Dav Class 8 Science chapter 5 Question Answer Friction Question Number- A, B, C, D, E.

♥Class 8

      Chapter 5 

Friction ( Question and Answers )

A. Fill in the blanks.

1. Oppose/ resist

2. Great

3. Increase

4. Decrease/reduce

5. Resistance

6. Streamlined/Aerodynamic

B. State true or false for the following.

1. False

2. False

3. True

4. False

5. True

6. False

7. True

C. Tick (√) the correct answer.

1. Last diagram on the right

2. Static friction

3. Rolling, sliding, static

4. Heat energy

5. Sprinkle talcum powder on the wooden surface

6. By putting a second identical block on top of the given block.

D. Answer the following questions in brief.

1. (a) Static friction is a self adjusting force of friction that exists between a stationary object

and surface on which it has been kept.

(b) Sliding friction is a force that acts between an object and the surface when an object

is sliding.

2. (a) Rolling friction is a force of friction resisting motion when a body rolls.

(b) Fluid friction is a frictional force exerted by fluids against moving objects.

3. Friction is caused by the 'interlocking of irregularities 'in the surfaces of the two objects

which are in contact with each other.

4. (a) The value of force of limiting friction is 3F

(b) In case a and b the applied force is below the limiting force.

(c ) The direction of frictional force is always opposite to motion.

Case a - Static

Case b - Static

Case c - Sliding

5. Rolling Friction

6. The objects are given special shapes when they are moving in fluids because these

objects experience fluid friction which act opposite in the direction of motion.

These are called Streamlined shapes.

E. Answer the following questions.

1. Whenever an object moves or tends to move over surface of another object a force

comes into play , this force acts parallel to the surface of contact and opposes relative

motion. This opposing force is Friction. It depends upon

a.Nature of two surfaces in contact.

b. force pressing the two surfaces together.

Example :A box placed on smooth surface and less mass will be easy to move but a

box placed on rough surface with greater

mass will experience more friction.

2. In case C (glass sheet) the ball will move for longest distance . It is because

  Glass surface is smooth

  Less interlocking

  Less friction

  Great emotion

3. We know that friction is caused by the interlocking of irregularities in the two surfaces .In

case of static friction ,interlocking between two objects is very strong but in case of

sliding friction, there is a less time of getting interlocked with each other. Thus the sliding

friction is less than static friction.

4. (a)As we stepped on a banana peel, then there is no grip between our feet and the

banana peel due to very less friction ,so we can accidentally slip.

(b)It is easier to push a lighter box because friction depends on the force pressing the

two surfaces together .The heavy box will exert more force so friction will be more

.Lighter box will exert less force and thus exerts less friction.

(c)The force needed to start a cart is greater than the force needed to keep it moving in a

uniform speed because once motion starts the contact points on one surface do not get

enough time to lock into the contact point of another surface.

(d)Sportsmen use shoes with spikes in order to avoid getting slip in the field.The spikes

in the shoes increase the frictionn so he or she can run on the ground easily.

(e) Ball bearing is used in machines to reduce friction and to support load.

(f) Tyres of the vehicles are designed to increase the friction between road and the

wheel. Due to wear and tear in the tyres by regular use, the friction between road and

the tyre reduces and the tyres become more slippery.

In order to avoid this we need to change the tyres regularly.

(g) Machine parts are frequently oiled or greased to increase the efficiency of machines

as oil or grease will reduce the friction.

(h)Sport cars, aeroplanes and boats have streamlined shape to overcome the fluid

friction.

(i)For increasing friction the metal chains are wrapped on tyres in icy areas which

increases the grip and increase friction that helps the car to stop after applying the break

in icy areas.

(j)It is always easier to roll the body then to slide it over a surface .So it is convenient to

pull the luggage fitted with rollers.

5.

  When we walk we push the ground under our feet in backward direction,the friction

then provides forward reaction and it is a main cause of motion.

  Vehicles run on road due to friction between tyres and road.

  Friction also helps in writing by moving a pen on a paper.

 Not Important 

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