a banana peel.
Answer: banana peel has a smooth surface as a result the friction between our feet and the peel will be less and we tend to slip
(b) It is easier to push a lighter box than a similar heavy box on the same floor.
Answer: as friction is dependent on with the amount of force applied it is very obvious that in case of a smaller box or a lighter box the force applied will be lesser as a result the resultant frictional force on it will be much lesser than a similar box which is heavier
(c) The force, needed to start a cart, is (somewhat) greater than the force needed to keep it moving with a uniform speed.
Answer: The force required to start a card is greater than the force needed to keep it moving because to start the car one must overcome the static friction that exists due to the interlocking between the swordfish and the wheel of the cart once the cards start moving the contact made by the wheel with the surface is much lesser and it results to lesser interlocking thus the frictional force is also less
(d) Sportsmen use shoes fitted with spikes.
Answer: sportsmen use spiked shoes because it increases the friction and thus prevents them from slipping away in the field
(e) Ball-bearings are used in machines.
Answer: It is known to us that rolling friction is smaller than sliding friction thus in car engines and machines sliding friction is applied by using ball bearing so that the amount of friction is less.
(f) Tyres of the vehicles need to be changed regularly.
Answer: Tires tend to get worn out when used for a long period of time as a result the force of friction decreases because the contact and interlocking between the wheel and the surface is not as good as it was before
(g) Machines parts are frequently oiled or greased.
Answer: machine parts are frequently greased or oiled so that the surface remains smooth and there isn’t much friction against each othe
(h) Sport cars, aeroplanes and boats are designed to have a streamlined shape.
Answer:. the streamline shape make sure that the body is brought in the middle and pointed at the ends so that it can overcome fluid friction this allows the bodies to use lesser energy and move with greater force and speed
(i) Metal chains are wrapped on tyres when they run on icy roads.
Answer:this is done so that the friction between the wheel and the icy surface increases also it helps the card in applying the brakes suddenly
(j) Rollers are used for transporting luggage.
Answer: since rolling friction is lesser than sliding friction it is easier to use rollers that help in rolling the luggage across the surface causing the person to apply lesser force
5.) State and explain, with examples, how friction can help the cause of motion in certain situations.
Answer: Friction is necessary in our day-to-day life as many tasks that we perform are aided by friction. the friction between the ground and our feet enable us to walk while walking we push the ground below us to create an opposing force that helps us in walking .