Class 8 Science

# Friction

## Extra Questions

Question 1: It is easier to open a bottle cap by gripping it with a towel. Why?

Answer: Rough surface of towel helps in increasing friction, and thus provides a better grip. The towel, thus, makes it easier to open the bottle cap.

Question 2: Why is it difficult to drive on a wet road?

Answer: It is very difficult to drive on a wet road because of less friction. A thin film of water is made over the surface of road. This thin film of water greatly reduces friction. Due to this, the driver finds it difficult to control the vehicle while driving on a wet road.

Question 3: How does sand paper help in increasing friction?

Answer: 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.

Question 4: Why is it difficult to write on a glass surface?

Answer: Glass surface is so smooth that it does not provide friction to the tip of the pen. Hence, it is difficult to write on a glass surface.

Question 5: Why is friction said to be a necessary evil?

Answer: Friction is all pervasive. Even a simple task, like writing on a page cannot be done without friction. Walking on road wonâ€™t be possible because friction. But friction causes wear and tear of materials, and wastage of energy. Thus, friction is beneficial as well as harmful. Hence, friction is called a necessary evil.

Question 6: Boats and aeroplanes are made in streamlined shape. Why?

Answer: A shape which is wide in middle and tapered at ends is called a streamlined shape. Such a shape reduces drag or fluid friction. Hence, a streamlined body can easily move through a fluid. Birds and fish are naturally endowed with streamlined body. Boats and airplanes are also made streamlined so that they can easily move through air or water.

Question 1: Discuss different types of friction.

Answer: Following are the three types of friction:

1. Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
2. Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.
3. Rolling Friction: When an object rolls over a surface, the friction created is called rolling friction. Rolling friction is less than sliding friction.

Question 2: Discuss the benefits of friction.

Answer: Benefits of friction are as follows:

• It is not possible to even walk on the road, in the absence of friction. Walking on ice is almost impossible because of lack of friction.
• Friction is necessary to keep a ladder against a wall. The ladder will not find grip against the wall in the absence of friction.
• It is very difficult to drive on a wet road because of less friction. A thin film of water is made over the surface of road. This thin film of water greatly reduces friction. Due to this, the driver finds it difficult to control the vehicle while driving on a wet road.

Question 3: Discuss the harms of friction.

Answer: Following are the harms of friction:

• It results in wear and tear of shoes and tyres, machine parts, clothes, etc. It causes monetary loss.
• Heat is generated by friction, which results in wastage of energy. Moreover, a significant amount of energy is wasted in overcoming friction.

Question 4: Suggest some methods to increase friction.

Answer: 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.

Question 5: Suggest some methods to reduce friction.

Answer: Some methods to reduce friction are as follows:

• Lubricating oil and grease are used in machines to reduce friction between moving parts.
• Some machines use air cushion for reducing friction because use of oil is not ideal in such machines.
• Graphite powder is used in some machines to reduce friction.
• Ball bearing coverts sliding friction into rolling friction. You have read that rolling friction is less than sliding friction. Thus, ball bearing helps in reducing friction.