Class8.L-7.Force and Friction

EXERCISE

A.Choose the correct option for each of the following .

1.Which among the following is an example of a contact force ?

a) Friction (✔)

b) Gravitation

c) Magnetic

d) Electrostatic

2.Force can

a)stop a moving body

b)change the shape of a body

c)change the direction of a moving body

d)All of these  (✔)

3. Identify the non-contact force that would best suit in lifting and placing large amount of iron scraps in a workplace.

a)Magnetic  (✔)

b)Gravitational

c)Electrostatic

d)Muscular

4.Which among the following is not always a disadvantage of friction?

a)Friction heats up parts of machines

b)Friction produces wear and tear

c)Friction decreases the speed of a moving body  (✔)

d) Friction makes movement difficult

5.Birds have a streamlined body shape that helps them to

a) reduce air friction to fly with less effort  (✔)

b) apply more effort to overcome air friction

c) reduce their speed during flight

d) fly at higher altitudes

B.State whether the following statements are true (T) or false (F) .

1. All contact and non-contact forces exhibit all effects of force. (T)

2. Gravitational force is a contact force.  (F)

3. It is not possible to walk in the absence of friction. (T)

4. Electrostatic force is a non-contact force. (T)

5. Friction can be reduced but cannot be eliminated. (T)

C.Fill in the blanks using the correct words.

1. Gravitational force is always a force of- pull .

2. Muscular force is an example of- contact force .

3. A ball thrown upwards comes back; this is due to the

- gravitational force of the Earth.

4. A vehicle slows down when brakes are applied because of

- friction .

5. Sprinkling a little talcum powder on the carom board helps to

- reduce the friction.

D.Answer the following questions in one sentence each.

1. Give one example of a contact force.

ANS:-Friction

2. Give one example of a non-contact force.

ANS:-Gravitational force.

3. Mention one situation in life where friction is necessary.

ANS:-Walking.

4. Mention two methods to minimize friction.

ANS:-

a)By using Ball bearings .

b)By using Lubricants.

5. Mention two methods to increase friction.

ANS:-

a) By increasing the roughness of surfaces.

b)By increasing the mass of the object in motion .

E.Answer the following questions briefly.

1.What are contact forces and non-contact forces?

 ANS:-

Contact force: A force acting between the two bodies that requires a physical contact between them is called a contact force . E.g. Muscular force, Frictional force

Non-contact force: A force acting between the two bodies that does not require any physical contact between them is called a non-contact force. E.g. Gravitational force, magnetic force .

2. What are sliding friction and rolling friction? Which is more?

ANS:-

Sliding friction: The friction acting between the surfaces when an object is slides over a surface is called sliding friction .

Rolling friction: The frictional force acting between the surfaces when an object rolls over a surfaces is called rolling friction.

Sliding friction is more than the rolling friction.

3.Mention three disadvantages of friction.

ANS:-Three disadvantages of friction are:

a)When a machine runs for a long time because of friction it get heated.

b)It causes unnecessary wastage of energy and reduces the efficiency of machine .

c)Moving machinery parts get worn out when used continuously because of friction.

4.What is static friction ?

ANS:-The friction acting between two surfaces in contact when there is no relative motion between them is called static friction .

5.Mention two ways of increasing friction .

ANS:- Same answer as D.(5)

F.Answer the following question in detail .

1.Define force .Explain the different effects of force .

ANS:-  A puss or a pull can be defined as force.

Different effects of force are :

a)Force can make a stationary body move or can increase the speed of a moving body. When we push a table it will move in the direction of applied force. If we push a moving trolley , it will move faster with high speed.

b)A force can make a moving body stop. In cricket a fielder stretch out his arm to apply force to catch the ball.

c)Force can change the direction of moving object .In cricket a batsman hits the ball and changes the direction of motion of the ball.

e)A force can change the shape of a body . When we press a rubber ball the shape of the ball changes.

2.Explian contact forces and non-contact forces with examples .

ANS:-  Some of the contact force are:

a)Muscular force : This force is the result of the action of muscles. Lifting a heavy box needs force applied by our hands.

b)Frictional force : It is a resistive force between two bodies that are in contact and motion such that it opposes the motion . A ball rolling on the ground comes to rest after some time because of the friction acting between the ball and the ground.

Some of the non-contact force are :

a)Gravitational force : It is a pulling force of earth on a object towards its surface. For example, because of this force a ball thrown up comes down.

b)Electrostatic force: It is a force that act between two charge particles .Charges are two types - positive and negative. This force may be attractive or repulsive depending on the nature (positive or negative) of the charges. Unlike charges attract and like charges repeal each other.

c)Magnetic force : It is a force that act between two magnets or  between a magnet and a magnetic material. A magnets has two poles-north and south. Like pole repeal and unlike pole attract each other.

3. Describe one activity to show that sliding friction is more than rolling friction. ANS:- To perform this activity the required materials are - A flat wooden plank, a small wooden block, a small wooden cylinder and two notebooks.

Procedure:

1. Place the wooden plank on the table.

2. Keep one/two notebooks below one end of the wooden plank to make an inclined plane as shown below.

3. Place the wooden block at the top of the inclined wooden plank. The block will not move at all. If we push the block gently it will   move a little and then quickly comes to rest.

4. Next, place the wooden cylinder at the top of the inclined wooden plank. We will see that it roll down on its own without any push to it.

Hence we can conclude that Sliding friction is greater than rolling friction.

4. ‘Friction is a necessary evil’. Explain. 

ANS:-  Friction is said to be necessary evil because it has some advantages as well as some disadvantages.

      Friction help us to walk on a surface, to hold object in our hand, to transfer energy from motor to different equipment through belt, to reduce the speed of vehicle by applying brakes etc. Without friction our normal daily life would not be possible.

     On the other hand it causes heating of machines when runs for long time, wear and tear of machinery parts , unnecessary  wastage of energy etc  which are wasteful to us.

            So, we can say that friction is a necessary evil.

5.Describe the methods to minimize friction .

ANS:-  We can reduce friction by a number of methods discussed below.

a) Friction can be reduced by making the two surfaces in contact smooth.

Smoothening reduces the roughness on the two surfaces and reduces the interlocking between the irregularities, thus decreasing the friction.

b) By applying some materials like oils, grease, graphite powder (Lubricants) between the two surfaces, help the surfaces to slide easily over one another, minimizing the interlocking of the irregularities on the surfaces.  

c) Rolling friction is less than sliding friction.  Usually, ball bearings are used to minimize friction by this method to provide the rolling effect.

d) Friction caused due to air (drag) can be reduced by making the object (vehicles) in the desired streamlined shape. This method is used in aircraft.

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