Force

gravitational force

Reason — It is the force of gravity on a body due to which earth continuously attracts the body towards itself therefore it tends to fall vertically downwards.

mass

Reason — A force does not change the mass of the body on which it is applied.

both the magnitude and direction

Reason — A force is expressed by stating both its magnitude and direction.

opposes motion

Reason — Friction is the force that opposes the relative motion between the two surfaces in contact with each other.

making the surfaces wet

Reason — Friction is reduced by making the surfaces wet since when liquid is applied between the surfaces, it fills the fine pores or depressions in the surfaces and thus reduces friction.

has all the above disadvantages

Reason — Disadvantages of friction are :

1. Friction opposes the motion of a body, so it decreases the efficiency (i.e., more force is needed to move a body).
2. Friction causes wear and tear in the moving parts.
3. Friction produces heat.

the surfaces are made dry

Reason — Friction can be increased by the following three ways :

1. By making the surfaces rough
2. By using dry surfaces
3. By increasing the weight

A is true but R is false

Explanation

Assertion (A) is true because when roads become wet after rain, the rainwater forms a thin layer between the tyres and the road surface.
This layer fills the irregularities of the road and acts like a lubricant, reducing the interlocking between the tyre and road.
As a result, the friction between them decreases, making it easier for the tyres to slip.

Reason (R) is false because rolling friction does not increase; in fact, the effective friction between the tyre and road decreases due to the presence of water.

Therefore, A is true but R is false.

A is false but R is true

Explanation

Assertion (A) is false because lubricants do not increase friction; they reduce friction between surfaces.

Reason (R) is true because lubricants form a smooth layer between the two surfaces in contact which decreases the effective friction between the surfaces.

Therefore, A is false but R is true.

(a) Force is applied as push or pull.

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

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

(d) A moving football when kicked, its direction of motion changes.

(e) On applying brakes on a moving car, its speed slows down.

(f) We use ball bearings to reduce the friction.

(g) Friction opposes motion.

(h) Lubricants are used to reduce friction.

(i) Friction causes wear and tear of moving parts of a machine.

(a) False because friction is the force that opposes the relative motion between the two surfaces in contact with each other.

(b) False since weight, being a force, is expressed in gravitational unit kgf or gf whereas kg or g is the unit of mass.

(c) True as if a force is applied on a moving body in a direction other than its direction of motion, the direction of motion of the body changes.

(d) False because a force does not change the mass of the body on which it is applied.

(e) False as friction is advantageous to us in almost all activities of our life, for example it helps us in walking or rotating a wheel etc.

(f) True since static friction > sliding friction > rolling friction

(g) True as gases offer less friction than liquids, while liquids offer less friction than solids.

(h) False because a dry rough road offers more friction than a wet oily road due to large numbers of pores or depressions in the dry road as compared to wet oily road.

Force.

Examples are given below :

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

(ii) Force as pull : A rickshaw man pulls on the rickshaw to move it

(iii) Force as stretch : On stretching a spring, it elongates.

(iv) Force as squeeze : On squeezing a tube of gum or toothpaste, its shape changes.

A force is that cause which changes the state of rest or the state of motion of a body or changes the size and shape of the body. A force has both magnitude and direction.

When a force is applied on a stationary body, the body begins to move. For example, a car originally at rest when pushed, begins to move.

Force can have following effects on a moving body:

1. If the force is applied in the direction opposite to the direction of motion then it can:
   1. stop the moving body.
   2. slow down the moving body.
2. If the force is applied in the same direction as the direction of motion then it can make a moving body to move faster.
3. If the force is applied in a direction other than its direction of motion, the direction of motion of the body changes.

When a force is applied on a body which is not allowed to move, it gets deformed, i.e., the shape or size of the body changes. For example, on squeezing a piece of rubber, its shape changes.

(a) A ball lying on the ground moves, when it is kicked.

(b) A moving cricket ball is stopped by the fielder when he catches it, by applying a force with his hands in a direction opposite to the motion of ball.

(c) Speed of a moving vehicle is slowed down by applying brakes. The brakes provide a force in a direction opposite to the direction of motion.

