NCERT 8th Class CBSE Science: Friction

Question: Give two situations where it is desirable to increase friction.

Answer: Two situations where it is desirable to increase friction are:

1. Tyres have designs and patterns with grooves on the surface to increase fraction with the road. This presents slipping of the tyres on the wet road.
2. Spikes are provided in the soles of shoes used by players and athletes to increase friction so that they get a firm grip on the ground.

Friction 8th Science – Question: What is ‘streamlining’? How is it useful?

Answer: Streamlining is a design that offers the least resistance to the flow of a fluid, especially air or water. Friction is minimum for a streamlined shape, which is rounded in the front and narrow at the back.

It is useful as:

1. The bodies of aeroplane, missiles, rockets, cars are streamlined to reduce friction with air called air resistance.
2. Ship also have streamlined shapes to reduce friction with water.

Question: Discuss three situations in daily life where friction is an advantage?

Answer: Three situations in daily life where friction is an advantage:

1. It would be impossible for us to walk without friction. It is because of friction between our feet or shoes and the ground that we are able to walk by pushing our feet against the ground. Without friction, we would slip.
2. Brakes on cycles or cars work because of friction. When brakes are applied, the ‘shoes’ of the brakes rub against the wheels. The friction between them reduces the speed.
3. Friction causes nails and screws to hold on to walls, write on paper and even the lighting of matchstick.

Question: Explain with examples the disadvantages of friction.

Answer: Friction has many disadvantages such as:

1. It produces heat in various parts of machines. In this way some useful energy is wasted as heat energy. This reduces the proper functioning of machines and reduces there life.
2. Friction wears out the rubbing surfaces. The soles of our shoes, moving parts of a machine, tyres of vehicles wear out in few months due to friction.
3. Friction generates heat which is harmful. The heat produced in fast moving machines is very high and can damage the machine. like in cars if we do not add coolants the car does not work properly, engine can be jammed.

Question: Explain four methods by which friction between two solid surfaces in contact can be reduced.

Answer: The four methods by which friction between two solid surfaces in contact can be reduced are:

1. By polishing – Polishing a rough surface smoothens it and reduces the friction.
2. By lubrication – Lubricating the surface with oil, grease or graphite decreases the friction between two surfaces.
3. By Streamlining the object – The bodies of aeroplane, birds, fish, missiles, rockets, ships all have streamlined shape to reduce the frictional force exerted by fluids like water and air.
4. By using rolling friction – It is easier to roll an object than to slide it. Rolling friction is therefore less than sliding friction. For example – wheels in vehicles, luggage etc. and ball bearings which are used in machines to reduce friction.

Question: Why do you think rolling friction is less than sliding friction.

Answer: Rolling friction is less than sliding friction as in rolling friction the surface in contact is less than the sliding friction.