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C. B and D 

A spring balance can measure both the force acting on it as well as the weight of the object.

The correct answer is (a) μ < μ', M < M'  

Explanation: 

The force applied by the boy on the box is equal to the force of friction between the shoes of the boy and the floor which is equal to µMg. 

Force of friction between the box and the floor = µ'M'g. 

To slide the box µMg>µ'M'g 

→    µM>µ'M' This is not possible only if  µ<µ', M<M' 

The correct answer is (a) μ < μ', M < M'  

Explanation: 

The force applied by the boy on the box is equal to the force of friction between the shoes of the boy and the floor which is equal to µMg. 

Force of friction between the box and the floor = µ'M'g. 

To slide the box µMg>µ'M'g 

→    µM>µ'M' This is not possible only if  µ<µ', M<M' 

Force of friction will be zero as the net force on the box is zero.

Threading in tyres provide friction for cars or buses so that they can move safely therefore friction is important for transport.

D) Heat shield reduces heat produced by friction

No, we cannot. Friction can never be entirely eliminated because no surface is perfectly smooth and some irregularities are always there.

1. We are using grease, oil, wax and other lubricants in the vehicles and machinery to reduce the friction.

2. Ball bearings are also used to reduce the friction. 

3. Engine oil, gear oil, brake oil, clutch oil, etc. are used in vehicles. 

4. Shock up oil is also used. 

5. Levers are reducing the friction. 

6. In trolleys and other vehicles which should carry heavy loads, the wheels are fixed.

Correct option is D) Grease

Correct option is B) lubricants

1. Adding lubricants to machinery parts. 

2. Using roller type ball bearings. 

3. Using air pressure to separate moving parts. 

4. Using magnetic repulsion to separate moving parts.

5. Using materials that tend to slide easily over each other. 

6. Regulate the temperature of the machine. 

7. The use of drive shafts.

If we do not reduce friction in machines, then friction causes overheating and wearing out of moving parts of machinery.

The substances which reduce friction are called lubricants.

1. Rub your palms against each other for a few minutes.

2. You can feel heat in your palms. 

3. Hence we can say that friction produces heat.

Ways of reducing friction:

a) Polishing: Decreasing irregularities of surface (or) smoothening the surface.

b) Lubricants: Using lubricants which will decrease the friction.

c) Ball bearings: Using ball bearings in machinery.

d) Steam lining: Automobiles and aeroplanes are steam lined to reduce the friction due to air.

Lubricants (oil or grease) are applied to rubbing surfaces of machines because when the parts rub against each other, heat is produced. To avoid the wearing and tearing of the machine parts, lubrication is done. Because it reduces the friction and thus less heat is produced.

Some methods to reduce friction are as follows:

• Rubber grips are put on bicycle handles, electrical tools and on many other tools. Increased friction helps in giving better grip on these objects.
• A surface can be made rough with the help of sand paper; in order to increase friction.
• Soles of shoes are made rough to increase friction. This helps in providing better grip on surface while walking.
• Treads of tyres are deliberately made so that a good road grip can be achieved with the help of friction.
• Holding a bottle cap with a thick towel helps in increasing friction. Increased friction makes it easier to open the bottle cap.

Fluid friction can be minimized by giving a suitable shape to bodies moving in fluids.

Wheels are important because they are used for transportation. Heavy machines can be moved from one place to another with the help of wheels because wheels can be rolled and the amount of force is also comparatively less.

The two methods of increasing friction are:–

I. Making both the surfaces rough which is increasing the irregularities.

II. Making use of Sliding friction rather than Rolling friction.

Lubrication means applying some additive to the parts of a machine to reduce the friction. Its important because it reduces the friction between the parts of the machine. But excess lubrication can generate more heat.

Example- Castor Oil, Grease.

A smooth surface can be made rough with the help of sandpaper. There are more irregularities on a rough surface than on a smooth surface. Thus, sand paper helps in increasing friction; by increasing the roughness of a surface.

A) to decrease friction

C. sliding friction into rolling friction 

Because rolling friction allows to move heavier objects from one place to other easily.

B) sliding friction

C) rolling friction

By applying any lubricant , say grease, one can reduce friction between those machine parts which rub against each other to avoid wear and tear.

a) Yes, the reason is when we add a wooden block to the existing wooden block or apply force by pressing vertically, the normal force increases and hence we find increase in frictional force.

b) Friction is proportional to the normal force, i.e., friction °c normal force.

The friction between sand paper and metal sheet is very large, compared to that between the ordinary paper and the metal sheet. Thus the sand paper is able to remove the outer dull layer from the metal sheet more effectively and makes it more shining.

B) static friction

This is because the paper surface offers more friction to the pencil rubber when we rub on the paper, some bits of rubber come on the paper.

D) Sliding friction

(a) A pencil is made up of graphite. The paper surface offers more friction to the pencil when we write on the paper, in this way when the pencil is rubbed on the paper, some bits of graphite come on the paper and we are able to write. But glass offers no friction to the pencil, therefore we can’t write on a glass with pencil.

(b) Climbing a greasy pole is very difficult because the greasy pole offers very less friction to the feet of the person climbing on it. Du to which, the person slips down again and again.

The banana peel has fibers on it which makes it smooth and when we place our foot on it the smooth surface is in contact with the ground reducing the friction and the peel goes forward with even low force and takes our feet also with it, so we slip.

C. it enables rubber pads to be rubbed off 

Friction does wear and tear of the objects.

A car spins on icy road because the treads of the car can no longer maintain the proper friction to keep it moving. Due to ice, the friction reduces and the car spins.

B) Shape of the body

Frictional force is responsible the wearing of footsteps of over-bridges.

B) Assertion is correct, Reason is incorrect.

Friction enables us to fix nails in a wall and knots to be tied.