Lakhmir Singh & Manjit Kaur: Friction- 1 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.1. Name the force which always opposes motion.

The friction always opposes the motion.

Q.2. Why does a fast car slow down if its engine is switched off?

When the engine of a car is switched off, then there is no energy available for the engine to oppose the frictional forces. Thus, the engine slows down and eventually comes to rest.

Q.3. Which type of surfaces produce
(a) least friction, and
(b) too much friction?

(a) Smooth surfaces offer least friction.
(b) Rough surfaces offer high friction.

Q.4. What is the direction of force of friction acting on a moving object?

The friction forces always act in the opposite direction of motion.

Q.5. Name a device which is used to measure force acting on an object.

Dynamometer is used to measure the forces acting on any object.

Q.6. What is a spring balance?

A spring balance is used to measure the weights of object with the help of tension force.

Q.7. Out of sliding friction, static friction and rolling friction:
(a) which one is the smallest?
(b) which one is the largest?

(a) Rolling friction is the smallest in magnitude.
(b) Static friction is the largest in magnitude.

Q.8. Which type of friction comes into play when a book kept on cylindrical pencils is moved by pushing?

Rolling friction comes into play when a book kept on cylindrical pencils is moved by pushing.

Q.9. Why is it more difficult to walk properly on a well-polished floor?

The well-polished floor offers very less resistance. Therefore, the shoes slip on it and its very difficult to walk properly.

Q.10. Why is it difficult to walk on a wet marble floor?

It is very difficult to walk on a wet floor because the friction is less and the required grip cannot be maintained.

Q.11. Which force is responsible for the wearing out of car tyres?

Frictional force is responsible the wearing of car tyres.

Q.12. What prevents you from slipping every time you take a step forward?

Frictional forces prevent us from slipping every time you take a step forward.

Q.13. Name the force which helps things to move and stop.

Frictional forces helps things to move and stop.

Q.14. What enables us to fix nails in a wall and knots to be tied?

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

Q.15. What makes the steps of foot over-bridges at Railway Stations to wear out slowly?

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

Q.16. What is done to increase friction between the tyres and road?

The tyre’s surface has treads on it. Treads improve the grip of the tyre on the road.

Q.17. Why do gymnasts apply a coarse substance to their hands?

Gymnasts apply a coarse substance to their hands to increase the frictional forces on their hands so that they can hold firmly.

Q.18. Why do kabaddi players rub their hands with dry soil?

Kabaddi players rub their hands with dry soil before playing so as to increase friction. This enables them to grip the other player’s hand properly.

Q.19. Name the device which is used between the hubs and axles of bicycle wheels to reduce friction.

Ball bearing is used between the hubs and axles of bicycle wheels to reduce friction.

Q.20. What is the purpose of using ball bearings in machines?

Ball bearing reduce friction between the support loads and axles of a vehicle.

Q.21. Name any two machines in which ball bearings are used.

Car and bicycle.

Q.22. Name the device which is attached to heavy luggage (such as a heavy suitcase) to move it easily by pulling.

Pulley.

Q.23. Name one example from everyday life where wheels (or rollers) are used to reduce friction.

Car

Q.24. Why does oiling the axles of a bicycle make the bicycle move more easily?

Oiling the-axles of a bicycle reduces the friction and thus the bicycle moves easily.

Q.25. State one way in which the friction between the wheel and its axle can be reduced.

The ball bearing help in reducing the friction between the wheels and its axle.

Q.26. Name two common lubricants.

Castor oil and Grease are the two common lubricants.
Note: A lubricant is a substance that smoothens a surface. It decreases the friction of a surface.

Q.27. Why do we sprinkle fine powder on carrom board?

Sprinkling fine powder on carrom board reduces friction between carom and striker.

Q.28. Which force gets reduced when the two surfaces in contact are polished to make them smooth?

Frictional force gets reduced when the two surfaces in contact are polished to make them smooth.

Q.29. Why is the surface of a slide polished to make it smooth?

The surface of a slide polished to make it smooth in order to reduce friction.

Q30. Name the force which increases when the two surfaces in contact are made more rough.

Frictional force increases when the two surfaces in contact are made more rough.

Q.31. What is the special name of frictional force exerted by fluids (like air or water)?

Drag is the special name of frictional force exerted by fluids.

Q.32. What is the name of ‘special shape’ which is given to objects moving through air (or water) to reduce drag?

The special shape is more like a point shaped so that they cut the fluid friction.

Q.33. Why are grooves provided in the soles of shoes?

Grooves in the soles of shoes help in increasing the friction when we walk on the roads. The grooves in the shoe sole help in maintaining a grip with the ground.

Q.34. Why are treads made in the surface of tyres?

Treads are made on the surface of tyres help in increasing the friction. These treads help in maintaining a firm grip on the road which reduces the risk of slipping on the road thus avoiding accidents.

Q.35. Fill in the following blanks with suitable words :
(a) Friction always opposes ________ between the surfaces in contact with each other.
(b) Sliding friction is ________ than the static friction.
(c) Friction produces________.
(d) Friction prevents our foot from ________ over the ground.
(e) Sprinkling of powder on the carom board ________ friction.
(f) Ball bearings reduce friction because they ________ rather than ‘slide.
(g) The friction when something moves through a liquid or gas is called ________.
(h) Cars and speedboats are ________ to reduce drag.
(i) Shapes that are designed to reduce air resistance are called ________ shapes.
(j) Objects which can move quickly through the water have a ________ shape.
(k) The shape of an aeroplane is similar to that of a ________ in flight.

(a) Friction always opposes motion between the surfaces in contact with each other
(b) Sliding friction is less than the static friction.
(c) Friction produces heat
(d) Friction prevents our foot from slipping over the ground
(e) Sprinkling of powder on the carom board reduces friction.
(f) Ball bearings reduce friction because they roll rather than ‘slide.
(g) The friction when something moves through a liquid or gas is called drag.
(h) Cars and speedboats are streamlined to reduce drag.
(i) Shapes that are designed to reduce air resistance are called streamlined shapes.
(j) Objects which can move quickly through the water have a streamlined shape.
(k) The shape of an aeroplane is similar to that of a bird in flight.