Selina Concise Class 6 Physics Chapter 3 Force Solution

Selina Concise Class 6 Physics Solution Chapter No. 3- ‘Force’ For ICSE Board Students.

A. Objective Questions Answers Solution:

(1) Write true or false for each statement:

(a) The frictional force acts in the direction of motion of body.

Ans. False

(b) The unit of weight is kilogram.

Ans. False

(c) A force can change the direction of motion of a moving body.

Ans. True

(d) A force increases the mass of the body when applied on it.

Ans. False

(e) The force of friction is always disadvantageous.

Ans. False

(f) The sliding friction is more than the rolling friction.

Ans. True.

(g) Liquids offer more friction than the gases.

Ans. True

(h) A wet oily road offers more friction than a dry rough road.

Ans. False.

(2) (a) Force is applied as ________ or _______

Ans. push, pull

(b) On squeezing a gum tube, its ______ changes.

Ans. shape

(c) On pulling a string, its _______ increases.

Ans. length

(d) A moving football when kicked, its _____ changes.

Ans. direction

(e) On applying brakes on a moving car, its speed _____

Ans. slows down

(f) We use ball bearing to ______ the friction.

Ans. reduce

(g) Friction _____ changes.

Ans. opposes

(h) Lubricants are used to ______ friction.

Ans. reduce

(i) Friction causes _____ of moving parts of a machine.

Ans. wear and tear.

(3) Match the following columns Solution:

Column A Column B
(a) Non Contact force (i) repeal
(b) Like poles (ii) kg
(c) Contact force (iii) Gravitational force
(d) Mass (iv) wear and tear
(e) Weight (v) force of friction
(i) Friction (vi) kgf

Ans:

Column A Column B
(a) Non Contact force (iii) Gravitational force
(b) Like poles (i) repeal
(c) Contact force (v) force of friction
(d) Mass (ii) kg
(e) Weight (vi) kgf
(i) Friction (iv) wear and tear

(4) Select the correct alternative Solution:

(i) A body falls downwards because of

(a) electrical force

(b) gravitational force

(c) mechanical force

(d) magnetic force

Ans. (b) gravitational force

(ii) A force does not change

(a) mass

(b) length

(c) shape

(d) state of motion

Ans. (a) mass

(iii) A force to be expressed correctly requires

(a) only the magnitude.

(b) only the direction

(c) acts in the direction of motion

(d) is always a nuisance

(iv) Friction

(a) promotes motion

(b) opposes motion

(c) acts in the direction of motion

(d) is always a nuisance

(v) Friction is reduced by

(a) making the surface wet

(b) making the surface dry

(c) making the surface rough

(d) sprinkling sand on the surface.

Ans. (a) making the surface wet

(vi) Friction

(a) causes wear and hear

(b) produces heat

(c) stops a moving body

(d) has all the above disadvantages

Ans. (d) has all the above disadvantages

(vii) Friction is increased if

(a) an oil is sprayed

(b) the surfaces are made yet

(c) the surfaces are made dry

(d) the surfaces are polished

Ans. (c) the surfaces are made dry

Short/ Long Answer Type Questions Answers Solution:

(1)  Name the term used for the push or pull.

Ans. Force.

(2) Give one example each of a force as (i) a push (ii) as pull (iii) a stretch and (iv) a squeeze.

Ans. The examples of each are following below:

(i) a push: To open a door, we push it.

(ii) as pull: To move a grass roller on a lawn it is pulled by a gardener.

(iii) a stretch: When a rubber string is stretched its length increases.

(iv) a squeeze: On squeezing a toothpaste tube its shape changes.

(3) Explain the meaning of the term force.

Ans. The push or pull on an object is called force.

(4) What effect can a force have on a stationary body?

Ans. A force can move a stationary body. When force is applied on a stationary object bthen the body begins to move.

(5) What effects can a force have on a moving body?

Ans. A force can make a moving body to move faster.

(6) What effect can a force produce on a body which is not allowed to move?

Ans.  A force can stop a moving body.

(7) Give one example each to indicate that the application of a force.

(a) produces motion.

(b) stops motion

(c) slows down motion

(d) changes the direction of motion

(e) deforms a body

Ans. (a) produces a motion: A ball lying on the ground moves, when its kicked.

(b) stops motion: A moving cricket ball is stopped by the fielder when he catches it, by applying a force.

(c) slows down motion: Pulls the swing in the opposite direction.

(d) changes the direction of motion: In tennis and badminton the direction of motion of the ball or cock is changed by hitting it in opposite direction.

(e) deforms a body: On squeezing a piece of rubber, its shape changes, on stretching a rubber string its length increases.

(8) state the effect produced by a force in the following cases:

(a) The sling of a rubber catapult is stretched

(b) A man pushes a heavy cart

(c) A player uses his stick to deflect the ball

(d) A cyclist applies brakes

(e) A spring is compressed.

Solution: The effects are following:

(a) A force can change the shape or size of a body.

(b) By applying force it will moves.

