Lakhmir Singh & Manjit Kaur: Friction- 2 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.36. When we try to push a very heavy box kept on ground, it does not move at all. Which force is preventing this box to move forward? Where does this force act?

Whenever we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

Q.37. Suppose your writing table (or desk) is tilted a little. A book kept on the table starts sliding down. Draw a diagram to show the direction of force of friction acting on the book.

Q.38. Which will cause more friction: a rough surface or a smooth surface? Why?

A rough surface will cause more friction than a smooth surface. Because, rough offers more resistance to the other surface in contact as compared to the smooth surface.

Q.39. Explain why sliding friction is less than static friction.

The sliding friction is less than static friction because, in sliding friction, the interlinking of the two surfaces is for very short time.
Friction mainly arises due to the interlocking of irregularities between two surfaces, as shown below in the diagram:

In smooth surfaces, there are very fewer irregularities, thus friction is very very less. But, on rough surfaces, the irregularities are very large.
In sliding friction, the time of interlocking is very less, as one object slides over another object. Whereas, in static friction, one object is moved over another object thus the irregularities time is more. Thus, the two surfaces could not properly come in contact with each other in sliding due to which the magnitude of static friction is more than sliding friction.

Q.40. What is meant by ‘rolling friction’?

Rolling friction is the friction or drag which opposes the motion when a body rolls.

Q.41. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why?

Seema will apply a larger force, because heavier objects generates more frictional force due to the two surfaces being pressed together harder.

Q.42. Why does a man slip when he steps on a banana peel thrown on the road?

While walking on the road, a man slips if he steps on a banana peel because the banana peel acts a lubricant between the road and the man’s shoe which reduces the friction. Thus, man slips on it.

Q.43. Car wheels often spin on icy roads. Explain why.

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.

Q.44. Explain why:
(a) A pencil will write on paper but not on glass.
(b) climbing a greasy pole is very difficult.

(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.

Q.45. Why does a matchstick light when we strike it on a rough surface ?

When a matchstick is rubbed on a rough surface, the surface produces friction. And we know friction produces heat, therefore the matchstick lightens up.

Q.46. Why is it difficult to light a matchstick by striking it on a smooth surface ?

The smooth surface offers very less friction, due to which very less heat is produced by it and it is not sufficient to lighten up the matchstick.

Q.47. Which parts wear away first in shoes? Give a reason for your answer.

The heels of the shoes get wear and tear first than the rest of the shoe sole. This is because, the heel of the shoe is interacted with the ground more than the other part and due to friction, the treads of the shoe sole wear out.

Q.48. Why do brake pads of bicycles have to be replaced quite often?

The brake pads of bicycles have to be replaced quite often because they gather more dirt, dust and the filthy matter on itself. So to allow proper functioning, they have to be replaced.

Q.49. A pencil eraser loses tiny pieces of rubber each time you use it. Why does this happen ?

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.

Q.50. What happens when you rub your hands vigorously for a few seconds ? Why does this happen ?

By rubbing our hands against each other we create friction between our hands and this friction produces heat. Therefore, out hands become warm.

Q.51. Explain how, friction enables us to walk without slipping.

When step forward and walk, we are applying force (denoted by “F”) on the ground. Now, in return ground also applies the reaction force to us. But we can fall if friction (denoted by “f”) is not present as the large ground can apply a large force . This is explained in the diagram:

Q.52. Which is easier to hold in hand: a kulhar (earthen pot) or a glass tumbler? Why?

It is easier to hold a kulhar (an earthen pot) because it more rough surface as it is made from mud. Mud offers more friction than glass and we are able to from a grip on the earthen plot.

Q.53. How does a bicycle stop when its brakes are applied ?

When we apply brake sot the bicycle, a force of friction generates between the rim of the bicycle and the brakes. The friction is produced in opposite direction of motion. Thus, the bicycle is stopped.

Q.54. Explain why, the soles of our shoes wear out gradually.

The shoes rub on the ground while walking. The ground offers friction due to which the sole of shoes wear and tear out.

Q.55. Why do tyres of cars wear out gradually ?

The tyres of the car wear out due to friction produced between he tyres and the car when a car is moving.

Q.56. State two advantages and two disadvantages of friction.

Disadvantages of Friction:

• Friction always opposes he motion.
• Friction causes wearing and tearing.

Advantages of Friction:

• Friction help us to walk.
• A horse can’t pull a cart without friction.

Q.57. Explain why, sportsmen use shoes with spikes.

The sports people use shoes with spikes, spikes increase the friction with the ground and does not let the person slip or fall when running fast.

Q.58. How will you reduce friction between those machine parts which rub against each other? Give the simplest method.

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

Q.59. What is meant by lubrication ? Why is it important?

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.

Q.60. Explain why, wheels are so useful.

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

Q.61. Why are lubricants (oil or grease) applied to rubbing surfaces of machines?

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.

Q.62. Explain with the help of diagrams, how the use of oil reduces friction between two surfaces in contact with each other.

If we oil between the two surfaces, say part1 and part 2 of a machine, then the oil layer will allow less contact of the two surfaces with each other as you can see in the figure below:

In this way, the friction between the two surfaces is decreased.

Q.63. Why are cars, aeroplanes and rockets streamlined ?

The cars, aeroplanes and rockets are streamlined so as to reduce the fluid friction. The air drag obstructs the vehicle to move forward and the fuel is consumed mainly for opposing this opposite frictional force. Therefore, the objects are given special shape called streamline shape, which itself cuts the air frictions.

Q.64. Explain why, a speedboat has a streamlined shape.

A speedboat has a streamlined shape so as to reduce the fluid friction as it moves in the waters.

Q.65. What are fluids? Name two common fluids.

Fluids are those whose which do not have any shape and flow like liquids. Example- Air and Blood.