(d) In hockey, a player applies force with a hockey stick to change the direction of motion of the ball.

(e) On squeezing a piece of rubber, its shape changes.

(a) The sling of a rubber catapult is stretched → changes the shape of the sling and increases it's length.

(b) A man pushes a heavy cart → changes the state of motion so cart starts moving or speeds up.

(c) A player uses his stick to deflect the ball → changes direction of motion of the ball.

(d) A cyclist applies brakes → the speed of the cycle slows down and may come to rest.

(e) A spring is compressed → shape changes and length of the spring decreases.

The two kinds of forces are:

1. Contact forces
2. Non contact forces

(a) A coolie lifts a luggage : Muscular force

(b) A bicycle comes to rest slowly when the cyclist stops pedalling : Frictional force

(c) A stone falls from a roof : Gravitational force

(d) A comb rubbed with silk attracts bits of paper : Electrostatic force

(e) A string hangs with a load : Tension force

(f) A horse moves a cart : Muscular force

(g) A magnet attracts an iron pin : Magnetic force

(h) A boy opens a door : Muscular force

(i) An apple falls from a tree : Gravitational force

(j) A man rows a boat : Muscular force

The weight of a body is the force by which earth attracts that body, so the weight of a body is also called its force due to gravity.

Friction is the force that opposes the relative motion between the two surfaces in contact with each other.

When we stop pedalling a bicycle, it gradually slows down and stops. This is because an opposing force acts on it. That force, which resists motion and brings the bicycle to rest, is friction.

The force of friction arises because of the interlocking of the irregular projections on the two surfaces in contact.

Two factors which directly affect the force of friction are:

1. The smoothness of the surface.
2. The nature of medium (solid, liquid or gas) in which the body moves.

When the truck is loaded then there will be more friction between the truck and the road as friction depends upon the weight of a moving body on a surface such that greater the weight of the moving body on a surface, more is the force of friction on the body by the surface.

A wooden surface offers more friction than a glass surface because a wooden surface has larger number of fine pores or depressions in its surface as compared to glass, having a smooth surface and hence low irregularities.

Three kinds of friction are as follow :

1. Static friction,
2. Sliding friction, and
3. Rolling friction

Disadvantages of friction are :

1. Friction opposes the motion of a body, so it decreases the efficiency (i.e., more force is needed to move a body).
2. Friction causes wear and tear in the moving parts.
3. Friction produces heat.

Ways of reducing friction are :

1. By making the surfaces smooth.
2. By use of lubricants.
3. By the use of ball bearings

Examples that show friction is a necessary evil :

1. In absence of friction, we would not be able to walk on road. It is the reaction of the force of friction which makes us move ahead.
2. Friction helps in supporting a ladder inclined to a wall.
3. Friction is necessary to light a match-stick.

(i) Static friction: The maximum opposing force between the object and the surface in contact with it, so long as the object remains stationary even on applying the external force, is called the static friction.

(ii) Sliding friction: When the body begins to slide on a surface, the force exerted by the surface on the object is called the sliding friction.

(iii) Rolling friction: When an object rolls over a surface, the force which opposes the rolling motion of the object is called the rolling friction.

Descending order is :

Static friction > sliding friction > rolling friction

Gravitational force is the force of attraction that exists between any two objects having mass which is the same force that pulls objects towards the centre of the Earth and keeps planets in their orbits around the Sun.

The weight of a body is the gravitational force with which the Earth attracts it, which is why it always acts vertically downwards.

For example, when leaves or fruits fall from a tree, they come down towards the ground due to the gravitational force exerted by the Earth.

When we apply the brakes on a bicycle, the brake pads press tightly against the rims of the wheels. This contact creates friction, which is the force that opposes the motion of the rotating wheels.
Friction is the force that opposes motion. It slows down the wheels and eventually stops the bicycle. But when friction acts, it also produces heat. That is why the rims become hot after braking.