(a) A force can change the direction of motion of a moving body.

(d) A force can slow down a moving body.

(e) A force can change the shape or size of a body.

(9) Name the two kinds of forces in nature.

Ans. The two kinds of forces in nature are:

(i) Contact forces.

(ii) Non Contact forces.

(10) Name the type of force which acts in the following cases:

(a) A coolie lifts a luggage.

(b) A bicycle comes to rest slowly when the cyclist stops pedaling.

(c) A stone falls from a roof.

(d) A comb rubbed with silk attracts bits of paper.

(e) A string hangs with a load.

(f) A horse moves a cart.

(g) a magnet attracts an iron pin

(h) A boy opens a door

(i) An apple falls from a tree

(j) A man rows a boat.

Ans. (a) Contact force.

(b) Contact force.

(c) Gravitational force.

(d) Electrostatic force.

(e) Magnetic force.

(f) Contact force.

(g) magnetic force.

(h) contact force.

(i) Gravitational force.

(j) Contact force.

(11) What do you mean by the gravitational force? Give an example to illustrate it.

Ans. Gravitational force between two bodies at a distance such as the force of attraction between two heavenly bodies, the force of attraction on a body by the earth etc.

Example:  An apple falls from a tree.

(12) Define the term ‘weight of a body’

Ans. The weight of a body is the force with which the earth pulls the body.

(13) What do you understand by the term friction?

Ans. Friction is a force that opposes motion.

(14) Give an example to illustrate the existence of force of friction.

Ans. If we stop pedaling our bicycle, it gradually slows down and ultimately it stops after travelling a certain distance.

(15) What is the cause of friction?

Ans. The cause of friction is the interlocking of the irregular projections on the two surfaces in contact.

(16) State two factors which directly affect the force of friction.

Ans. The two factors which directly affect the force of friction:

(a) the smoothness (or roughness) of the surfaces in contact

(b) The weight of the sliding (or rolling) body.

(17) In which case will there be more friction between the truck and the road : when the truck is empty or when it is loaded.

Ans. When it is loaded.

(18) Which offers more friction on a body: a glass surface or a wooden surface?

Ans. A wooden surface.

(19) Name the three kinds of friction.

Ans.  The three kinds of friction is: Static friction, Sliding friction, Rolling friction.

(20) List the three disadvantages of friction.

Ans. The three disadvantages of friction are following:

(i) It opposes the motion of a body, so it decreases the efficiency.

(ii) It causes wear and tear in the moving parts.

(iii) It produces heat.

(21) When apply the brakes, the bicycle stops and the rim of the wheel becomes hot. Explain the answer.

Ans. It is our common experience that, when we apply the brakes, the bicycle stops and the rim of the wheel becomes hot. It is due to friction that the moving parts of a machine get warm.

(22) The eraser gets smaller and smaller as you use it more and more. Explain the reason.

Ans. It is our common experience that, by using more and more, eraser gets smaller. It is due to friction as we know friction causes wear and tear. By using gradually it wear out due to friction between the eraser and copy or books.

(23) List three ways of reducing friction.

Ans.  The three ways of reducing friction are:

(i) Making the surface smooth.

(ii) By the use of lubricants or oil.

(iii) Use of Ball bearing.

(24) It is difficult to open an inkpot with greasy or oily hands. Explain.

Ans. It is difficult to open an inkpot with greasy or oily hands because friction is reduced in machines by greasy or oily hands. That’s it.

(25) It is difficult to walk on a wet road. Explain.

Ans. It is difficult to walk on a wet road, This is because the surfaces of the ground made smooth.

(26) Give three example to illustrate that friction is a necessary evil.

Ans. There are countless necessity of friction in our daily life. The three necessity of friction are given below:

(i) Without friction we would not able to walk on road. It is the reaction of the force of friction which makes us move faster.

(ii) When the road becomes wet after rain, friction is reduced and hence the road becomes slippery.

(iii) It would be difficult to sit on a chair without friction or brush our teeth using a tooth brush.

(27) Define (i) Static friction (ii)( Sliding friction and (iii) Rolling friction.

Ans. (i) Static Friction: The maximum force exerted by a surface on a body so long as it remains stationary is called the force of static friction.

(ii) Sliding Friction: The minimum force required to keep the body moving over a surface such that it moves equal distances in equal intervals of time is called the force of sliding friction.

(iii) Rolling Friction:  The minimum force required to roll a body on a surface is called the force of rolling friction.

(28) Arrange the following in descending order of magnitude: static friction; sliding friction and rolling friction.

Ans. Static friction > Sliding friction > rolling friction.

(29) A body needs a force F1 just to start motion on a surface. A force F2 to continue its motion and a force F3 to roll on the surface. What is (i) the static friction (ii) sliding friction (iii) rolling friction? State whether F2 is equal , less than or greater than (i) F1 and (ii) F3

Ans. F1 is the static friction.

F2 is the sliding friction.

F3 is the rolling friction.

F2 is less than F1.

F2 is greater than F3

_____ X  _____


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