When we rub an eraser on paper, friction acts between the surface of the eraser and the paper. This frictional force opposes the motion and causes tiny particles of the eraser material to wear away along with the graphite particles from the pencil marks. Repeated rubbing gradually removes more and more of the eraser’s material. This continuous loss of material due to friction is why the eraser becomes smaller over time.

It is difficult to open an inkpot with greasy or oily hands because the friction between our fingers and the inkpot lid becomes very low.

Friction is necessary to get a good grip and apply the force needed to turn and open the lid. When our hands are oily, the surfaces become slippery, reducing friction significantly.

As a result, we cannot hold the lid firmly or apply enough force effectively, which makes it hard to open the inkpot.

It is difficult to walk on a wet road because the water reduces the friction between our feet (or shoes) and the road surface.

Friction is necessary to grip the ground and push backward while walking.

When the road is wet, the thin layer of water acts as a lubricant, making the surface slippery and this decrease in friction causes our feet to slip more easily, making it harder to walk steadily and safely.

(i) Since F₁ is needed to start the motion which means it behaves like static friction.

So,

Static Friction → F₁

(ii) Since F₂ is needed to continue the motion which means it behaves like sliding friction.

So,

Sliding Friction → F₂

(iii) Since F₃ is needed to roll on the surface which means it must be rolling friction.

So,

Rolling Friction → F₃

(a) Since F₁ is static friction and F₂ is sliding friction and as static friction is greater than sliding friction hence, F₂ is less than F₁.

(b) Since F₃ is rolling friction and F₂ is sliding friction and as sliding friction is greater than rolling friction hence, F₂ is greater than F₃.

Friction is both a necessity and an evil because it has useful effects as well as undesirable consequences. There are a lot of examples to justify the statement.

Friction as a Necessity (Useful Effects) :

1. Helps in walking – Without friction between our feet and the ground, we would slip and fall.
2. Enables writing – Friction between pen/pencil and paper allows us to write.
3. Stops vehicles – Brakes work due to friction between brake pads and wheels.
4. Objects can be held – We can hold and grip things because of friction between our fingers and the object.

Friction as an Evil (Harmful Effects) :

1. Wears out machines – Continuous rubbing causes wear and tear in machine parts.
2. Reduces efficiency – Friction converts useful mechanical energy into heat, lowering efficiency.
3. Causes overheating – Moving parts in engines and machines heat up due to friction.
4. Makes movement harder – Extra force is needed to overcome friction while moving heavy objects.

So, from above examples it is visible that Friction is essential for many daily activities, but it also causes damage and energy loss, which is why it is considered both a necessity and an evil

both (2) and (3)

Reason — For a rocket like Chandrayaan to be successfully launched from Earth's surface into space, it must overcome:

(a) Gravitational force of Earth – This is the primary force pulling the rocket downward, and a significant amount of thrust is needed to overcome it.

(b) Frictional force of Earth's atmosphere – As the rocket ascends, it encounters air resistance (friction), especially in the lower layers of the atmosphere.

So, the rocket must overcome both gravity and atmospheric friction during launch.

Static, sliding, Rolling

Reason — Static friction > sliding friction > rolling friction

Glass tumbler

Reason — When mustard oil is applied on hands, it acts as a lubricant, reducing roughness and making the hands slippery. Also, the friction between glass surfaces and other objects is less because glass is smoother than most materials. So, holding an object with oily hands is very difficult due to reduced friction. Since a glass tumbler itself is also very smooth and slippery, it becomes even harder to grip firmly with oily hands.

(i) The car moved most easily on the smooth marble floor because it required the least effort and took the least time which indicates minimum friction on the smooth surface.

(ii) The carpeted floor exerts maximum friction because it required the highest effort and the car took the longest time thus, rough surfaces like carpets offer more resistance to motion.

(iii) If the weight of the car increases, the frictional force will increase on all surfaces. As a result, the car will become harder to pull and will take more time to cover the same distance on each surface.

(iv) Two advantages of friction :

• Friction helps us walk without slipping.
• Friction allows us to write on paper.

(v) Effects of force shown in the experiment :

• Force is used to move an object from rest.
• Force can change the speed of an object (car moves faster or slower depending on friction).